RE: working age of Ni-Mh
from what I have seen, they don't get warm until over charged at a fairly high charge rate. The extra current has to go some where and heat is one way for the battery to vent extra current.
I would guess that a really fast charge could cause the batter to get warm, not real sure about that.
I know that when I raced cars we charge fast and hard and we would take them off charge when warm.