Don’t bother. 9v NiMh frankly are lousy batteries. Not LSD, very low capacity, high discharge……just awful. Really, I cannot emphasize that too much. Dead end. Don’t go there. If you have……run away.
Recycled all of mine when I discovered 9v (8.4v really, 2S) Li-On. WAY better. No comparison. Decent capacity, hold a charge for a long time, have decent output if required. Seem to have a pretty good life-span too, at least the ones I have. I even use them in smoke alarms, but I do have to stay on top of the charge, about every 6 months.