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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

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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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RickS avatar image RickS Erik Sporns commented ·
Hi Eric

I'm not even trying to calculate load, I'm just referring to what happens when nothing is running, fridge is off, no lights... only what is factory installed directly to the battery.

These are VMaxTank SLR-125 batteries, I have 2 of them

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