Cells in parallel do they really self balance?

[JonathanW]

Example 4 280ah cells in parallel. Slightly different soc, common bus bars. Can a single cell “run away” if in parallel? Or no matter the charge voltage ie 3.5v it cant run away with the other three sharing the load? Hard to measure at the cell since it’s averaging the other cells.

Also at the bottom of soc can a cell dip below 2.5v it the others are at say 3v?

 

[sunshine_eggo]

Example 4 280ah cells in parallel. Slightly different soc, common bus bars. Can a single cell “run away” if in parallel?

No.

Also at the bottom of soc can a cell dip below 2.5v it the others are at say 3v?

No.

Current flows due to voltage difference. You might have seen, V = I * R.

If a cell in parallel is somehow a little lower than the others, current will flow from all others into the low cell.

If a cell in parallel is somehow a little higher than the others, current will from from that cell into all others.

Cells in parallel are essentially just a bigger single cell.

 

[JonathanW]

Thank you! Seems cells in parallel are safer than single cells. Obviously a bms is supposed to cut off high cells. I was using a daly and well it didn’t cut it off and had a couple cells climb to 3.8-4.0v

I have 4 seplos bms to use for a 16s 4p bank but im considering 4p16s seems the cells would have less chance of that happening again. Im probably just paranoid now with daly experience.

 

[sunshine_eggo]

Thank you! Seems cells in parallel are safer than single cells.

Not necessarily. A single bad cell that shorts will have all the capacity of all the parallel cells dumping into it.

A single cell in series with others simply goes to zero.

Obviously a bms is supposed to cut off high cells.

and low cells.

I was using a daly and well it didn’t cut it off and had a couple cells climb to 3.8-4.0v

Either it was defective, and in my opinion DALY failures seem really common, OR you bought (or were shipped) the wrong BMS. Many BMS are designed for 3.7V chemistry with cutoffs at 4.20V.

Brief and infrequent jaunts to 3.8-4.0V shouldn't cause measurable damage. Some of the server rack batteries don't engage protection until 3.9V.

I have 4 seplos bms to use for a 16s 4p bank but im considering 4p16s seems the cells would have less chance of that happening again. Im probably just paranoid now with daly experience.

To my knowledge all BMS prohibit placing BMS in parallel with other BMS.

The likely safer prospect is 4X separate 16S batteries each with their own BMS.

 

[JonathanW]

Not necessarily. A single bad cell that shorts will have all the capacity of all the parallel cells dumping into it.

A single cell in series with others simply goes to zero.



and low cells.



Either it was defective, and in my opinion DALY failures seem really common, OR you bought (or were shipped) the wrong BMS. Many BMS are designed for 3.7V chemistry with cutoffs at 4.20V.

Brief and infrequent jaunts to 3.8-4.0V shouldn't cause measurable damage. Some of the server rack batteries don't engage protection until 3.9V.



To my knowledge all BMS prohibit placing BMS in parallel with other BMS.

The likely safer prospect is 4X separate 16S batteries each with their own BMS

Good to hear about the cells. Thank you for the input.