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