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Replace just 1-2 bad li-on cells? Answered

I've got a fairly new DeWalt 20V Lipo Battery pack that got a bit of water inside it's case, causing 1 of the 10 cells inside to go bad.

Pertinent info:
* Battery pack is configured as 5x2 (5 series stages of two parallel pairs each)
* Other cells are in near-new condition.
* OEM Factory battery exact matches are rare and quite overpriced.  (1500mah x 15A discharge rate @ $10-20 shipped.)
* Due to improvement in battery tech, I can get batteries that outperform OEM in every spec for 30-40% of what the OEM costs (2500mah x 20A discharge rate @ $4-6 shipped.)
* The battery pack is always charged on a cell-balancing charger.

Assumptions:  (Correct me if I'm wrong here, especially on the 2nd one)
* The partner cell to the dead one is now suspect, due to being undercharged (down to <.1V)  by it's dead partner.
* I should be able to just replace the bad cell and it's partner with the higher spec'd batteries and it shouldn't be a problem

Is this workable, or will having non-identical cells for 2 of the 10 cells cause problems?  (I'm totally fine with the pack only being as strong as the weakest link, and only getting OEM performance out of the replaced cells.)

I'd rather not have to replace all 10 cells, but I'd rather do that than replace 2, and -then- have to replace all 10 because the combination didn't work.

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Downunder35m

3 years ago

I did some "battery repairs" in the past with very mixed results.
For that reason I tend to say that the more power a battery pack can supply and the more the cells it has, the better it is to completely replace it.

Finding a suitable replacement is not easy, finding one that perfectly matches the existing and partially worn batteries next to impossible.
When I replaced one or two cells from old NiCd packs there was not much negative happening.
But for these NiMh one the stories is different.
Replaced one weak cell from a cordless drill and was lucky enough that the original values had been marked on the old cells.
All single cells, including the new one were charged and discharged multiple times in a dedicated charger (for single cells) to ensure they are ok.
But with normal use the capacity would soon go down again like it was before the fix.
Another checked turned up two more of the old cells going bad.
In the end I replaced 6 out of 10 cells until the rest kept working and performing normally.

IMHO the problem is within the different internal resistances of the cells, causing them to heat up differently.
So the "weakest" one will heat up most and of course experience a premature dead.
Especially after water damage I would be careful with just replacing one cell.
The electrolyte in the batteries is not sealed as good as in single cells that you use in your flashlight.
The water can enter the battery and cause all sortf of problems, but in time it always causes more corrosion within the battery pack.
Especially those cells just packed in paper or with vent holes are best replaced completely.