RE: NIMH Questions
If soldering you may have ruined that cell if you touched it for very long. I understand there are plastic dividers inside that can melt. Remember soldering irons are in the 400+ range of temps.
when I solder batteries I use a very hot iron, then touch the battery for a brief second to let the solder melt. If you hold a soldering iron on for very long the heat has time to build on the cell and conduct to the inside.
Make sure you scuff your battery posts clean with steel wool or fine sand paper before tinning them with solder so it will stick quickly. Otherwise you have to hold the soldering iron on and 'cook' the solder on with lots of heat.
If your charger can charge one cell, try charging just that cell.
If not, when charging, does that cell get warmer quicker than the rest? That could be an indicator that cell could be bad.