Hi all and knowing there are some scientists / mathematicians walking
amongst us ... ;-)

I'm trying to see if there is a (rough even) way of measuring the
Depth Of Discharge voltage / cutoff point versus current load value
for a specific lead acid battery.

The manufacturers have provided me the following values (@ 50% DOD).

11.75V @ 93A (12.4V initial)
12.05V @ 18.2A (12.7V initial)
12.10V @ 4.65A (12.75V initial)

(The general chart starts at the values marked 'initial' so that would
be for those currents but at 0% DOD but I'm not sure if that is
relevant).

Now, it's obviously not a straight line (Peukert's law), but can you
extrapolate a graph (or create a formula that would be more useful for
my project) from just 3 points please?

Maybe Peukert's law itself would give those with a brain wired
differently than mine the answer? ;-)

https://en.wikipedia.org/wiki/Peukert%27s_law#Formula

Cheers, T i m

p.s. The nearest *I* could get to an answer would be some graph paper
and a Flexicurve. ;-)

On 14/09/2017 20:14, T i m wrote:
Hi all and knowing there are some scientists / mathematicians walking
amongst us ... ;-)

I'm trying to see if there is a (rough even) way of measuring the
Depth Of Discharge voltage / cutoff point versus current load value
for a specific lead acid battery.

The manufacturers have provided me the following values (@ 50% DOD).

11.75V @ 93A (12.4V initial)
12.05V @ 18.2A (12.7V initial)
12.10V @ 4.65A (12.75V initial)

(The general chart starts at the values marked 'initial' so that would
be for those currents but at 0% DOD but I'm not sure if that is
relevant).

Now, it's obviously not a straight line (Peukert's law), but can you
extrapolate a graph (or create a formula that would be more useful for
my project) from just 3 points please?

Maybe Peukert's law itself would give those with a brain wired
differently than mine the answer? ;-)

https://en.wikipedia.org/wiki/Peukert%27s_law#Formula

Cheers, T i m

p.s. The nearest *I* could get to an answer would be some graph paper
and a Flexicurve. ;-)


Probably no simple answer because it all depends on the shape of the curve.

I'm not familiar with that particular "law" (I use the term advisedly)
but exponentials are buggers to deal with (especially when, as in this
case, it is obviously only an approximation: exponentials are fine for
radioactive decay, but it will certainly have limits in this case).

I think my approach would be to try to collect some data for your
specific battery, and try to work with that. Quite possibly with a
flexicurve, or with some sort of polynomial fit if there was more data
available.

Sometimes you can over-think a problem, you have to work with the data
and tools available.

As an example, compare the different "lines" of a Spitfire or a Vanwall
with the various front wings of current F1 cars. Modern stuff has the
benefit of serious CFD and wing-tunnel testing. And it is justified in
that context.

"newshound" wrote in message
o.uk...
On 14/09/2017 20:14, T i m wrote:
Now, it's obviously not a straight line (Peukert's law), but can you
extrapolate a graph (or create a formula that would be more useful for
my project) from just 3 points please?

p.s. The nearest *I* could get to an answer would be some graph paper
and a Flexicurve. ;-)


Probably no simple answer because it all depends on the shape of the
curve.

I'm not familiar with that particular "law" (I use the term advisedly) but
exponentials are buggers to deal with (especially when, as in this case,
it is obviously only an approximation: exponentials are fine for
radioactive decay, but it will certainly have limits in this case).

I think my approach would be to try to collect some data for your specific
battery, and try to work with that. Quite possibly with a flexicurve, or
with some sort of polynomial fit if there was more data available.

I'd say the best approach would be to find a transformation (eg y=log(x) or
y=sqrt(x)) which gives a good, well-correlated straight line. Then
extrapolate that and do an inverse transformation (eg antilog or x-squared)
on the predicted value. Obviously the more data points you have, the better
prediction you can make and the better you can construct a least-squares
regression line for extrapolation and then back-transformation.

T i m wrote:
Hi all and knowing there are some scientists / mathematicians walking
amongst us ... ;-)

I'm trying to see if there is a (rough even) way of measuring the
Depth Of Discharge voltage / cutoff point versus current load value
for a specific lead acid battery.

The manufacturers have provided me the following values (@ 50% DOD).

11.75V @ 93A (12.4V initial)
12.05V @ 18.2A (12.7V initial)
12.10V @ 4.65A (12.75V initial)

(The general chart starts at the values marked 'initial' so that would
be for those currents but at 0% DOD but I'm not sure if that is
relevant).

Now, it's obviously not a straight line (Peukert's law), but can you
extrapolate a graph (or create a formula that would be more useful for
my project) from just 3 points please?

Maybe Peukert's law itself would give those with a brain wired
differently than mine the answer? ;-)

https://en.wikipedia.org/wiki/Peukert%27s_law#Formula

Cheers, T i m

p.s. The nearest *I* could get to an answer would be some graph paper
and a Flexicurve. ;-)



Three points is a bit tough, 6 points fine.
Plot using excel. then use ADD Trendline to try different type fits
then on the options tab, you can get it to display the equation of the
trend line.

On 14/09/2017 21:28, NY wrote:
"newshound" wrote in message
o.uk...
On 14/09/2017 20:14, T i m wrote:
Now, it's obviously not a straight line (Peukert's law), but can you
extrapolate a graph (or create a formula that would be more useful for
my project) from just 3 points please?

p.s. The nearest *I* could get to an answer would be some graph paper
and a Flexicurve. ;-)


Probably no simple answer because it all depends on the shape of the
curve.

I'm not familiar with that particular "law" (I use the term advisedly)
but exponentials are buggers to deal with (especially when, as in this
case, it is obviously only an approximation: exponentials are fine for
radioactive decay, but it will certainly have limits in this case).

I think my approach would be to try to collect some data for your
specific battery, and try to work with that. Quite possibly with a
flexicurve, or with some sort of polynomial fit if there was more data
available.

I'd say the best approach would be to find a transformation (eg y=log(x)
or y=sqrt(x)) which gives a good, well-correlated straight line. Then
extrapolate that and do an inverse transformation (eg antilog or
x-squared) on the predicted value. Obviously the more data points you
have, the better prediction you can make and the better you can
construct a least-squares regression line for extrapolation and then
back-transformation.

Sorry, but trying to get the "right" straight line from three points is
pretty meaningless. Batteries are intrinsically non-linear. You can
(sort of) rely on an exponential for radioactive decay when you are
starting with maybe 10^20 atoms. Batteries are more complicated because
you have competing processes.

On Thu, 14 Sep 2017 21:20:39 +0100, newshound
wrote:

snip

Probably no simple answer because it all depends on the shape of the curve.

Catch 22 then. ;-(

I'm not familiar with that particular "law" (I use the term advisedly)

Well, I think it translates to this:

t=H(C/IH)k

(k is a power)

whe
H is the rated discharge time
C is the rated capacity at the discharge rate (Ah)
I is the discharge current
t is the actual time to discharge the battery in hours
K is the Peukert constant.

The value of k is normally between 1.1-1.25 for Gel

(found somewhere on the net).

So, say I this battery: http://www.mkbattery.com/images/8GU1H.pdf

That datasheet may give us some more information if I had the brain to
make use of it, eg it says it's:

36Ah at C/100 (so when discharged at 0.36A for 100 hours)
31.6Ah at C/20 (so when discharged at 1.58A for 20 hours)
26.8Ah at C/5 (so when discharged at 5.3A for 5 hours)

However, I would be running 3 of those in parallel so I'm assuming my
max 30A load would just pull the discharge time further round the
graph (in the more linear section).

And this gives the discharge time versus current (to 10.5V I'm told
which is 100% discharged):

http://www.mkbattery.com/gel_specs.php?model=8GU1H

Could we use any of those to give us values the formula for that
particular battery?


but exponentials are buggers to deal with (especially when, as in this
case, it is obviously only an approximation: exponentials are fine for
radioactive decay, but it will certainly have limits in this case).

Understood.

I think my approach would be to try to collect some data for your
specific battery, and try to work with that.

You mean physically? The issue as I see it is that I don't know what
the Low Voltage Cutoff voltages should be for any other currents than
the ones I listed originally so I'm not sure UI can (and sorta my
dilemma). ;-(

Quite possibly with a
flexicurve, or with some sort of polynomial fit if there was more data
available.

Understood.

Sometimes you can over-think a problem, you have to work with the data
and tools available.

I'm trying to underthink the problem as anything else makes my brain
hurt. ;-(

As an example, compare the different "lines" of a Spitfire or a Vanwall
with the various front wings of current F1 cars. Modern stuff has the
benefit of serious CFD and wing-tunnel testing. And it is justified in
that context.

Understood.

Basically I could put the 3 batteries in my boat, run the electric
outboard till it slows to a point that's unusable and charge them back
up when I get home. However, while I'm so doing I could damage them
and they weren't cheap. ;-(

So, I could assume the worse case discharge (30A) and buy / build a
simple voltage alarm that triggers at 'some voltage' (that I think
represents 50% depth of discharge) but what if I'm not going a full
speed and only drawing 10-15A when the voltage wouldn't be anything
like as low as when using 30A?

The idea being, once I know the rules I might be able to include that
into the calculations to provide a more dynamic / real-world cutoff /
alarm point? (Now I know they might be other variables like
temperature and aging but I can also compensate for those).

Cheers, T i m

On Thu, 14 Sep 2017 21:28:31 +0100, "NY" wrote:

snip

I'd say the best approach would be to find a transformation (eg y=log(x) or
y=sqrt(x)) which gives a good, well-correlated straight line. Then
extrapolate that and do an inverse transformation (eg antilog or x-squared)
on the predicted value. Obviously the more data points you have, the better
prediction you can make and the better you can construct a least-squares
regression line for extrapolation and then back-transformation.

Yeah, I was going to say that but thought it was obvious. ;-)

Cheers, T i m

On Thursday, 14 September 2017 21:40:21 UTC+1, Bob Minchin wrote:
T i m wrote:
Hi all and knowing there are some scientists / mathematicians walking
amongst us ... ;-)

I'm trying to see if there is a (rough even) way of measuring the
Depth Of Discharge voltage / cutoff point versus current load value
for a specific lead acid battery.

The manufacturers have provided me the following values (@ 50% DOD).

11.75V @ 93A (12.4V initial)
12.05V @ 18.2A (12.7V initial)
12.10V @ 4.65A (12.75V initial)

(The general chart starts at the values marked 'initial' so that would
be for those currents but at 0% DOD but I'm not sure if that is
relevant).

Now, it's obviously not a straight line (Peukert's law), but can you
extrapolate a graph (or create a formula that would be more useful for
my project) from just 3 points please?

Maybe Peukert's law itself would give those with a brain wired
differently than mine the answer? ;-)

https://en.wikipedia.org/wiki/Peukert%27s_law#Formula

Cheers, T i m

p.s. The nearest *I* could get to an answer would be some graph paper
and a Flexicurve. ;-)



Three points is a bit tough, 6 points fine.
Plot using excel. then use ADD Trendline to try different type fits
then on the options tab, you can get it to display the equation of the
trend line.

If you do this, be aware that you might need to format the trendline eqn to get a sufficient number of decimal places. Else can get large errors.

On Thu, 14 Sep 2017 21:41:42 +0100, Bob Minchin
wrote:

T i m wrote:
Hi all and knowing there are some scientists / mathematicians walking
amongst us ... ;-)

I'm trying to see if there is a (rough even) way of measuring the
Depth Of Discharge voltage / cutoff point versus current load value
for a specific lead acid battery.

The manufacturers have provided me the following values (@ 50% DOD).

11.75V @ 93A (12.4V initial)
12.05V @ 18.2A (12.7V initial)
12.10V @ 4.65A (12.75V initial)

(The general chart starts at the values marked 'initial' so that would
be for those currents but at 0% DOD but I'm not sure if that is
relevant).

Now, it's obviously not a straight line (Peukert's law), but can you
extrapolate a graph (or create a formula that would be more useful for
my project) from just 3 points please?

Maybe Peukert's law itself would give those with a brain wired
differently than mine the answer? ;-)

https://en.wikipedia.org/wiki/Peukert%27s_law#Formula

Cheers, T i m

p.s. The nearest *I* could get to an answer would be some graph paper
and a Flexicurve. ;-)



Three points is a bit tough, 6 points fine.

That was my fear Bob. ;-(

Plot using excel. then use ADD Trendline to try different type fits
then on the options tab, you can get it to display the equation of the
trend line.

Yeah, easy for you to say but because I've never produced a
spreadsheet in my life ... ;-(

As posted elsewhere, would this help:

t=H(C/IH)k

(k is a power)

whe
H is the rated discharge time
C is the rated capacity at the discharge rate (Ah)
I is the discharge current
t is the actual time to discharge the battery in hours
K is the Peukert constant.

The value of k is normally between 1.1-1.25 for Gel

https://en.wikipedia.org/wiki/Peukert%27s_law#Formula

I'm not sure what H would be as it's a function of the other
variables?
So, we know C at three points.
We know I will be between 0 and 30 (we have 3 with other variables).
We can take the Peukert constant as 1.2

My brain hurts ... ;-(


Cheers, T i m

On Thu, 14 Sep 2017 22:20:20 +0100, pamela wrote:

snip

That Peukert's Law gives the total capacity (varying with load) but
to do what you want don't you need the specific curve for that type
of battery showing voltage against time (which is what your
Flexicurve would be approximating)?

Yes / no / pass / confused. ;-(

What I *need* for my purposes is the formula I can insert into an
Arduino micro controller 'sketch' that will read the current current
g, read the current volts and apply the formula to determine if
those volts for that current equals a discharge level that
approximates 50%.

We have Peukert's Law, we have some voltages that equal a 50% DOD for
three currents and some other bits of info that might allow someone
good at such things to get somewhere near. ;-)

eg, As my outboard draws a maximum of 30A we (only / already) have two
points (currents, from the battery manufacturer) that I could have
kick in as / when I crossed those points that represent 50% DOD,

12.05V @ C5 (18.2A)
12.10V @ C20 (4.65A)

My 2 x 16 LCD display will show the battery voltage, the current drawn
and the current Low Voltage Disconnect threshold from the magic
formula. The idea being I can get a sense of how long I can continue
to run at that speed (and I may be able to actually display that time
as early on in the battery discharge voltage curve it's a reasonably
straight line and that line *could* easily be plotted by me ITRW) ;-)

Cheers, T i m

T i m wrote:

On Thu, 14 Sep 2017 21:41:42 +0100, Bob Minchin
wrote:

T i m wrote:
Hi all and knowing there are some scientists / mathematicians walking
amongst us ... ;-)

I'm trying to see if there is a (rough even) way of measuring the
Depth Of Discharge voltage / cutoff point versus current load value
for a specific lead acid battery.

The manufacturers have provided me the following values (@ 50% DOD).

11.75V @ 93A (12.4V initial)
12.05V @ 18.2A (12.7V initial)
12.10V @ 4.65A (12.75V initial)

(The general chart starts at the values marked 'initial' so that would
be for those currents but at 0% DOD but I'm not sure if that is
relevant).

Now, it's obviously not a straight line (Peukert's law), but can you
extrapolate a graph (or create a formula that would be more useful for
my project) from just 3 points please?

Maybe Peukert's law itself would give those with a brain wired
differently than mine the answer? ;-)

https://en.wikipedia.org/wiki/Peukert%27s_law#Formula

Cheers, T i m

p.s. The nearest *I* could get to an answer would be some graph paper
and a Flexicurve. ;-)



Three points is a bit tough, 6 points fine.

That was my fear Bob. ;-(

Plot using excel. then use ADD Trendline to try different type fits
then on the options tab, you can get it to display the equation of the
trend line.

Yeah, easy for you to say but because I've never produced a
spreadsheet in my life ... ;-(

As posted elsewhere, would this help:

t=H(C/IH)k

(k is a power)

whe
H is the rated discharge time
C is the rated capacity at the discharge rate (Ah)
I is the discharge current
t is the actual time to discharge the battery in hours
K is the Peukert constant.

The value of k is normally between 1.1-1.25 for Gel

https://en.wikipedia.org/wiki/Peukert%27s_law#Formula

I'm not sure what H would be as it's a function of the other
variables?
So, we know C at three points.
We know I will be between 0 and 30 (we have 3 with other variables).
We can take the Peukert constant as 1.2

My brain hurts ... ;-(


Cheers, T i m

H, C and (I at the standard discharge rate, call it Istd) are
constants, measured at the standard conditions for quoting battery
capacity (whatever they are). If you know the capacity that the
battery is sold at, say 36 Ah, and the discharge current it is
standardised at then the capacity C is a constant 36. Istd is the
standard discharge current and it is constant, and H is equal to C/Istd
and also a constant. This leaves only the particular 'I' you are using
as a variable and a fairly simple computation. I am guessing here, but
the equation only makes sense if H and C are constants. I am surprised
they do no t use 'i' for the independent variable rather than 'I'
though.

BICBW



--

Roger Hayter

T i m Wrote in message:
On Thu, 14 Sep 2017 22:20:20 +0100, pamela wrote:

snip

That Peukert's Law gives the total capacity (varying with load) but
to do what you want don't you need the specific curve for that type
of battery showing voltage against time (which is what your
Flexicurve would be approximating)?

Yes / no / pass / confused. ;-(

What I *need* for my purposes is the formula I can insert into an
Arduino micro controller 'sketch' that will read the current current
g, read the current volts and apply the formula to determine if
those volts for that current equals a discharge level that
approximates 50%.

We have Peukert's Law, we have some voltages that equal a 50% DOD for
three currents and some other bits of info that might allow someone
good at such things to get somewhere near. ;-)

eg, As my outboard draws a maximum of 30A we (only / already) have two
points (currents, from the battery manufacturer) that I could have
kick in as / when I crossed those points that represent 50% DOD,

12.05V @ C5 (18.2A)
12.10V @ C20 (4.65A)

My 2 x 16 LCD display will show the battery voltage, the current drawn
and the current Low Voltage Disconnect threshold from the magic
formula. The idea being I can get a sense of how long I can continue
to run at that speed (and I may be able to actually display that time
as early on in the battery discharge voltage curve it's a reasonably
straight line and that line *could* easily be plotted by me ITRW) ;-)

Cheers, T i m


Buy a battery & do the measurements in anger yourself?

--
Jim K


----Android NewsGroup Reader----
http://usenet.sinaapp.com/

"T i m" wrote in message
...
On Thu, 14 Sep 2017 22:20:20 +0100, pamela wrote:

snip

That Peukert's Law gives the total capacity (varying with load) but
to do what you want don't you need the specific curve for that type
of battery showing voltage against time (which is what your
Flexicurve would be approximating)?

Yes / no / pass / confused. ;-(

What I *need* for my purposes is the formula I can insert into an
Arduino micro controller 'sketch' that will read the current current
g, read the current volts and apply the formula to determine if
those volts for that current equals a discharge level that
approximates 50%.

We have Peukert's Law, we have some voltages that equal a 50% DOD for
three currents and some other bits of info that might allow someone
good at such things to get somewhere near. ;-)

eg, As my outboard draws a maximum of 30A we (only / already) have two
points (currents, from the battery manufacturer) that I could have
kick in as / when I crossed those points that represent 50% DOD,

12.05V @ C5 (18.2A)
12.10V @ C20 (4.65A)

My 2 x 16 LCD display will show the battery voltage, the current drawn
and the current Low Voltage Disconnect threshold from the magic
formula. The idea being I can get a sense of how long I can continue
to run at that speed (and I may be able to actually display that time
as early on in the battery discharge voltage curve it's a reasonably
straight line and that line *could* easily be plotted by me ITRW) ;-)

I reckon your flexicurve is the best/easiest idea.
Bend it to suit what you have, bung the results into a look-up table and
interpolate between points.
Even if you have a formula I doubt it will be accurate using real world
batteries.

Actually i'm unlucky with batteries even though i'm careful with them.
I don't actually turn off the wipers when i go under a bridge but i'm
careful.
E.g, only sit at traffic lights using sidelights.

On Thu, 14 Sep 2017 23:14:47 +0100 (GMT+01:00), jim k wrote:

snip

Buy a battery & do the measurements in anger yourself?

Oh dear. ;-(

1) I have 3 of the batteries.

2) The answer to the question isn't 'measurable'.

3) Thanks for your help (not).


Cheers, T i m

On Thu, 14 Sep 2017 23:22:06 +0100, "bm" wrote:

snip

My 2 x 16 LCD display will show the battery voltage, the current drawn
and the current Low Voltage Disconnect threshold from the magic
formula. The idea being I can get a sense of how long I can continue
to run at that speed (and I may be able to actually display that time
as early on in the battery discharge voltage curve it's a reasonably
straight line and that line *could* easily be plotted by me ITRW) ;-)

I reckon your flexicurve is the best/easiest idea.

It seem the only idea that I can cope with so far. ;-)

Bend it to suit what you have, bung the results into a look-up table and
interpolate between points.

Yup.

Even if you have a formula I doubt it will be accurate using real world
batteries.

You might be right but I'm not really expecting 'accurate' but a
reasonable guide. The thing is I could just go with / for a preset
'worst case' low voltage alarm value but I thought while I was
developing this Arduino based solution, why not see if I can make to
software do what it's good at?

Actually i'm unlucky with batteries even though i'm careful with them.

Ok?

I don't actually turn off the wipers when i go under a bridge but i'm
careful.

Yeah, that can be a big issue ... for electric cars ... ;-)

E.g, only sit at traffic lights using sidelights.

I've done that when 'nursing' a car or motorbike home that has a known
bad battery ... that and taking even more care to keep my foot off the
foot brake (1, 2, 3 or 6 (inc towing) 21W brake lights) and not having
indicators on (2 or 3 if towing) flashing 21W with a 50:50 duty cycle.

So, I'll keep looking to see if I can obtain any parallel information
from other manufacturers of similar capacity, chemistry and quality
batteries to help fill in the missing pieces.

Cheers, T i m

On Thu, 14 Sep 2017 23:17:32 +0100, (Roger Hayter)
wrote:

snip

Yeah, easy for you to say but because I've never produced a
spreadsheet in my life ... ;-(

As posted elsewhere, would this help:

t=H(C/IH)k

(k is a power)

whe
H is the rated discharge time
C is the rated capacity at the discharge rate (Ah)
I is the discharge current
t is the actual time to discharge the battery in hours
K is the Peukert constant.

The value of k is normally between 1.1-1.25 for Gel

https://en.wikipedia.org/wiki/Peukert%27s_law#Formula

I'm not sure what H would be as it's a function of the other
variables?
So, we know C at three points.
We know I will be between 0 and 30 (we have 3 with other variables).
We can take the Peukert constant as 1.2

My brain hurts ... ;-(


H, C and (I at the standard discharge rate, call it Istd) are
constants, measured at the standard conditions for quoting battery
capacity (whatever they are).

The thing is I'm not sure there is such a thing Roger, being the
battery capacity varies based on the rate of discharge (and what I'm
trying to pin down).

If you know the capacity that the
battery is sold at, say 36 Ah, and the discharge current it is
standardised at then the capacity C is a constant 36.

Well, I know the capacity at 3 discharge rates because it's on the
data sheet:

It's 36Ah at the C/100 rate (0.36A), it's 31.6Ah at the C/20 rate
(1.58A) and it's 26.8 at the C/5 rate (5.32A).

I will be discharging it (if using one on it's own or 'them' if using
all 3 in parallel) between 0 and 30A, so seeing a capacity between C/0
(= infinity, and better than C/100) and ??/30 (as I don't know the
capacity is when the battery(ies) is/are discharged at 30A)?

http://www.mkbattery.com/images/8GU1H.pdf

And we know some other things about the battery from the general data
sheet:

http://www.mkbattery.com/documents/1..._GEL_v7_r2.pdf

and I know the voltages that represent 50% DOD for 3 currents because
I have taken them from their graph (bottom chart on P6):

http://www.mkbattery.com/documents/1...I&O)_MK_r1.pdf

for 3 Istd is the
standard discharge current and it is constant, and H is equal to C/Istd
and also a constant.

Ok (I think)? ;-(

This leaves only the particular 'I' you are using
as a variable and a fairly simple computation.

But if the capacity is a non linear function of the current drawn ...

I am guessing here, but
the equation only makes sense if H and C are constants. I am surprised
they do no t use 'i' for the independent variable rather than 'I'
though.

Well maybe it isn't as simple as both of us would like / hope Roger.
;-(

Cheers, T i m

T i m wrote:

On Thu, 14 Sep 2017 23:17:32 +0100, (Roger Hayter)
wrote:

snip

Yeah, easy for you to say but because I've never produced a
spreadsheet in my life ... ;-(

As posted elsewhere, would this help:

t=H(C/IH)k

(k is a power)

whe
H is the rated discharge time
C is the rated capacity at the discharge rate (Ah)
I is the discharge current
t is the actual time to discharge the battery in hours
K is the Peukert constant.

The value of k is normally between 1.1-1.25 for Gel

https://en.wikipedia.org/wiki/Peukert%27s_law#Formula

I'm not sure what H would be as it's a function of the other
variables?
So, we know C at three points.
We know I will be between 0 and 30 (we have 3 with other variables).
We can take the Peukert constant as 1.2

My brain hurts ... ;-(


H, C and (I at the standard discharge rate, call it Istd) are
constants, measured at the standard conditions for quoting battery
capacity (whatever they are).

The thing is I'm not sure there is such a thing Roger, being the
battery capacity varies based on the rate of discharge (and what I'm
trying to pin down).

If you know the capacity that the
battery is sold at, say 36 Ah, and the discharge current it is
standardised at then the capacity C is a constant 36.

Well, I know the capacity at 3 discharge rates because it's on the
data sheet:

It's 36Ah at the C/100 rate (0.36A), it's 31.6Ah at the C/20 rate
(1.58A) and it's 26.8 at the C/5 rate (5.32A).

I will be discharging it (if using one on it's own or 'them' if using
all 3 in parallel) between 0 and 30A, so seeing a capacity between C/0
(= infinity, and better than C/100) and ??/30 (as I don't know the
capacity is when the battery(ies) is/are discharged at 30A)?

http://www.mkbattery.com/images/8GU1H.pdf

And we know some other things about the battery from the general data
sheet:

http://www.mkbattery.com/documents/1..._GEL_v7_r2.pdf

and I know the voltages that represent 50% DOD for 3 currents because
I have taken them from their graph (bottom chart on P6):

http://www.mkbattery.com/documents/1...I&O)_MK_r1.pdf

for 3 Istd is the
standard discharge current and it is constant, and H is equal to C/Istd
and also a constant.

Ok (I think)? ;-(

This leaves only the particular 'I' you are using
as a variable and a fairly simple computation.

But if the capacity is a non linear function of the current drawn ...

I am assuming the C and the H in your equation are constants but the
actual capacity varies according to your equation, which is definitely
non-linear.




I am guessing here, but
the equation only makes sense if H and C are constants. I am surprised
they do no t use 'i' for the independent variable rather than 'I'
though.

Well maybe it isn't as simple as both of us would like / hope Roger.
;-(


I'm sure you're right, but if you know C and H at one value of I then
your equation should let you work out the time to discharge at another
value of I, within the limits of how accurate the equation is. If the
equation is meaningful at all the only one item on the RHS is a
variable. Admittedly you have to guess a value of k, but you could try
two values and see how much difference it makes.


--

Roger Hayter

"newshound" wrote in message
...
I'd say the best approach would be to find a transformation (eg y=log(x)
or y=sqrt(x)) which gives a good, well-correlated straight line. Then
extrapolate that and do an inverse transformation (eg antilog or
x-squared) on the predicted value. Obviously the more data points you
have, the better prediction you can make and the better you can construct
a least-squares regression line for extrapolation and then
back-transformation.

Sorry, but trying to get the "right" straight line from three points is
pretty meaningless. Batteries are intrinsically non-linear. You can (sort
of) rely on an exponential for radioactive decay when you are starting
with maybe 10^20 atoms. Batteries are more complicated because you have
competing processes.

Yes, I never suggested that the data would be linear. I'd be highly
surprised if it was anything like linear. But if you can find a way of
transforming the non-linear data so it becomes linear, then you can work out
the best-fit line and predict future (transformed) values, from which you
can back-transform to get the corresponding real value. Easier to do it that
way that to try to fit a flexicurve to a curved line so you can predict an
out-of-range value (extrapolating) as opposed to predicting an in-range
value (interpolating).

On Fri, 15 Sep 2017 10:32:18 +0100, (Roger Hayter)
wrote:

T i m wrote:

On Thu, 14 Sep 2017 23:17:32 +0100, (Roger Hayter)
wrote:

snip

Yeah, easy for you to say but because I've never produced a
spreadsheet in my life ... ;-(

As posted elsewhere, would this help:

t=H(C/IH)k

(k is a power)

whe
H is the rated discharge time
C is the rated capacity at the discharge rate (Ah)
I is the discharge current
t is the actual time to discharge the battery in hours
K is the Peukert constant.

The value of k is normally between 1.1-1.25 for Gel

https://en.wikipedia.org/wiki/Peukert%27s_law#Formula

I'm not sure what H would be as it's a function of the other
variables?
So, we know C at three points.
We know I will be between 0 and 30 (we have 3 with other variables).
We can take the Peukert constant as 1.2

My brain hurts ... ;-(


H, C and (I at the standard discharge rate, call it Istd) are
constants, measured at the standard conditions for quoting battery
capacity (whatever they are).

The thing is I'm not sure there is such a thing Roger, being the
battery capacity varies based on the rate of discharge (and what I'm
trying to pin down).

If you know the capacity that the
battery is sold at, say 36 Ah, and the discharge current it is
standardised at then the capacity C is a constant 36.

Well, I know the capacity at 3 discharge rates because it's on the
data sheet:

It's 36Ah at the C/100 rate (0.36A), it's 31.6Ah at the C/20 rate
(1.58A) and it's 26.8 at the C/5 rate (5.32A).

I will be discharging it (if using one on it's own or 'them' if using
all 3 in parallel) between 0 and 30A, so seeing a capacity between C/0
(= infinity, and better than C/100) and ??/30 (as I don't know the
capacity is when the battery(ies) is/are discharged at 30A)?

http://www.mkbattery.com/images/8GU1H.pdf

And we know some other things about the battery from the general data
sheet:

http://www.mkbattery.com/documents/1..._GEL_v7_r2.pdf

and I know the voltages that represent 50% DOD for 3 currents because
I have taken them from their graph (bottom chart on P6):

http://www.mkbattery.com/documents/1...I&O)_MK_r1.pdf

for 3 Istd is the
standard discharge current and it is constant, and H is equal to C/Istd
and also a constant.

Ok (I think)? ;-(

This leaves only the particular 'I' you are using
as a variable and a fairly simple computation.

But if the capacity is a non linear function of the current drawn ...

I am assuming the C and the H in your equation are constants but the
actual capacity varies according to your equation, which is definitely
non-linear.

Quite.




I am guessing here, but
the equation only makes sense if H and C are constants. I am surprised
they do no t use 'i' for the independent variable rather than 'I'
though.

Well maybe it isn't as simple as both of us would like / hope Roger.
;-(


I'm sure you're right, but if you know C and H at one value of I then
your equation should let you work out the time to discharge at another
value of I, within the limits of how accurate the equation is.

Ok ...

If the
equation is meaningful at all the only one item on the RHS is a
variable. Admittedly you have to guess a value of k, but you could try
two values and see how much difference it makes.

Well yes, as you say, k for gel batteries lies in between two fairly
confined values and reasonably different to other battery chemistries
so should be a reasonable starting point.

So, would you mind helping me plug some values in from what we know
because I really suffer a form of blindness when it comes to such
things (always have).

t=H(C/IH)k


(k is a power)

whe
H is the rated discharge time
C is the rated capacity at the discharge rate (Ah)
I is the discharge current
t is the actual time to discharge the battery in hours
K is the Peukert constant.

The value of k is normally between 1.1-1.25 for Gel

Does this help?

http://batteryuniversity.com/learn/a...ttery_runtime-

Cheers, T i m

On Fri, 15 Sep 2017 10:46:28 +0100, "NY" wrote:

"newshound" wrote in message
...
I'd say the best approach would be to find a transformation (eg y=log(x)
or y=sqrt(x)) which gives a good, well-correlated straight line. Then
extrapolate that and do an inverse transformation (eg antilog or
x-squared) on the predicted value. Obviously the more data points you
have, the better prediction you can make and the better you can construct
a least-squares regression line for extrapolation and then
back-transformation.

Sorry, but trying to get the "right" straight line from three points is
pretty meaningless. Batteries are intrinsically non-linear. You can (sort
of) rely on an exponential for radioactive decay when you are starting
with maybe 10^20 atoms. Batteries are more complicated because you have
competing processes.

Yes, I never suggested that the data would be linear. I'd be highly
surprised if it was anything like linear. But if you can find a way of
transforming the non-linear data so it becomes linear, then you can work out
the best-fit line and predict future (transformed) values, from which you
can back-transform to get the corresponding real value. Easier to do it that
way that to try to fit a flexicurve to a curved line so you can predict an
out-of-range value (extrapolating) as opposed to predicting an in-range
value (interpolating).

So, as you sound like you know what you are talking about, can you
plug some real values in and see what we get please?

I'm happy to answer any questions re numbers and points etc.

I know the off load starting voltage is 13.10 volts.
I know the voltage that represents 50% (and other percentages) for 3
different currents.

We have the formula that should make of it all but I don't have the
understanding of how to take what we have and make use of it ITRW (if
it's possible)?


Cheers, T i m

T i m wrote:

On Fri, 15 Sep 2017 10:32:18 +0100, (Roger Hayter)
wrote:

T i m wrote:

On Thu, 14 Sep 2017 23:17:32 +0100, (Roger Hayter)
wrote:

snip

Yeah, easy for you to say but because I've never produced a
spreadsheet in my life ... ;-(

As posted elsewhere, would this help:

t=H(C/IH)k

(k is a power)

whe
H is the rated discharge time
C is the rated capacity at the discharge rate (Ah)
I is the discharge current
t is the actual time to discharge the battery in hours
K is the Peukert constant.

The value of k is normally between 1.1-1.25 for Gel

https://en.wikipedia.org/wiki/Peukert%27s_law#Formula

I'm not sure what H would be as it's a function of the other
variables?
So, we know C at three points.
We know I will be between 0 and 30 (we have 3 with other variables).
We can take the Peukert constant as 1.2

My brain hurts ... ;-(


H, C and (I at the standard discharge rate, call it Istd) are
constants, measured at the standard conditions for quoting battery
capacity (whatever they are).

The thing is I'm not sure there is such a thing Roger, being the
battery capacity varies based on the rate of discharge (and what I'm
trying to pin down).

If you know the capacity that the
battery is sold at, say 36 Ah, and the discharge current it is
standardised at then the capacity C is a constant 36.

Well, I know the capacity at 3 discharge rates because it's on the
data sheet:

It's 36Ah at the C/100 rate (0.36A), it's 31.6Ah at the C/20 rate
(1.58A) and it's 26.8 at the C/5 rate (5.32A).

I will be discharging it (if using one on it's own or 'them' if using
all 3 in parallel) between 0 and 30A, so seeing a capacity between C/0
(= infinity, and better than C/100) and ??/30 (as I don't know the
capacity is when the battery(ies) is/are discharged at 30A)?

http://www.mkbattery.com/images/8GU1H.pdf

And we know some other things about the battery from the general data
sheet:

http://www.mkbattery.com/documents/1..._GEL_v7_r2.pdf

and I know the voltages that represent 50% DOD for 3 currents because
I have taken them from their graph (bottom chart on P6):

http://www.mkbattery.com/documents/1...I&O)_MK_r1.pdf

for 3 Istd is the
standard discharge current and it is constant, and H is equal to C/Istd
and also a constant.

Ok (I think)? ;-(

This leaves only the particular 'I' you are using
as a variable and a fairly simple computation.

But if the capacity is a non linear function of the current drawn ...

I am assuming the C and the H in your equation are constants but the
actual capacity varies according to your equation, which is definitely
non-linear.

Quite.




I am guessing here, but
the equation only makes sense if H and C are constants. I am surprised
they do no t use 'i' for the independent variable rather than 'I'
though.

Well maybe it isn't as simple as both of us would like / hope Roger.
;-(


I'm sure you're right, but if you know C and H at one value of I then
your equation should let you work out the time to discharge at another
value of I, within the limits of how accurate the equation is.

Ok ...

If the
equation is meaningful at all the only one item on the RHS is a
variable. Admittedly you have to guess a value of k, but you could try
two values and see how much difference it makes.

Well yes, as you say, k for gel batteries lies in between two fairly
confined values and reasonably different to other battery chemistries
so should be a reasonable starting point.

So, would you mind helping me plug some values in from what we know
because I really suffer a form of blindness when it comes to such
things (always have).

t=H(C/IH)k


(k is a power)

whe
H is the rated discharge time
C is the rated capacity at the discharge rate (Ah)
I is the discharge current
t is the actual time to discharge the battery in hours
K is the Peukert constant.

The value of k is normally between 1.1-1.25 for Gel

Does this help?

http://batteryuniversity.com/learn/a...culate_battery
_runtime-

Cheers, T i m

It does occur to me I am solving a problem (how to calculate discharge
time) that you are not interested in. You were originally interested
in discharge voltage at 50% discharge, as a point to stop using the
battery.

Looking at your original figures for voltage at 50% discharge, I think
they are adequately modelled within the range 0A to 93A by assuming an
open circuit 50% discharge voltage of 12.12 and a series resistance of 4
milliohms. This would give voltages for the cases you mention of
4.65A 12.10
18.2A 12.05
93A 11.75



Which is suspiciously close to their figures. Identical to this degree
of precision! I wouldn't be surprised if they used the same method to
calculate them. I did this by assuming that the open circuit voltage
at 50% discharge is independent of current (not probably true
dynamically) and the effective series resistance is the same for all
currents, mentally extropolating to 0A and dividing the apparent voltage
drop from the 0A level by current to give a notional resistance,
slightly different in each case, and choosing the one nearest the high
current value. I.e. handwaving plus mental arithmetic.

But this model (12.12 volts minus current times .004) seems to be a very
good fit up to 93A. And all linear!


Peurket doesn't help you with this - it tells you how soon you'll get
there, but not what criteria to use for 50% discharge. I don't
believe it allows for potential battery recovery either, so is probably
too conservative. But it does give the same answer as the makers want
you to have.

None of the information you have helps with saying whether this relation
will still be valid above 93A, but perhaps it should be good up to the
rated maximum current of the batteries??



--

Roger Hayter

Roger Hayter wrote:



None of the information you have helps with saying whether this relation
will still be valid above 93A, but perhaps it should be good up to the
rated maximum current of the batteries??

Caveat is that battery voltage drops at high current due to reversible
changes, so using an underlying open circuit fixed voltage for detecting
a given discharge point is too conservative. But, as I say, the makers
use a linear relation up to 93A so perhaps it is not outrageously wrong.
In any case, unless they tell you how you can't allow for this
accurately.

--

Roger Hayter

"T i m" wrote in message
...
On Thu, 14 Sep 2017 23:22:06 +0100, "bm" wrote:

snip

My 2 x 16 LCD display will show the battery voltage, the current drawn
and the current Low Voltage Disconnect threshold from the magic
formula. The idea being I can get a sense of how long I can continue
to run at that speed (and I may be able to actually display that time
as early on in the battery discharge voltage curve it's a reasonably
straight line and that line *could* easily be plotted by me ITRW) ;-)

I reckon your flexicurve is the best/easiest idea.

It seem the only idea that I can cope with so far. ;-)

Bend it to suit what you have, bung the results into a look-up table and
interpolate between points.

Yup.

Even if you have a formula I doubt it will be accurate using real world
batteries.

You might be right but I'm not really expecting 'accurate' but a
reasonable guide. The thing is I could just go with / for a preset
'worst case' low voltage alarm value but I thought while I was
developing this Arduino based solution, why not see if I can make to
software do what it's good at?

Actually i'm unlucky with batteries even though i'm careful with them.

Ok?

I don't actually turn off the wipers when i go under a bridge but i'm
careful.

Yeah, that can be a big issue ... for electric cars ... ;-)

E.g, only sit at traffic lights using sidelights.

I've done that when 'nursing' a car or motorbike home that has a known
bad battery ... that and taking even more care to keep my foot off the
foot brake (1, 2, 3 or 6 (inc towing) 21W brake lights) and not having
indicators on (2 or 3 if towing) flashing 21W with a 50:50 duty cycle.

So, I'll keep looking to see if I can obtain any parallel information
from other manufacturers of similar capacity, chemistry and quality
batteries to help fill in the missing pieces.

Cheers, T i m

Please state the type no. of the battery; if more than one how they are
connected; what current range you will be using.
--
Dave W

On Fri, 15 Sep 2017 14:04:56 +0100, "Dave W"
wrote:

snip

Please state the type no. of the battery;

They are MK 8GU1H

http://www.mkbattery.com/images/8GU1H.pdf
http://www.mkbattery.com/documents/9350MK_GEL_v7_r2.pdf
http://www.mkbattery.com/gel_specs.php?model=8GU1H
http://www.mkbattery.com/documents/1...I&O)_MK_r1.pdf


if more than one how they are
connected;

3, in parallel.

what current range you will be using.

0 to 30A (typically 15A).

Cheers, T i m

On Fri, 15 Sep 2017 13:04:46 +0100, (Roger Hayter)
wrote:

snip

It does occur to me I am solving a problem (how to calculate discharge
time) that you are not interested in.

Correct. The reason is because the *time* will vary with load and the
load will vary with the situation (this is an electric outboard motor
on a boat and not some lights in a caravan etc). ;-)

You were originally interested
in discharge voltage at 50% discharge, as a point to stop using the
battery.

Correct, as a formula I could apply to an Arduino based 'display / low
voltage' alarm.

Looking at your original figures for voltage at 50% discharge, I think
they are adequately modelled within the range 0A to 93A by assuming an
open circuit 50% discharge voltage of 12.12 and a series resistance of 4
milliohms. This would give voltages for the cases you mention of
4.65A 12.10
18.2A 12.05
93A 11.75

Ok ...


Which is suspiciously close to their figures.

They are indeed. ;-)

Identical to this degree
of precision!

Magic! shrug ;-)

I wouldn't be surprised if they used the same method to
calculate them. I did this by assuming that the open circuit voltage
at 50% discharge is independent of current (not probably true
dynamically) and the effective series resistance is the same for all
currents, mentally extropolating to 0A and dividing the apparent voltage
drop from the 0A level by current to give a notional resistance,
slightly different in each case, and choosing the one nearest the high
current value. I.e. handwaving plus mental arithmetic.

Ok, I follow most of that because it's 'fuzzy logic'. ;-)

But this model (12.12 volts minus current times .004) seems to be a very
good fit up to 93A. And all linear!

Hmm, should it be though (as as you say) it wouldn't be in practice)?

Peurket doesn't help you with this - it tells you how soon you'll get
there, but not what criteria to use for 50% discharge.

Oh. ;-(

I don't
believe it allows for potential battery recovery either, so is probably
too conservative.

Understood. I can only reasonably expect any gauge to work when the
load is reasonably continuous (even if not consistent).

But it does give the same answer as the makers want
you to have.

Ok.

None of the information you have helps with saying whether this relation
will still be valid above 93A, but perhaps it should be good up to the
rated maximum current of the batteries??

Ok, well all I'm interested in is how it would work up to 30A when
spread across the 3 batteries in parallel Roger. ;-)

So, sorry, I think I must have missed the conclusion. ;-(

Cheers, T i m

On Fri, 15 Sep 2017 13:21:32 +0100, (Roger Hayter)
wrote:

Roger Hayter wrote:



None of the information you have helps with saying whether this relation
will still be valid above 93A, but perhaps it should be good up to the
rated maximum current of the batteries??

Caveat is that battery voltage drops at high current due to reversible
changes, so using an underlying open circuit fixed voltage for detecting
a given discharge point is too conservative.

Agreed, and should only be done after resting the battery for 24 hours
or so (I read somewhere).

But, as I say, the makers
use a linear relation up to 93A so perhaps it is not outrageously wrong.

But how does that fit with what they say here?

http://www.mkbattery.com/images/8GU1H.pdf

Or are you also taking that into consideration?

Capacity at C/100 = 36 Ah
Capacity at C/20 = 31.6 Ah
Capacity at C/5 = 26.8 Ah

They are saying that between that range of currents there is a 10Ah
capacity difference, that's over 30% of it's average capacity?
Realistically I would only be interested to see how much the capacity
changed between say 10 and 30A. *If* the capacity doesn't change much
between those particular values then it may not be worth including.
*If*. ;-)

In any case, unless they tell you how you can't allow for this
accurately.

Ok, but if I understand the above info correctly the difference in
capacity changes significantly within the range of currents I intend
to use therefore (in my mind anyway) making it worth of inclusion in
any gauge trying to display the cutoff voltage for a given depth of
discharge? As you say, we need the 'how' and I was hoping we might be
able to extrapolate that from the information we *have* been given?

Thanks very much for your feedback on this Roger. ;-)

Cheers, T i m

T i m wrote:

On Fri, 15 Sep 2017 13:21:32 +0100, (Roger Hayter)
wrote:

Roger Hayter wrote:



None of the information you have helps with saying whether this relation
will still be valid above 93A, but perhaps it should be good up to the
rated maximum current of the batteries??

Caveat is that battery voltage drops at high current due to reversible
changes, so using an underlying open circuit fixed voltage for detecting
a given discharge point is too conservative.

Agreed, and should only be done after resting the battery for 24 hours
or so (I read somewhere).

But, as I say, the makers
use a linear relation up to 93A so perhaps it is not outrageously wrong.

But how does that fit with what they say here?

http://www.mkbattery.com/images/8GU1H.pdf

Or are you also taking that into consideration?

Capacity at C/100 = 36 Ah
Capacity at C/20 = 31.6 Ah
Capacity at C/5 = 26.8 Ah

They are saying that between that range of currents there is a 10Ah
capacity difference, that's over 30% of it's average capacity?
Realistically I would only be interested to see how much the capacity
changed between say 10 and 30A. *If* the capacity doesn't change much
between those particular values then it may not be worth including.
*If*. ;-)

In any case, unless they tell you how you can't allow for this
accurately.

Ok, but if I understand the above info correctly the difference in
capacity changes significantly within the range of currents I intend
to use therefore (in my mind anyway) making it worth of inclusion in
any gauge trying to display the cutoff voltage for a given depth of
discharge? As you say, we need the 'how' and I was hoping we might be
able to extrapolate that from the information we *have* been given?

Thanks very much for your feedback on this Roger. ;-)

Cheers, T i m

It depends whether you are trying to predict capacity at a given
current, which is where your Peurkert equation comes in. Or just
finding out when you have reached 50% capacity. The latter will be
indicated by the voltage they mention, and the fact that it was reached
proportionately sooner with a lower effective capacity at high current
is irrelevant to the problem of detecting when it has been reached.
The battery has *been* inefficient at high current and it is too late to
do anything about it.

This phenomenon is separate from the *temporary* loss of capacity after
a period of high current. The only way to deal with this is to stop
discharging the battery well before 100% use of capacity, which is what
you are being advised to do when you use 50% as an end point. This
makes sense to me though it is a bit speculative.


--

Roger Hayter

T i m wrote:

On Fri, 15 Sep 2017 13:04:46 +0100, (Roger Hayter)
wrote:


None of the information you have helps with saying whether this relation
will still be valid above 93A, but perhaps it should be good up to the
rated maximum current of the batteries??

Ok, well all I'm interested in is how it would work up to 30A when
spread across the 3 batteries in parallel Roger. ;-)

So, sorry, I think I must have missed the conclusion. ;-(

Cheers, T i m

50% discharge voltage /= 12.12 - (discharge current [while stable for
a few sec] X 0.00133)

VSMOP if you can measure current and voltage continuously.

The most difficult bit will be finding out what is a useful value of 'a
few sec'.



--

Roger Hayter

"Roger Hayter" wrote in message
...
T i m wrote:

On Fri, 15 Sep 2017 13:21:32 +0100, (Roger Hayter)
wrote:

Roger Hayter wrote:



None of the information you have helps with saying whether this
relation
will still be valid above 93A, but perhaps it should be good up to the
rated maximum current of the batteries??

Caveat is that battery voltage drops at high current due to reversible
changes, so using an underlying open circuit fixed voltage for detecting
a given discharge point is too conservative.

Agreed, and should only be done after resting the battery for 24 hours
or so (I read somewhere).

But, as I say, the makers
use a linear relation up to 93A so perhaps it is not outrageously wrong.

But how does that fit with what they say here?

http://www.mkbattery.com/images/8GU1H.pdf

Or are you also taking that into consideration?

Capacity at C/100 = 36 Ah
Capacity at C/20 = 31.6 Ah
Capacity at C/5 = 26.8 Ah

They are saying that between that range of currents there is a 10Ah
capacity difference, that's over 30% of it's average capacity?
Realistically I would only be interested to see how much the capacity
changed between say 10 and 30A. *If* the capacity doesn't change much
between those particular values then it may not be worth including.
*If*. ;-)

In any case, unless they tell you how you can't allow for this
accurately.

Ok, but if I understand the above info correctly the difference in
capacity changes significantly within the range of currents I intend
to use therefore (in my mind anyway) making it worth of inclusion in
any gauge trying to display the cutoff voltage for a given depth of
discharge? As you say, we need the 'how' and I was hoping we might be
able to extrapolate that from the information we *have* been given?

Thanks very much for your feedback on this Roger. ;-)

Cheers, T i m

It depends whether you are trying to predict capacity at a given
current, which is where your Peurkert equation comes in. Or just
finding out when you have reached 50% capacity. The latter will be
indicated by the voltage they mention, and the fact that it was reached
proportionately sooner with a lower effective capacity at high current
is irrelevant to the problem of detecting when it has been reached.
The battery has *been* inefficient at high current and it is too late to
do anything about it.

This phenomenon is separate from the *temporary* loss of capacity after
a period of high current. The only way to deal with this is to stop
discharging the battery well before 100% use of capacity, which is what
you are being advised to do when you use 50% as an end point. This
makes sense to me though it is a bit speculative.

Yes, I was wondering why 50% was specified.
Or maybe I missed it.

On Fri, 15 Sep 2017 23:08:41 +0100, (Roger Hayter)
wrote:

snip

Ok, but if I understand the above info correctly the difference in
capacity changes significantly within the range of currents I intend
to use therefore (in my mind anyway) making it worth of inclusion in
any gauge trying to display the cutoff voltage for a given depth of
discharge? As you say, we need the 'how' and I was hoping we might be
able to extrapolate that from the information we *have* been given?


It depends whether you are trying to predict capacity at a given
current, which is where your Peurkert equation comes in.

No, I wouldn't say I was trying to predict anything (at this point
anyway g), but calculate it from the current characteristics?

Or just
finding out when you have reached 50% capacity.

Yeah, that's the badger. ;-)

The latter will be
indicated by the voltage they mention,

(at the discharge rates they mention, so far ...)

and the fact that it was reached
proportionately sooner with a lower effective capacity at high current
is irrelevant to the problem of detecting when it has been reached.

Ok ...

The battery has *been* inefficient at high current and it is too late to
do anything about it.

Ok, so, let's say that the battery would reach 50% depth of discharge
if we were going along on speed 1 (of 5) in 3 hours. The alarm beeps
and I know I might damage the battery any further so stop (the device
has done it's job).

Then I fit the second battery but it's starting to get dark so we want
to get home quicker so I put it on speed 3 that will have the alarm
going off in 50 minutes (again, the device has done it's job).

The point is that in each case the only information that will be used
to determine the battery voltage but with a rider that is a function
of the current current being drawn.

And this isn't me imagining anything or making things up, this is
scientifically proven fact that is measured and well published.

This phenomenon is separate from the *temporary* loss of capacity after
a period of high current.

Of course.

The only way to deal with this is to stop
discharging the battery well before 100% use of capacity, which is what
you are being advised to do when you use 50% as an end point.

Quite.

This
makes sense to me though it is a bit speculative.

And why I'm trying to apply *some* science to whatever I end up with?

If I set the low voltage threshold to a voltage that would represent
50% DOD when discharged at 5A the alarm *will* sound prematurely when
running at a higher current. Now I could take that on the grounds that
at least it wouldn't over-discharge the batteries but I wouldn't get
the full capacity either. ;-(

I'm not asking or expecting any solution to be an accurate measure of
the DOD, just that I think I should be able to better than using one
set voltage over a range of currents when we *know* the voltage will
vary as a function of the current drawn?

Cheers, T i m

On Fri, 15 Sep 2017 23:19:25 +0100, "bm" wrote:
snip

Yes, I was wondering why 50% was specified.
Or maybe I missed it.

50% is often given as the maximum depth of discharge (DOD) as it
typically represents a reasonable output of energy whilst giving a
reasonable battery life.

http://www.mkbattery.com/images/8GU1H.pdf

Cheers, T i m

"T i m" wrote in message
...
On Fri, 15 Sep 2017 23:19:25 +0100, "bm" wrote:
snip

Yes, I was wondering why 50% was specified.
Or maybe I missed it.

50% is often given as the maximum depth of discharge (DOD) as it
typically represents a reasonable output of energy whilst giving a
reasonable battery life.

http://www.mkbattery.com/images/8GU1H.pdf

Ok.

On Fri, 15 Sep 2017 23:08:42 +0100, (Roger Hayter)
wrote:

T i m wrote:

On Fri, 15 Sep 2017 13:04:46 +0100, (Roger Hayter)
wrote:


None of the information you have helps with saying whether this relation
will still be valid above 93A, but perhaps it should be good up to the
rated maximum current of the batteries??

Ok, well all I'm interested in is how it would work up to 30A when
spread across the 3 batteries in parallel Roger. ;-)

So, sorry, I think I must have missed the conclusion. ;-(


50% discharge voltage /= 12.12 - (discharge current [while stable for
a few sec] X 0.00133)

Ok, they look like numbers O could see in an Arduino sketch. ;-)

VSMOP if you can measure current and voltage continuously.

I can (TAIAP), several times a second. But we aren't measuring Ah are
we, we are (only needing to) measure the instantaneous voltage and
compare that with the instantaneous current and from those two
calculate the instantaneous low voltage thresholds (rinse / repeat)?

eg, It will (very predictably) cope with the (example) scenario of me
going continuously at speed one (5A) for three hours before the alarm
sounds. It will also / equally easily cope with me going along at
speed 3 for 50 minutes (15A) or speed 5 for 20 minutes (30A).

So stage one is using the (non linear) runtime / 'voltage that equals
50% DOD' divisor (that operates increasingly as the current goes up
ITRW) as a constant over the entire battery use. Stage two is to
dynamically calculate that LVD voltage as a function of voltage
current and do so once every second. Stage 3 might be to store and
hold and average those values over say 60 seconds to protect against a
few second burst of speed 5 tripping the alarm whilst generally going
at speed 3.

The most difficult bit will be finding out what is a useful value of 'a
few sec'.

See above? shrug

Cheers, T i m

On Sat, 16 Sep 2017 00:08:47 +0100, "bm" wrote:


"T i m" wrote in message
.. .
On Fri, 15 Sep 2017 23:19:25 +0100, "bm" wrote:
snip

Yes, I was wondering why 50% was specified.
Or maybe I missed it.

50% is often given as the maximum depth of discharge (DOD) as it
typically represents a reasonable output of energy whilst giving a
reasonable battery life.

http://www.mkbattery.com/images/8GU1H.pdf

Ok.

Now, as an aside, because we may only go electric outboarding 10 times
a year, we may decide to push the battery to say 100% DOD (which is
near 10.5V, *not* 0V etc) because, according to the chart, even at
100% DOD those batteries should offer 450 cycles which would mean 45
years use at that usage rate!

However, the closer you go to 100% DOD the greater chance of doing
irreversible damage to the battery, something I'd prefer to avoid.

That said, once we have the potentially 'arbitrary' 50% DOD covered,
it would only be a matter of changing a couple of numbers to allow any
program / formula / calculation to adapt accordingly and take it to 60
or 70% DOD. Better run time at the expense or a lower cycle life.

Cheers, T i m

"T i m" wrote in message
...
On Sat, 16 Sep 2017 00:08:47 +0100, "bm" wrote:


"T i m" wrote in message
. ..
On Fri, 15 Sep 2017 23:19:25 +0100, "bm" wrote:
snip

Yes, I was wondering why 50% was specified.
Or maybe I missed it.

50% is often given as the maximum depth of discharge (DOD) as it
typically represents a reasonable output of energy whilst giving a
reasonable battery life.

http://www.mkbattery.com/images/8GU1H.pdf

Ok.

Now, as an aside, because we may only go electric outboarding 10 times
a year, we may decide to push the battery to say 100% DOD (which is
near 10.5V, *not* 0V etc) because, according to the chart, even at
100% DOD those batteries should offer 450 cycles which would mean 45
years use at that usage rate!

However, the closer you go to 100% DOD the greater chance of doing
irreversible damage to the battery, something I'd prefer to avoid.

That said, once we have the potentially 'arbitrary' 50% DOD covered,
it would only be a matter of changing a couple of numbers to allow any
program / formula / calculation to adapt accordingly and take it to 60
or 70% DOD. Better run time at the expense or a lower cycle life.

Well then you need to balance the cost of batteries against enjoyment.
Same as with our campervan.
If it dies, I buy another. Peanuts in the scheme of things.
Ok so we have no TV for a night, the world doesn't end.
Oh wait,
Monday, Wednesday or Friday the world DOES end

T i m wrote:

On Fri, 15 Sep 2017 23:08:41 +0100, (Roger Hayter)
wrote:

snip

Ok, but if I understand the above info correctly the difference in
capacity changes significantly within the range of currents I intend
to use therefore (in my mind anyway) making it worth of inclusion in
any gauge trying to display the cutoff voltage for a given depth of
discharge? As you say, we need the 'how' and I was hoping we might be
able to extrapolate that from the information we *have* been given?


It depends whether you are trying to predict capacity at a given
current, which is where your Peurkert equation comes in.

No, I wouldn't say I was trying to predict anything (at this point
anyway g), but calculate it from the current characteristics?

Or just
finding out when you have reached 50% capacity.

Yeah, that's the badger. ;-)

The latter will be
indicated by the voltage they mention,

(at the discharge rates they mention, so far ...)

and the fact that it was reached
proportionately sooner with a lower effective capacity at high current
is irrelevant to the problem of detecting when it has been reached.

Ok ...

The battery has *been* inefficient at high current and it is too late to
do anything about it.

Ok, so, let's say that the battery would reach 50% depth of discharge
if we were going along on speed 1 (of 5) in 3 hours. The alarm beeps
and I know I might damage the battery any further so stop (the device
has done it's job).

Then I fit the second battery but it's starting to get dark so we want
to get home quicker so I put it on speed 3 that will have the alarm
going off in 50 minutes (again, the device has done it's job).

The point is that in each case the only information that will be used
to determine the battery voltage but with a rider that is a function
of the current current being drawn.

And this isn't me imagining anything or making things up, this is
scientifically proven fact that is measured and well published.

This phenomenon is separate from the *temporary* loss of capacity after
a period of high current.

Of course.

The only way to deal with this is to stop
discharging the battery well before 100% use of capacity, which is what
you are being advised to do when you use 50% as an end point.

Quite.

This
makes sense to me though it is a bit speculative.

And why I'm trying to apply *some* science to whatever I end up with?

If I set the low voltage threshold to a voltage that would represent
50% DOD when discharged at 5A the alarm *will* sound prematurely when
running at a higher current. Now I could take that on the grounds that
at least it wouldn't over-discharge the batteries but I wouldn't get
the full capacity either. ;-(

This is where I think you are slipping into a fallacy. Although the
capacity has been less at the higher current, you have *lost* that
capacity by using a higher current and you can't get it back. Even
though 50% DoD has been reached proportionaliy sooner, it *has* been
reached and you can't reclaim the loss of capacity, except by
over-discharging the battery. This is why for battery management, as
opposed to journey management, the reduction of capacity with higher
current is irrelevant to you. You still want to stop at 50% DoD however
soon you reach it.







I'm not asking or expecting any solution to be an accurate measure of
the DOD, just that I think I should be able to better than using one
set voltage over a range of currents when we *know* the voltage will
vary as a function of the current drawn?

Cheers, T i m


--

Roger Hayter

T i m wrote:

On Fri, 15 Sep 2017 23:08:42 +0100, (Roger Hayter)
wrote:

T i m wrote:

On Fri, 15 Sep 2017 13:04:46 +0100, (Roger Hayter)
wrote:


None of the information you have helps with saying whether this relation
will still be valid above 93A, but perhaps it should be good up to the
rated maximum current of the batteries??

Ok, well all I'm interested in is how it would work up to 30A when
spread across the 3 batteries in parallel Roger. ;-)

So, sorry, I think I must have missed the conclusion. ;-(


50% discharge voltage /= 12.12 - (discharge current [while stable for
a few sec] X 0.00133)

Ok, they look like numbers O could see in an Arduino sketch. ;-)

VSMOP if you can measure current and voltage continuously.

I can (TAIAP), several times a second. But we aren't measuring Ah are
we, we are (only needing to) measure the instantaneous voltage and
compare that with the instantaneous current and from those two
calculate the instantaneous low voltage thresholds (rinse / repeat)?

eg, It will (very predictably) cope with the (example) scenario of me
going continuously at speed one (5A) for three hours before the alarm
sounds. It will also / equally easily cope with me going along at
speed 3 for 50 minutes (15A) or speed 5 for 20 minutes (30A).

So stage one is using the (non linear) runtime / 'voltage that equals
50% DOD' divisor (that operates increasingly as the current goes up
ITRW) as a constant over the entire battery use. Stage two is to
dynamically calculate that LVD voltage as a function of voltage
current and do so once every second. Stage 3 might be to store and
hold and average those values over say 60 seconds to protect against a
few second burst of speed 5 tripping the alarm whilst generally going
at speed 3.

The most difficult bit will be finding out what is a useful value of 'a
few sec'.

See above? shrug

Cheers, T i m

Sounds good to me. The other thing I thought of was to calculate the
inequality every 100milliseconds, and trip when it remained low every
time for a 100 times in succession. Same answer as your method I
should think.
--

Roger Hayter

On Sat, 16 Sep 2017 00:41:32 +0100, "bm" wrote:

snip

That said, once we have the potentially 'arbitrary' 50% DOD covered,
it would only be a matter of changing a couple of numbers to allow any
program / formula / calculation to adapt accordingly and take it to 60
or 70% DOD. Better run time at the expense or a lower cycle life.

Well then you need to balance the cost of batteries against enjoyment.

Quite and I am ... except this is also a bit of a project in itself
(electric propulsion).

Same as with our campervan.
If it dies, I buy another. Peanuts in the scheme of things.

Campervan or campervan battery? FWIW the 3 batteries cost nearly as
much as one of the boats so in our case the batteries are a
significant part of the cost of the whole package. And there is a
difference between 'died' and 'death by neglect or abuse' (even if non
intentional). It would be like you not ever bothering to check the
engine oil and knowing it uses some.

Ok so we have no TV for a night, the world doesn't end.

No, but if we are stuck without power 2 hours away from the car, in an
open dinghy, in the rain it *would* be a big PITA.

Cheers, T i m

On Sat, 16 Sep 2017 00:50:48 +0100, (Roger Hayter)
wrote:

snip

And why I'm trying to apply *some* science to whatever I end up with?

If I set the low voltage threshold to a voltage that would represent
50% DOD when discharged at 5A the alarm *will* sound prematurely when
running at a higher current. Now I could take that on the grounds that
at least it wouldn't over-discharge the batteries but I wouldn't get
the full capacity either. ;-(

This is where I think you are slipping into a fallacy. Although the
capacity has been less at the higher current, you have *lost* that
capacity by using a higher current and you can't get it back.

Yep, I understand that.

Even
though 50% DoD has been reached proportionaliy sooner, it *has* been
reached

No, it won't have been reached if the meter is using an arbitrary low
voltage threshold though?

and you can't reclaim the loss of capacity, except by
over-discharging the battery.

Of course (if that was the case).

This is why for battery management, as
opposed to journey management, the reduction of capacity with higher
current is irrelevant to you. You still want to stop at 50% DoD however
soon you reach it.

*Agreed*. Now, what low voltage is *correct* for what current, as one
voltage can't be correct for all currents can it (or they wouldn't
publish a graph showing different voltage cutoff points for different
currents presumably)?

Please don't get embroiled in the thoughts that I want to actually
monitor the actual battery capacity because I don't ... I just want to
do a bit more that use a single 'catch-all' values for what is already
known to be a conditional range. ;-)

Cheers, T i m

p.s. I have various electronic measurement tools including a watt hour
meter but it's no use as a form of protection tool and it doesn't
compensate for the varying capacities at varying loads. It could give
me a 'worse case' value though (30A x the voltage x the period
measured as a percentage (50% say) of the potential at that rate).

On Sat, 16 Sep 2017 00:50:49 +0100, (Roger Hayter)
wrote:

T i m wrote:

On Fri, 15 Sep 2017 23:08:42 +0100, (Roger Hayter)
wrote:

T i m wrote:

On Fri, 15 Sep 2017 13:04:46 +0100, (Roger Hayter)
wrote:


None of the information you have helps with saying whether this relation
will still be valid above 93A, but perhaps it should be good up to the
rated maximum current of the batteries??

Ok, well all I'm interested in is how it would work up to 30A when
spread across the 3 batteries in parallel Roger. ;-)

So, sorry, I think I must have missed the conclusion. ;-(


50% discharge voltage /= 12.12 - (discharge current [while stable for
a few sec] X 0.00133)

Ok, they look like numbers O could see in an Arduino sketch. ;-)

VSMOP if you can measure current and voltage continuously.

I can (TAIAP), several times a second. But we aren't measuring Ah are
we, we are (only needing to) measure the instantaneous voltage and
compare that with the instantaneous current and from those two
calculate the instantaneous low voltage thresholds (rinse / repeat)?

eg, It will (very predictably) cope with the (example) scenario of me
going continuously at speed one (5A) for three hours before the alarm
sounds. It will also / equally easily cope with me going along at
speed 3 for 50 minutes (15A) or speed 5 for 20 minutes (30A).

So stage one is using the (non linear) runtime / 'voltage that equals
50% DOD' divisor (that operates increasingly as the current goes up
ITRW) as a constant over the entire battery use. Stage two is to
dynamically calculate that LVD voltage as a function of voltage
current and do so once every second. Stage 3 might be to store and
hold and average those values over say 60 seconds to protect against a
few second burst of speed 5 tripping the alarm whilst generally going
at speed 3.

The most difficult bit will be finding out what is a useful value of 'a
few sec'.

See above? shrug

Cheers, T i m

Sounds good to me. The other thing I thought of was to calculate the
inequality every 100milliseconds, and trip when it remained low every
time for a 100 times in succession. Same answer as your method I
should think.


Agreed ... as we are talking runtimes in the 'hours' etc. So, how to
achieve that ... ;-)

We know two LV values in the current range I need, so what formula do
we use to calculate the rest?

ITRW the current is never likely to drop below 5A (ignoring stop times
when the battery voltage might recover and cannot be quantified) and
shouldn't go above 30A unless the prop gets stalled or fouled etc.

So from that we know that at 50% DOD, is seen at 12.05V @ 18.2A and
12.10V @ 4.65A (so we could use that as our lower RW limit) and, not
that I would ever use it, it would be 11.75V @ 93A (but that might
help our graph / formula). Now, I'm not sure if you said that works
out to be a straight line (then the figures are obviously bogus) and
if not, do they fit in with the capacities stated at the various
discharge rates:

36Ah at C/100 (so when discharged at 0.36A for 100 hours)
31.6Ah at C/20 (so when discharged at 1.58A for 20 hours)
26.8Ah at C/5 (so when discharged at 5.3A for 5 hours)

http://www.mkbattery.com/documents/1...I&O)_MK_r1.pdf

Thought: Now, in my real world tests (across all three batteries
(singularly)) I was able to see 5A for 4 hours down to 11.2V (that
*should* represent 50% DOD at that current).

It was done manually but worked out like this (measurements taken
every 10 minutes or so):

12.90, 12.72, 12.64, 12.46, 12.38, 12.30, 12.24, 12.18, 12.13, 12.08,
12.04, 11.99, 11.95, 11.90, 11.85, 11.80, 11.74, 11.68, 11.62, 11.55,
11.48, 11.41, 11.30, 11.21V

Irrespective of any published information, if you could extrapolate
that above log down to 0V, it might give us the shape of the graph
(and therefore the calculations) to continue?

Can we extrapolate any information from the chart on the bottom half
of P6 to help create our formula?

So the questions is, once we have worked out how to compute the values
for that range, are they still significant enough to bother with? I
can only decide that once I know what it is. ;-)

'You can manage (or at least go some way towards managing) what you
can measure (for', in this case). ;-)

Cheers, T i m