Bad cell in parallel group - What will happen?

Degrom

1 µW
Joined
Jul 18, 2018
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3
I am just wonder what would happen with a group of cells that are connected in parallel that has a single bad cell.

Usually a BMS are hooked up to groups of cells and not to individual cells.

Would the hole group go down and struggle to charge?

Or is it hard to detect faulty cells in a parallel setup?
 
Depends what happens to the cell.

An internal short will cause the cell's voltage to drop through self-discharge. Depending on the impedance of the short, the cell's voltage will drop quickly or slowly. In the case of a high impedance short, current will flow from the adjacent cells in parallel into the dud cell, lowering their overall voltage. This will continue until all of the paralleled cells hit zero volts.

If the impedance of the short is low, the cell will get hot itself, while the adjacent cells (say 5 in parallel) will deliver 5 x more current into that one cell. Internal fusing will cut the dud cell off in this instance.
 
So by comparing the voltage of the cell groups, it will indicate that one of the groups is having issues.

From there I can pinpoint the individual cell...
 
Usually you will have balancing problems. If you take the pack apart you might find something obvious like a loose solder connection. But usually when I start testing the suspect cells, I always find a cell that self discharges too fast. Just one cell like that will cause balancing problems.

If you charge the cells to 4.20 and 1 hour later , one of the cells reads 4.10, thats usually your problem.
 
You will lose one row of your series cells because that cell makes battery to loose some capacity and bms will cut the voltage
 
I take it the next question then is how does the effected group of cells effect the other groups that are linked in series?

Does the overall voltage of the pack drop...
 
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