I can't speak with any authority on the NiMH vs. NiCd substitution, except that I've replaced them on low-draw devices and used the same charger. After 20 years of doing this, I can't claim to have destroyed any NiMH, though I can certainly appreciate the self-limiting of overcharge on NiCd cells.
On some small tools taking NiCds, however, I've substituted Li-ion rechargeables (3.6v->3 NiCd) and used the low-cost charger PCBs that are usually sold as 3 or 4 in a snap-apart package. Those have worked well for me. I wonder if changing a couple of resistors on those could suit NiMH charging. FWIW, I've salvaged the Li-ion cells from discarded vaping e-cigs. Those generally are fairly high quality and run about 600 maH capacity at 3.6V. Why they're used in a non-rechargable application is anyone's guess. --Chuck
