Series battery at different voltages?

[Stephen InGeorgia]

I just upgraded my inverter from 12v to 24v to better handle my well pump. The batteries were arranged in parallel: AmpereTime 12v 200ah Plus models with a 200a BMS. They have been in parallel for 6 months. Upon disconnecting they both read 13.34V so I put them in series and programmed the MPPT and set the SunGold inverter to Lifepo4. Inverter started up and charged them as I'd expect for the first time. Switched over to battery priority and let the MPPT take over to charge the battery. About two hours later got an overvoltage protect on the MPPT. Currently the batteries are reading 13.34 and 14.14.
I dialed back the MPPT charge voltage to 27.5V and got it less frequently, but still no good. The suspicious battery at 14.14 would jump up to ~18V then back to ~14V when I was watching it with the voltmeter.

Ideas?

 

[MurphyGuy]

Could be charge hysteresis or one weak battery.

Let them sit without charge or discharge for about an hour then remeasure voltage at terminals.

If you put a weak battery in series with a strong battery, the weak one will have a higher voltage when charging, and a lower voltage on discharge.

You might also be pumping in too many charging amps, which would greatly amplify the effect.

 

[400bird]

You should put those 12v batteries in parallel and charge them to full and let the cells balance. You connected them at 13.34 which is 3.3 volts/cell so there's no guarantee the two batteries were at the same SOC.

 

[Stephen InGeorgia]

Could be charge hysteresis or one weak battery.

Let them sit without charge or discharge for about an hour then remeasure voltage at terminals.

If you put a weak battery in series with a strong battery, the weak one will have a higher voltage when charging, and a lower voltage on discharge.

You might also be pumping in too many charging amps, which would greatly amplify the effect.

That makes sense. On a few occasions I had a large voltage drop and after the inverter stopped it returned. I'm guessing a bad battery or bms.

 

[time2roll]

Consider a balancer similar to this: https://www.aliexpress.us/item/2251832773013010.html