LFP cells gone bad

[OffGridInTheCity]

In my very 1st few months (6 yrs ago) I had a 7s50p NMC battery on my 1st solar system (200w PV, EPEVER, Reliable Inverter).
I thought - I'll just measure the pack voltages each morning..... don't need a BMS.... but I learned

After a few weeks, the morning measurement showed significant different pack voltages at rest. So I started measuring A LOT more and found that some packs were dropping to 2.5v (or lower - who knows?) and then bouncing up to 3.3v after the inverter turned off. Had no clue with 1 measurement (or even several) in 24hrs... over the charge/discharge sequence.

YIKES - a BMS is absolutely needed as one needs to monitor voltages of the lithium-ion cells/packs in series all the time.. when part of an automated charge/discharge application such as a solar system which cannot be done by a human + action (alerts as a minimum, battery current cut-off is good) is valuable based on the measurements if they get out of line - e.g. 2.5v for NMC is not good!.

 

[Browneye]

We don't use a bms on radio control lipo packs, any number of cells. However we DO use a balance charger - specific to the chemistry type. That's what the individual cell leads are for. Although packs are pretty durable these days and many can stand very high discharge and charge rates, no more than 1C charging is still recommended. The chargers are configurable. LIPO WILL go up in flames - and the fire WILL NOT go out until they completely burn. Plenty of kids have burned down their family's garage. Charging in a fireproof bag is still recommended. I've been messing with them for a half-dozen years without a problem, but I follow the protocols.

For lifepo4 cells a bms is required to manage and protect the individual cells. This forum turned me onto Overkill Solar, which are JBD bms's. Maybe get one of those before something bad happens again. There are plenty of fire threads, even on here. One was a bus, spectacular.

 

[OffGridInTheCity]

We don't use a bms on radio control lipo packs, any number of cells.

Agreed, there is a category of battery where you can control the charge via balanced chargers.... and RC is often single use discharge followed by a balanced charge - done manually.

I use this approach for my 12,000lb winch battery made with headways.... needs to support up to 600a? I balance charged it and have a BattGo 8s to monitor manually if desired but if I use it, it will be single use (10mins?) to get unstuck and then balanced charged manually later on. I'll likely never use it but take it boondocking sometimes just in case.

Powerwall / daily automated charge/discharge situations or EVs or things like this are fundamentally different and really need a BMS.

 

[terrango]

did you test the 13 for capacity, internal resistance and self-discharge? if the answer is "no" to any, you don't know they're good.

Internal resistance is basically equivalent to a short. Everytime i try to charge the cells, the voltage hovers around .74V and as soon as you remove the power source it drops to zero. Im pretty sure these cells are garbage.

 

[sunshine_eggo]

Internal resistance is basically equivalent to a short. Everytime i try to charge the cells, the voltage hovers around .74V and as soon as you remove the power source it drops to zero. Im pretty sure these cells are garbage.

Cells that won't hold voltage are clearly bad. I'm talking about the "good" cells that do.