New bank overcharged cell

[ZDub77]

Ladies and gents,

Long time reader first time poster.

Finally received my 8 200ah cells and am looking forward to building 2 12v batteries with them. While waiting for the correct sized ring terminals to come in to hook up the BMS, I started charging up the cells to top balance them. I had cells in a 4s configuration with the power supply set to 14.4v. I planned to hook all up in parallel to top off to 3.65. When it had finished bringing the bank up to 14.4 I had 3 cells at 3.33v and 1 at 4.4v. The overcharged cell did not swell or get warm so I’m hoping it’s not to badly damaged.
What would cause this? Bad cell? Something I did wrong?
My plan is to discharge the overcharged cell and wait till I hook up the bms to charge them anymore

 

[upnorthandpersonal]

I had cells in a 4s configuration with the power supply set to 14.4v. I planned to hook all up in parallel to top off to 3.65. When it had finished bringing the bank up to 14.4 I had 3 cells at 3.33v and 1 at 4.4v.

Yeah, don't charge 4 unbalanced cells without a BMS.

What happened is the following:
- 4 cells of unknown state of charge, one higher than the other three overall
- charging starts, the highest cell gets to full
- the rest of the cells are still low, so overall voltage stays below 14.4V
- the highest cell is now overcharged and goes over 4V, the rest still sits at 3.3V or thereabout, overall voltage still below 14.4V

This is why you need to top balance in the first place, so that you have them all at a known state of charge.

If you want to do what you wanted to do, you need a BMS so you can disconnect once one of the cells hits 3.6V. Then you parallel, set the power supply to 3.6V and then connect to top them all off at 3.6V.

 

[ZDub77]

Thanks. They were all pretty close to each other when I started charging but I won’t do that again. I’ll just hook them all up in parallel and let them top balance that way

 

[ZDub77]

Thanks. They were all pretty close to each other when I started charging but I won’t do that again. I’ll just hook them all up in parallel and let them top balance that way

I guess I should have said close on voltage but an unknown SOC

 

[DJSmiley]

As you've noticed, never charge without a BMS

Voltage isn't an indicator of SOC. Firstly, the curve is almost flat, so 1mV is a huge difference in SOC. More important: Accuracy of 99% of the normal multimeters isn't that great. 0-100% SOC difference is almost within it acceptable tolerances...

Even without ring terminals you still can use the BMS, I did once, just clamped the wire below the nut of the busbar. Not recommended tho, but still way better than no BMS. (The connection might not be perfect and give a false reading if you're really unlucky)

 

[ZDub77]

As you've noticed, never charge without a BMS

Voltage isn't an indicator of SOC. Firstly, the curve is almost flat, so 1mV is a huge difference in SOC. More important: Accuracy of 99% of the normal multimeters isn't that great. 0-100% SOC difference is almost within it acceptable tolerances...

Even without ring terminals you still can use the BMS, I did once, just clamped the wire below the nut of the busbar. Not recommended tho, but still way better than no BMS. (The connection might not be perfect and give a false reading if you're really unlucky)

Thanks, the ring terminals will be here today. I'll post what the capacity test reveals once I finish the top balance and hook up the BMS. Hopefully I didn't damage the one cell to badly