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james-connell avatar image
james-connell asked

BMV showing 96% but voltage down at 12.1V

Hi, I’m trying to determine if my boats 220Ah (2x110Ah) battery bank is dead or if there is another issue at play.

I have a BMV712 and the low voltage alarm has been going off after about 1hr of disconnecting the charger. The BMV is initially showing 100% and has usually dropped into the high ninetys within an hour under normal loads, however the voltage is tailing off quickly and the alarm usually sounds within the hour. There is every chance I have a dead battery/batteries in the bank, however the charge percentage remaining high is making me question this. Does the BMV determine the charge percentage as a factor between battery capacity and load over time and use this to calculate remaining capacity percentage regardless of measured voltage, or is this also taken into account?

If the two are exclusive of each other this could explain why my low voltage alarm is going off when the BMV is showing almost full charge. If this is not the case I might have some more serious issues to investigate.

Many thanks,

James

BMV Battery Monitor
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2 Answers
JohnC avatar image
JohnC answered ·

Hi James. The SOC is of course a calculated figure, and BMV takes into account a number of things to achieve it. Batt Ah, peukert factor, round trip efficiency and the like. All are settable to find you a 'true' SOC, and usually require quite some tuning-in to reach 'confidence' stage in your SOC.

If your batts (or one of them) is sick, that process will break your heart. Often just self discharge over time will break the algorithm and you'll never see what you want.

12.1V indicates well-discharged, and not a good place to spend their time. If you're using default settings in the BMV (aside from Ah), then what you describe isn't so good. You could have the batts pro tested, but to diy it give the batts a good charge over a day or 2, lift a terminal to separate them, leave for a few hours (or overnight) with no load, and measure the V of each. >12.5V would be nice to see then, and a dud should be obvious.

Don't replace 1, do both. It's imbalance in parallel strings that kills.

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james-connell avatar image james-connell commented ·

Many thanks John. I’m really hoping it is a sick battery not anything else. Will try you trick to diagnose.


Thank you,


James

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mvader (Victron Energy) avatar image
mvader (Victron Energy) answered ·

Hi James, to add to John, the BMV does not take voltage into account.


A few reasons to see high soc percentage yet low voltage could be:

- when real capacity is less than configured capacity: broken or aged battery.

- when you have loads wired directly to battery negative terminal rather than on system side of the shunt: battery would be discharged but the bmv doesn’t know it

- when the soc counter is out of sync; to prevent that; make sure the battery is regularly charged fully and that the parameters are setup such that it syncs. See bmv manual and some youtube videos for details on that.

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