BMV SoC drifting with partial charging due to efficiency >99%

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brainy asked Nov 2, '19

BMV SoC drifting with partial charging due to efficiency >99%

Hi!

I have a BMV-700 on my LiFePO4 (100AH) Battery and i notice that the SoC is drifting heavily after days/weeks when i only have partial charging (Solar, only short trips with charge booster). Peukert is already set to 1.00 but it seems that the charging-efficiency is higher than 99% but in the App i only can set integer (not float) values for that. This results is too less AHs detected when charging. Last time i had a SoC show of 78%, then i plugged in a external charger, it loaded to 80% then the Voltage was 14.4 and current was 0A, so the battery was full. It then synchronised but it will not do that (and even can't!) when only have partial charge often. Any ideas? Also there should be a (hidden?) function in the BMV to calculate the charge efficiency, e.g: Fully load the battery with external charger. Discharge to let's say 50%, then activate the "calculate charge efficieny", plug in the charger again and then the BMV can calculate the efficiency (as it knows how many AHs were charged to reach 100% again). This should be possible, or not?

Can someone help me?

Regards

BMV Battery Monitor SOC

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Answer 1 · 2019-11-09T01:05:51Z

Epicycler answered · Nov 09, 2019 at 01:05 AM

Charge / Discharge efficiency depends on the charge and discharge current. For any SOC the coulombic efficiency is the ratio of discharge volts to charge volts. Its difficult to have a single number for efficiency that keeps the SOC accurate.

Peukert should not effect efficiency.

The BMV history shows Ah (and Wh) in and out so can supply the data you require. Read before and after and subtract. Over the life of my battery so far I think the efficiency calculates about 80%, which is patently wrong.

Simply, to maintain any sort of accuracy the SOC meter needs to reset at 100% periodically.

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