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? ;-) 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.

Predicting a graph from 3 (6?) values?

Sep 14, 2017 81 Replies

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Dave W 8 years ago

OK understood. You could use the voltage between the two ends of the current-carrying wire from the battery as input to your Arduino current sensing arrangement. The resistance of the lead is the current shunt.

Cheers, Dave

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T i m 8 years ago

Funny you should say that ... that's exactly what I did on the electric racing bike I built, measuring the voltage drop over the longest battery cable and after calibration, displaying it as current. ;-)

The thing with this project though is I want to try to keep it all modular / flexible to it would be better to keep the current measurement 'local' to any meters etc. I'm hoping to use a hall effect module with integrated amplifier chip to make it easier to interface with the Arduino etc.

Also, with mostly 20mm^2 copper leads (2 x 2m) from the battery to the outboard I'm hoping that at my typical 15A draw I'll not get sufficient voltage drop for it to be worth measuring (although I know I will etc). ;-)

Cheers, T i m

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