Battery voltage imbalance when connected in series

[DaveM]

Hello - I'm trying to troubleshoot a problem I'm having, and I'm not sure where to look. I have two 206ah SOK batteries connected in series which are part of a 24v MPP Solar all-in-one system. I charged both batteries independently with a NOCO Genius 10 charger, and then connected them overnight in parallel to balance. Once balanced, I connected them in series to the MPP, and let the MPP fully charge them.



I was doing some load testing and let the system run until it cut off. But now when I reconnect to grid power, the batteries won't charge via the MPP.
When checking the voltage of the batteries while still connected in series, I get 12.23v (it should be in the 22-23v range).



When I disconnect the cables from the batteries and check them independently, one reads 9.77v and the other reads 13.11v.

So my questions are: 1) shouldn't the low voltage disconnect on the battery's BMS have prevented the one from dropping down to 9.77v? Secondly, what would cause the two batteries to drift so far apart in voltage? It's only been about a week since I balanced the batteries.

This is the second time this has happened. I'm trying to build a small emergency power backup system, so I'd like to find out if I've got a bad component or if I need to adjust my settings on the MPP. Any help would be greatly appreciated.

 

[Bud Martin]

How long did it take you to charge each battery pack to full charge using that 10A charger?

 

[time2roll]

Charge the low battery again with NOCO and try again. It would seem you are doing everything correct. Could be a battery issue.
Although if fully charged 22 - 23 volts seems low or is this discharged? In this case charge them in series and see if the voltages come together at the top.

Is it too soon to recommend a 24v battery?
https://www.sokbattery.com/products/sok-24v-100ah-lifepo4-battery

 

[Steve_S]

12V should cutoff NO LOWER than 10V / 2.500Vpc. The SOK's apparently use a JBD BMS and may possibly have a WIFI or BlueTooth port but no module attached to use it (pretty dumb to not include it IMO) which would allow you to access the BMS and program it for use cases. IF any cell within a pack is below the Low Recovery point the BMS will disallow discharge and quite possibly could go into Fail Safe mode which means it has to be triggered manually to take charge again. IF SOK has an ON/OFF switch for the BMS that should do it. Unfortunately that means they would have to solder a switch wire from the BMS terminals for that, which again would be an Extra but should have been included by default for a prebuilt pack.

206AH @ 10A charge from 0%SOC will probably take 20-24 hours each.

Warning !
DO NOT CHarge these to 14.6V (per pack) and that will certainly result in High Volt Disconnects. There is NO POINT in charging above 3.550Vpc and even that will settle to 3.500 in < 1 hour. The working voltage curve is 12.0V - 13.6, Voltage below 2.900 and above 3.500 only represents roughly 5-7% of total overall capacity and the SOK batteries tend to be actually a little high in cell capacity to guarantee you get the 206AH as advertised.

Remember to program the Inverter, SCC accordingly for cutoff thresholds (which are only accurate IF you have calibrated your system to see the voltage drop between batteries & devices... Unlike LEAD, LFP like all Lithium Based batteries are super sensitive to Millivolt Changes specifically because of the flat voltage curve. If readings are even off by ONE VOLT, you could damage your batteries IF the BMS doesn't protect them and allows them to go too low (like yours just did).

Hope it helps, Good Luck

 

[DaveM]

Thanks. The low voltage battery is charging up again with the NOCO. Once it's charged, I will connect the two in parallel to balance again and once that's done will reconnect in series. I will check the parameters on the MPP All in One as well. I think what was puzzling me was that the one battery voltage dropped so low. I thought the low voltage disconnect on the BMS would have prevented that.

 

[DaveM]

Charge the low battery again with NOCO and try again. It would seem you are doing everything correct. Could be a battery issue.
Although if fully charged 22 - 23 volts seems low or is this discharged? In this case charge them in series and see if the voltages come together at the top.

Is it too soon to recommend a 24v battery?
https://www.sokbattery.com/products/sok-24v-100ah-lifepo4-battery

Yes, had those 24v batteries been available when I ordered mine in December, that is what I would have gone with. The 22-23v I referenced was where I would have expected the batteries to be at the low disconnect point.

 

[time2roll]

Thanks. The low voltage battery is charging up again with the NOCO. Once it's charged, I will connect the two in parallel to balance again and once that's done will reconnect in series. I will check the parameters on the MPP All in One as well. I think what was puzzling me was that the one battery voltage dropped so low. I thought the low voltage disconnect on the BMS would have prevented that.

OK I hope I did not mislead you. Since both are discharged... both need to be recharged to the same level. Probably need to fully charge both.

The way this is looking the minimum discharge should be closer to 24 volts. Need to adjust your equipment to disconnect sooner or you need more capacity to avoid going lower.

 

[Steve_S]

Also appreciate that LFP when under load will have a Voltage Sag. IE I turn on coffee maker which pulls 50A from 24V, the bank goes from 26.4V to 26.0 while brewing then recovers afterwards... So when configuring the BMS, and cutoff delays and cutoff thresholds this must be considered as well. Had the system been at 3.000Vpc (24.000V) the cells would have dropped by 0.4 bringing them to 2.600 and a possible BMS Trip for LVD if not configured to deal with load sags. Now to be clear, one pack or a bank like my 1190AH the same stuff happens regardless.

 

[JRUD]

Still wondering why his BMS did not shutoff on low voltage. I just had a very similar problem with a new battery. Dropped below 10v doing capacity test.