Account for Peukert's Law and temperature when Coulomb counting for SOC?

[BackwoodsEE]

I'm doing my own SOC calculation using the Rasbberry Pi I have hooked up to everything in the solar equipment shed. With a battery current sample every 2 seconds, trapezoidal integration, and a 97% Coulombic efficiency, I've got the Ah coming out matching up fairly close to the Ah going back in. But the battery is still pulling more than just float-service current by the time all the current gets put back in, even when accounting for the 97%. (These are 410 Ah Rolls AGMs, pretty new and only drawing about 0.2 A to maintain float voltage.)

I'm wondering if high-current draws might be accounting for the difference. A lot of those Amp-hours were from brief runs of high-powered stuff: well pump, microwave oven, etc. I know that reduces the effective capacity of the battery during that discharge cycle, but does it increase the amount of charge that has to be restored before the absorb current trails off to a given threshold? Or is the number of Amp-hours required for recharge the same and the battery reaches a lower SOC before you start charging it back up?

I'm also curious about the effect of temperature. Suppose I discharge the battery when it's cold (night) and then charge it up again when it's warm (day, inside equipment shed). Does the higher SOC during charging (due to warmer battery) somehow boost the recharge and reduce the number of Amp-hours required for recharge? Or, again, is it just an SOC thing where I could discharge the battery more when warmer but the recharge Amp-hours remains the same?

[littleharbor]

Great questions. I'm looking forward to some detailed answers. I have noticed the shunt based meters will go up to and even beyond 100%. The battery will still be accepting current beyond the meters 100% reading and the instructions even mention that can happen and that regardless what goes in, over and above the initial 100% reading the meter will basically start counting down when the battery begins discharging. Of course the meters only know what battery capacity they are programmed for which will soon be incorrect as the battery ages.

[Sunking]

Backwoods I will try to help out.

To start I am not sure where you came up with 97% efficiency? Charge efficiency changes minute by minute and to my knowledge no AGM ever reaches 97%. Perhaps 95% in perfect conditions and charge rate. Many lithium batteries are not that efficient. AGM batteries are more efficient than flooded but in the neighborhood of 90% when at room temps. Key word is temperature with respect to capacity. At 77 degrees a battery is at full rated capacity. As temps goes up, capacity increases. Thus as temps drop capacity decreases.

Lastly is Peukert Effect and Charge Efficiency. Ironically with lead acid the faster you charge, lowers charge efficiency. As for Peukert Law it still applies to AGM, not as severely as FLA, but still there. Example a Rolls R12-AGM is a 100 AH battery @ 20-hour rate. Goes down to 79 AH at 5-hour rate, and up to 110 AH @ 100-hour rate.

Just too many variables to account for and the reason why battery monitors for lead acid batteries are a waste of money IMO because they rarely even get it in the ball park and need re-calibrated with each charge cycle. You would get more accurate results using Internal Resistance and Temperature. Good luck figuring out the algorithm.

Now I can tell you how as an engineer to account for Peukert Law. Best way is to look at manufactures discharge curves at various rates, or make your own. Again an example using Rolls R12-AGM you can make a Peukert Correction Factor based on the deviation from 20-hour rate.

100 Hour = 1.1
20 Hour = 1
10 Hour = .86
5 Hour = .79

Even better is to make a Master Correction Factor for all Rolls or whatever manufacture you are using. Look at the AH for all AGM models and average them out. But as you can probable already tell is a WAG Monitor is a good as it will ever get. In just one year of use, all the numbers are garbage because your battery no longer has rated capacity. Eventually you will find better uses for your time like watching paint dry or grass growing.