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