Cell aging variance within same batch

[Philtao]

Hi, I made some observations that lead me to believe a cell is not aging as fast as the other ones.
Here is the setup: 300 Ah Winston cells (2016) setup as 2P4S (600 Ah 12 volt battery). Cells have sequential serial numbers.
Cells are never charged at more than 3.46 VPC. Float at 13.2V (or charger disconnected)

Charge graph (see attached):
this is the end of charge at 13.84 V (3.46 VPC) and charge current is just under 15 A
cells 4 seems to be less charged than cell 3 (120 mV less than cell 3) - edit: read 20mV

Discharge graph:
SOC is at 19% and current draw is about 15 A
cell 4 seems to have more capacity left than cell 3 (60 mV more than cell 3)

This observation would say that cell 4 has more capacity than the other cells. Has anyone seen similar aging variance?
I would be surprised if it comes from differences in the cells (same manufacturing batch and sequential serial numbers)

Any other suggestions / hypothesis that could explain such variance?

Edit: the cells internal resistances measured by the BMS are:
cell 1 = 0.26 mOhm
cell 2 = 0.29 mOhm
cell 3 = 0.27 mOhm
cell 4 = 0.33 mOhm
These have been stable for the last 6 months (when I started measuring)

 

[snoobler]

Hi, I made some observations that lead me to believe a cell is not aging as fast as the other ones.
Here is the setup: 300 Ah Winston cells (2016) setup as 2P4S (600 Ah 12 volt battery). Cells have sequential serial numbers.
Cells are never charged at more than 3.46 VPC. Float at 13.2V (or charger disconnected)

Charge graph (see attached):
this is the end of charge at 13.84 V (3.46 VPC) and charge current is just under 15 A
cells 4 seems to be less charged than cell 3 (120 mV less than cell 3)

I'm seeing less than 20mV - about .3455 to 3.465.

Discharge graph:
SOC is at 19% and current draw is about 15 A
cell 4 seems to have more capacity left than cell 3 (60 mV more than cell 3)

I'm seeing 50mV.

This observation would say that cell 4 has more capacity than the other cells.

The differences you're seeing are very small and well within any range I would expect from premium cells.

Has anyone seen similar aging variance?
I would be surprised if it comes from differences in the cells (same manufacturing batch and sequential serial numbers)

Unless these were truly matched cells, there is plenty of opportunity for manufacturing variations within a batch.

Any other suggestions / hypothesis that could explain such variance?

Edit: the cells internal resistances measured by the BMS are:
cell 1 = 0.26 mOhm
cell 2 = 0.29 mOhm
cell 3 = 0.27 mOhm
cell 4 = 0.33 mOhm
These have been stable for the last 6 months (when I started measuring)

I see absolutely nothing of concern.

Ensure BMS is set to balance during charge only, only above 3.4V and only if deviation is 30mV (you already meet those criteria, and balancing should not occur based on the info you've provided.

 

[Philtao]

Thanks @snoobler
Yes, my mistake charge unbalance is less than 20mV (not sure what I was smoking!) and yes they are top balanced.
I confirm that balancing takes place above 3.4V - currently starting with a differential of 15mV (just reduced it last week from 25mV).
It is the low SOC voltage differential that got my attention this morning. I will soon do a capacity test so I will have better data at lower SOC.