Good Nickel Cadmium cells do cool down slightly when charging actually, then warm up when they are full. Because of this, some chargers detect the temperature change to tell when the cells are full. Though NiMH also warm up when full, they don't cool down as notably as NiCd during charge.

I don't think you could get away with running NiCd batteries in the N900 or with a Li-ion charger, however.