I have a NIMH battery pack that is a 4.8v 1600mah. I want to replace the cells but can't find any 400mah replacement cells. Can I safely upgrade the cells to say 900mah per cell for a total of 3600mah without harming anything. The volts will be the same, just not sure how high I can go. I'm assuming all I'll have to deal with is longer charging times and that the mah difference wont matter on anything else. Am i right?
I'd just recommend buying a new pack.
You should replace all of the cells or just buy a new battery pack. The older cells will probably not last as long as the new cells. When their voltage drops the higher voltage that the newer cells will still be producing will mean a reverse voltage across the old cells. This will cause problems in the battery and probably in the device they're powering.
As long as they fit, you can safely use new identical NiMH cells of higher capacity when rebuilding your pack. Don't use cells with a different chemistry, or mix new and old, or your charger may not be able to charge them correctly. You have correctly assumed that your charging and runtimes will be correspondingly longer. Chances are your original cells had 1600 mAH capacity each. 4 of them in series would make a 4.8 volt pack. The mAH stays the same in that case. Don
