Voltage suddenly drop while trying to balance at 3.65V?

[AshleyL]

It is the semi-annual high voltage full balancing time again.

I have 3 Sinopoly 200Ah cells exhibit this issue.
The lug and terminal are properly tightened. Even used clamp.
Based on the cell specification, the Cut-off Current is 6.7A.

The voltage is steadily increasing until 3.65V @ 20A (CC stage done) and the current started to taper off.
But here is the strange thing during CV stage. Once the amperage lowers down to around 9A - 8A, the voltage cell suddenly dropped to 3.488V - 3.460V.
I used another charger for these cells, same issue. Used clamp to measure the amperage and yes, 8A-9A flowing in the cable.

Any idea why is this happening?

 

[sunshine_eggo]

@RCinFLA

This guy might.

 

[RCinFLA]

Misbalanced cells are starting out with differing SoC so some cells will take longer before their current tapers down.

Cell overpotential vs current is different between cells because cells when they are mismatched. This causes the cells requiring higher overpotential voltage for given current to lag behind on charging. For paralleled cells, they get less share of the total applied charging current so they charge slower.

8-9 amps into a 200 AH cell has about 30 to 45 millivolts of overpotential voltage so 3.488vdc is still on the edge of just getting to full charge.

Just be patient and let the charge current taper down.

 

[AshleyL]

Misbalanced cells are starting out with differing SoC so some cells will take longer before their current tapers down.

Cell overpotential vs current is different between cells because cells when they are mismatched. This causes the cells requiring higher overpotential voltage for given current to lag behind on charging. For paralleled cells, they get less share of the total applied charging current so they charge slower.

8-9 amps into a 200 AH cell has about 30 to 45 millivolts of overpotential voltage so 3.488vdc is still on the edge of just getting to full charge.

Just be patient and let the charge current taper down.

Hmm.....the current eventually taper down to 6.5A after overnight charging......but the cell voltage is at 3.490v, not 3.65V. I guess I better stop the charger since something is definitely not right here.

 

[Partimewages]

How and where are you measuring the voltage?

 

[AshleyL]

How and where are you measuring the voltage?

Using two diffferent chargers (selectable voltage and current).
One charger is using clamp type and the other one is using ring terminal + bolt + washer.
Both chargers shown the same issue with my 3 sinopoly cells. Measured the voltage via multimeter at the battery terminal itself, same reading as the charger.

The other 13 cells are fine.

 

[RCinFLA]

What happens to the cell open circuit unloaded voltage when you disconnect it at the 3.49v and let it sit overnight?

I'm thinking about a cell leakage short. Have you been charging below freezing?

 

[AshleyL]

What happens to the cell open circuit unloaded voltage when you disconnect it at the 3.49v and let it sit overnight?

I'm thinking about a cell leakage short. Have you been charging below freezing?

Since I have two chargers, I can only test 2 cells at once.
After more than 1 hour disconnection from charger, I measured those cells again via multimeter and the voltage settled at 3.33V for both.

As for the charging below freezing.......I live at equator......impossible to have snow here. No hot weather either.....

 

[RCinFLA]

Since I have two chargers, I can only test 2 cells at once.
After more than 1 hour disconnection from charger, I measured those cells again via multimeter and the voltage settled at 3.33V for both.

As for the charging below freezing.......I live at equator......impossible to have snow here. No hot weather either.....

Cells are likely bad unless you have a connection problem to explain that much voltage drop. With only 9 amp of charging current on 200AH cell, and overpotential bump being to 3.49v, should only drop to around 3.40-3.45v with no cell current, rested open circuit, and sit there.

See if the 3.33v drops more overnight with open circuit. If so, cell has a lot of leakage current.

Hope you trust your voltmeter accuracy though.

 

[AshleyL]

Cells are likely bad unless you have a connection problem to explain that much voltage drop. With only 9 amp of charging current on 200AH cell, and overpotential bump being to 3.49v, should only drop to around 3.40-3.45v with no cell current, rested open circuit, and sit there.

See if the 3.33v drops more overnight with open circuit. If so, cell has a lot of leakage current.

Hope you trust your voltmeter accuracy though.

2 chargers and 3 multimeters.....should be trustable?

If these 3 cells really have a lot of leakage current, I expect the cell to be hot or warm, but it is very cool to touch.
Let check the voltage on rest after 12 hours or next day.

 

[AshleyL]

Update after overnight idle:
Weird, these 2 cells voltage are at 3.33V. The third one on standby for time being.
So, can rule out cell internal current leakage....

Just in case, I measured IR too this morning, seems good. Low IR 0.2mOhm.

Guess it must be related to the cell aging.....