Can't charge one of my two batteries above 13.5 V

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Christian Bertschy asked Oct 26, '21

Can't charge one of my two batteries above 13.5 V

Hi everyone,

I'm trying to balance one of my two batteries (2x Lithium Batterie 12,8V/200Ah, BAT512120610) in my 24V system, but can't get it above 13.5 V. It needs to get to 13.8V to do so, based on the feedback I get on the Victron connect app:

I was able to balance the first battery:

The temperature difference is due to the one battery being further from the floor.

It seems there is an overall threshold that seems to prevent the second battery to get to 13.8V?
Any help is highly welcome and thanks in advance!

I have a the following setup:

2x Victron Energy Lithium Batterie 12,8V/200Ah - Light - Smart / BAT512120610
Victron Energy EasySolar C 24/1600/40-16 MPPT 100/50 / CEP241621010
Victron Energy VE Bus BMS / BMS300200000
Victron Energy BatteryProtect 12/24V-100A - SMART / BPR110022000
Victron Energy Digital Multi Control 200/200A GX (RJ45 90°) / DMC000200010R
Victron Energy BMV-712 - SMART / BAM030712000R
Victron Energy Venux GX

BMS battery

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Answer 1 · 2021-10-26T17:45:37Z

Matthias Lange - DE answered · Oct 26, 2021 at 05:45 PM

15V is pretty high.
Are the batteries still connected together? If yes disconnect them and charge them separately.

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Answer 2 · 2021-10-27T10:12:40Z

Christian Bertschy answered · Oct 27, 2021 at 10:12 AM

Hi @Matthias Lange - DE

Thanks for your answer! Yes they are still together. What would I need to change on my system if I only hook up one of the two? (i.e. it being then a 12V setup)

Would another approach be then to let both batteries discharge and then try to reload again?

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kevgermany ♦♦ commented · Oct 27, 2021 at 02:57 PM

The batteries are in series, so same current through both. If you don't fully charge separately, you'll not get it right because the same current goes through both.

You could consider a battery balancer to bring them to the same state of charge. This will then keep them close. Using a balancer to get these in balance will take a while, though as they're a long way out.

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Christian Bertschy kevgermany ♦♦ commented · Oct 27, 2021 at 03:27 PM

thanks, @kevgermany, ok, might have to consider the balancer. Is there a risk of battery damage with this setup at the moment? Esp. that the one battery goes to 15V frightens me. As far as I can read, the imbalance of the other battery wouldn't be a risk though.

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kevgermany ♦♦ Christian Bertschy commented · Oct 27, 2021 at 03:37 PM

I think you have a BMS in there which should prevent over-discharge. Lithium does not handle over voltage well. Damage is quite likely, especially if you continue like this.

One way you could consider it balance the batteries is to disconnect them completely. Then join negative to negative. Then connect a 12V bulb across the positives. Leave for a day or two, checking the individual voltages until they're equalised. The bulb will provide a safe resistance limiting the current between them.

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Christian Bertschy kevgermany ♦♦ commented · Oct 27, 2021 at 03:44 PM

Thanks, I just checked the balancer, seems like an easy install (would probably solve the problem also longterm). I assume due to the fact that one battery is closer to the floor and therefore tends to get colder quickly, I'd probably run into the same problem over time again.

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