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RickS avatar image
RickS asked

12.5v indicates 85%

Hello all,

New to us 2021 Forrest River Sunseeker 2250LE

I installed 2 new 125ah AGM batteries at the same time as installing the Victron BMV-712

Batteries wired in parallel, all Positive terminals go to battery 1, neg goes from battery 2 to the shunt, coach neg goes to the opposite side of the shunt. Battery Neg to Neg, Pos to Pos. Pretty basic install

I have setup the Victron app per all the data I can find, see attached

The other day, the app was telling met the batteries were at 85% yet the battery voltage was reading 12.5v after they had been setting for about 2 weeks.

If 12.5v is about 50% battery, shouldn't the Victron app have told me the batteries were at 50% or even better, 0% meaning there was no more useable battery life left w/o doing damage to the batteries?


What am I missing?

Thanks, Rick

screenshot-20210614-155540.jpg

screenshot-20210614-155548.jpg

screenshot-20210614-155559.jpg

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BMV Battery Monitor
screenshot-20210614-155540.jpg (120.1 KiB)
screenshot-20210614-155548.jpg (63.6 KiB)
screenshot-20210614-155559.jpg (51.0 KiB)
screenshot-20210614-155645.jpg (74.7 KiB)
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2 Answers
Erik Sporns avatar image
Erik Sporns answered ·

The bmv is keeping track of the amp hours going in and out of the battery. Voltage is used for sync (0% and 100%).

Did the batteries go through a full cycle with the bmv connected? If not, that would be a starting point. Other than that its playing with the peukert factor/charge efficiency, etc.

Also: Did a quick google search: 12,5V for a AGM is closer to 75% than to 50%.

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RickS avatar image
RickS answered ·

Hello Erik,

Yes the batteries have gone through a full charge cycle more than once actually. I also did a zero current calibration and the SOC sync when fully charged when I first setup everything up

It just seems weird to me that with 250ah, my batteries don't seem to last very long

For instance, we drove home yesterday for 2 hours, I plugged into 15amp shore power and with nothing running other than the battery switch on, today I'm at 86%

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Erik Sporns avatar image Erik Sporns commented ·
Lead based batteries can be a bit tricky, because of the peukert factor (= less Ah if you draw more power). What kind of loads do you have on you batteries, or how many amps do you draw on average? E.g. I have a 12v compressor switch and im draining 5% over night (400ah lithium) which would equate to roughly 10% on your bank.


Your batteries are either rated for c20 or more likely c100. Which means they get their full capacity by drawing 250ah/100 = 2,5amps. Everything above that and you wont get the full charge out of them.

What make and model of batteries are you using?

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