Temp compensation

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svpablo asked Nov 21, '19

Temp compensation

Hi

I have a BMV712 and 2 smart solar MPPT. I'm trying to configure the temp compensation and I'm getting a bit lost. Should I configure it on the BMV (temp coefficient) or on the mppt (temp compensation)? And what should be the value?

I have 12 batteries, each being 12v (12v system, 1200Ah capacity). The specs of the batteries are saying "Temp-Comp Factor (Charging) 5mV per °C per cell (2V)". So if every cell of 12v is 5mv, is the temp coefficient 30mv/°C? if so, how do you calculate the temp coefficient in %cap/°C ?

Thanks, I'm really confused on this and the manuals aren't very specific on the topic

sam

MPPT Controllers BMV Battery Monitor temperature compensation

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JohnC answered · Nov 22, 2019 at 05:08 AM

Hi Sam. Your 30mV is correct. It's applied in the mppts, and is a simple V correction to Abs and Float V's. No capacity adjustment needed.

That '%cap/C' figure is another animal. With that the BMV can adjust it's battery capacity figure to allow for temp, with subsequent better accuracy in reported SOC. That's a separate figure, one best supplied by the batt maker if it's available at all.

As an example, lets say you have batts at 35C. There they can hold more Ah, but need less V to charge. The mppts will adjust the charge V's downwards (note that mV is a -ve figure), and the BMV will raise the batt capacity in its calcs to give a better SOC.

Make sense?

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svpablo answered · Nov 22, 2019 at 04:54 PM

Thanks so much John. To make sure; the temp compensation is always negative?

I think it makes sense for the %cap/C. I understand the logic, but I don't see that in the spec sheet of the batteries, except that they graphed it? Am I missing something?

http://www.outbackpower.com/downloads/documents/energy_storage/energycell_nanocarbon/energycellnc_specsheet.pdf

106nc batteries.

MANY thanks!
sam

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JohnC ♦ commented · Nov 23, 2019 at 05:47 AM

Yes Sam, the temp comp in the mppts must show -ve. It's just to set the 'inverse' relationship between temp and target V. Higher temp = Lower V. Certainly for those batts.

Indeed they've graphed that other thing, but you can work out that single figure pretty closely. Use the C/20 line. Pick a range, say 10C to 30C, and read the capacity at each of those points. Just roughing it, 90% and 100% respectively? So ~10% Ah difference over 20C = 0.5 %cap/C. Then that figure goes in the BMV..

Try over a wider range too, but it's not a straight line so can never be perfect. But it'll be close..

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Answer 1 · 2019-11-23T13:24:31Z

svpablo answered · Nov 23, 2019 at 01:24 PM

Thanks soooo much John! Learned quite a bit! :) . Going back to read about peukert and tail current! :D

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