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jcarnouil asked

Midpoint voltage suddent deviation

Dear Victron fellows,

For the last few days the midpoint voltage on my installation went up to 6 % deviation from time to time, causing the BMV 702 alarm ring. My 48v storage consist of 4 x 12v carbon-lead batteries in series, so I suspect one battery to be at his end of life, but they are just 2 years old !.

My installation is offgrid, so the cycles are quite deep (75-80 % every day), as you may see on my public VRM :

https://vrm.victronenergy.com/installation/25746/advanced

Do you think one battery could be not enough charged when the BMV reach 100% and my MPPT 150/35 stop charging ?

Any help is welcome
Thanks in advance
- Jean-Claude

BMV Battery Monitor
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jcarnouil avatar image jcarnouil commented ·

A follow-up to my last post :

For the last few days I've managed to take care of not discharging too much the batteries. Even with that precaution, the middle point went down up to 8% from time to time.

So yesterday, I stopped my whole system and recharged the faulty battery for 3 hours (with the help of my generator and a Victron Blue Smart 12/5). The battery was charged at 5A for 3 hours and the charger went to absorption mode for about one hour.

After powering back the Multiplus II, the midpoint voltage was OK, with 0% difference, but soon after it came back to 0,5% and so on.

So I'm afraid that battery is gone, but being part of a series of 4, I'm not sure I can replace it without having soon problems with the other 3 batteries ?

As I said in my previous message, those carbon lead batteries are just 22 months old .. but maybe I stressed them to much, and also during winter and cloudy days they were not recharged à 100 % everyday ...

If you have any idea about how I could regenerate the "tired" battery, let me know please !

Here are some screenshots from VRM showing the evolution of the midpoint voltage for the last 24h, along with the corresponding SOC.

More detailed stats could be seem on my VRM public page.

Thanks for any help
- Jean-Claude


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sprinter2072 avatar image sprinter2072 commented ·

I just set up midpoint monitoring with my BMV 712, I have six 6v batteries so I connected the mids for each series - its a 12v system. Initially the deviation was above 2%, but I it settled down to somewhere to 0 and 1%. That seemed reasonable. I did test the bottom three battery with a multimeter and one the voltages ranged from 6.6ish on 2 and 7.1 on 1. It turns out what battery cable had a crimp for some time. I equalized to see if that would help.

Very soon after I started getting alarms when the midpoint deviation would jump above 2%. In the last few days it's gone over 6% at least 4-5 times per day but then settles back down. I turns out that this happened when the charger (either the solar or multiplus) is bulk charging. As soon as it moves into float, the deviation drops below 1%.

I think this is normal, but I can't find a definitive answer anywhere.

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1 Answer
Paul B avatar image
Paul B answered ·

Have you thought about installing a battery balancer each battery. this may save you replacing the whole bank as adding a balancer onto EACH battery will help re balance the out of balance battery all the time.

I dont know much about the carbon batteries and cant advise re there condition etc

but this solution may give you more time to save up for a new set of lithium phosphate batteries. much much better for Offgrid systems in my opinion.

It would be unadvisable to replace one battery in any lead acid bank of batteries and your batteries are still lead acid based. Flooded,Gel, AGm or Carbon are all Lead Acid based batteries.

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