If one cell is bad, the others are probably not far behind.

I'ld just replace the pack. If you replace one cell it will be different from the others and not charge well.

I had JR radio with Sanyo batts. Had an RX cell fail. Replaced pack. 2 moths later the TX pack failed. Must have been a bad lot. Replaced both and they worked for 10 years. Still test at 100% of capacity.