Replacing cordless drill batteries

If you were happy with the original (Ryobi seem to make decent and budget stuff) I'd first price up a replacement. It's often cheaper than re-celling. Of course if it was a poor battery to start with, decent cells will transform the performance of the drill. Found that out with a PPPro one.

No gotchas with using tagged cells - just make the joint as quickly as possible with a hot clean iron. The temp sensor is usually just glued or taped in position - put it in the same place on the replacement.

If you've got a meter you will probably find just one or two of the cells have gone s/c. Did a complete rebuild job on a hitachi battery pack and was fine for light drilling work but as soon as you started to apply more of a load the soldered battery connections melted! Remade the connections twice (I'm an electronic eng. so I know how to solder before anybody throws that in) then threw the battery pack in the bin!

- would have been a good repair if I could have got them welded somehow.

Err...

They are welded to prevent heat build up to the cell which may occur while soldering, and damage the cell.

A properly made overlapping solder joint will happily carry more current than the tags themselves. I've done hundreds without problems.

Indeed - there is no way those tags are meant to reach 160C + in normal operation. Something was definitely wrong there, to cause the soldered joints to melt.

I keep meaning to actually make a little mock-up of a cell (strong salt solution on the inside of a thin steel plate, with a thermocouple on it) to find out exactly how hot they get when using my preferred technique.

(basically clean very thouroughly, tin (0.5s) then attach a wire (0.5s)) The place where the wire is can be touched in under 2 seconds easily.

(I've posted a much more detailed explanation several times in the past)