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