I just bought some 1.2v 3700maH Vapex cells complete with tabs off of eBay then soldered them together. Had to cut open the pack's casing with a small modelling saw, drop the guts out, joined all the cells up, then epoxied it back together. This was on a Swiss made 12v Bosch and that thing has been copping a belting for 15 years now. Way nicer and better balanced drill than any of the new ones I've used, just not as quick as my 18v Li-Ion Hitachi.
Had to use "Baker's Soldering Fluid" to get the zinc plated tabs on the cells to accept solder though. They were very "solderphobic" prior to that stuff
The cells were $5 each and for $50 I had a better than new drill.