Initial balancing of CALB SE200FI 3,2V200AH, cell drift

[Walef]

Hello,
I'm building a 4S battery pack with CALB cells and a smart JBD 4S BMS board.
I received the cells as grade A ( new), all showed the same voltage level of about 3,2 Volts.
I'm currently in a working stage to lab' test the components and functions, battery and BMS.
To condition the cells I have parallel connected them as received and charged them up to 3,65V and let them rest for a minimum of 24 hours in connected condition.
As the next step I series connected the cells and conducted a capacity test 0,5C load, test confirmed 206 AH's capacity, and charged the pack again until it reached the max charge level of 14.5 Volts. The BMS cut off the charging process when the first cell reached 3.65 V OVP .

It became obvious while charging that one cell showed a higher voltage level from about 3,4 ave volts per cell onwards.

After charging I parallel connected the cells again and let them rest and zip balance for at least 24 hours, than I parallel connected the pack again with the BMS in place.
I conducted some moderate discharge and charging action to watch the voltage levels.

My observation is that one cell is alway ahead the three other cells of a minimum of 150mV.
The other 3 cells voltages are tight together.
This results that this one particular cell reaches the OVP at first (3,65V) and shuts off the charging process.

BMS balancer kicks in as expected at 50mV diff., balance turn on voltage at 3,4 V, however, not able to equalise the diff voltages in a timely manner.

I'm looking for your comments based on your experience if there is anything wrong in the initial balancing process or recommendations I could do to improve the voltage drift situation.

BTY: What is the voltage drift you see on your set ups and what are acceptable limits ?


Have a nice day,
Waldemar

 

[Supervstech]

I would set the charger to shoot for 3.55 per cell, (14.2V) and see how it charges. The battery will last longer, and it won’t cost much capacity.

What are the other cells reaching before charging stops?

 

[Walef]

I would set the charger to shoot for 3.55 per cell, (14.2V) and see how it charges. The battery will last longer, and it won’t cost much capacity.

What are the other cells reaching before charging stops?

I agree yo change the parameters when the pack gets installed in the RV.
However, this is a math exercise! When one of four cell reaches OVP at 3,65V and is 150mV ahead, the other cells are at about 3.45 - 3.50 Volts.
My question is what can I do to reduce the spread and minimise the drift?
In one of Will's video clips he made a statement that drift on new cells is in most cases a matter of incorrect balancing.
I need to get the proof!
Waldemar

 

[Ampster]

Clen all connections.nTest the capacity of that cell. It is probably less than 200 Amphrs. If you can't do that discharge the pack and observe that cell carefully since it will go lower than the others at the bottom.
To balance that cell get a big resistor and bleed off some charge so all cells get to 3.55 at the same time.

 

[Supervstech]

I agree yo change the parameters when the pack gets installed in the RV.
However, this is a math exercise! When one of four cell reaches OVP at 3,65V and is 150mV ahead, the other cells are at about 3.45 - 3.50 Volts.
My question is what can I do to reduce the spread and minimise the drift?
In one of Will's video clips he made a statement that drift on new cells is in most cases a matter of incorrect balancing.
I need to get the proof!
Waldemar

Wow, that is a lot of difference... that cell really needs inspection. You top balanced, and STILL got one cell to drift THAT far... it sounds like a weak cell, but for SURE, clean and check all connections.

Can you post a pic of the pack, indicating which cell is the weak one?

 

[Walef]

I drained the affected cell from 3,51 volts to 3,35 volts, diff. 150mV, in a controlled way and burned only 1,5 AH off. I expected more energie to get burned. I've no experience to determine weather this is surface charge only and in an expected range.
This brought all cells to the same voltage level 3,35V. - Same voltage level might not mean the same SOC, I'm aware off -
Therefore I parallel connected all cells and let them rest for a couple of hours.
To re-do the balancing exercise I'll parallel charge and step load them to 3,45V - rest & measure each cell - to 3,55V - rest & measure each cell - 3,60V - rest & measure each cell.
I believe this might be the most effectfull procedure to get all cells to same SOC.
In case you have different opinions or helpful comments please let me know.