Since this thread has gotten to be much about battery technology I feel free to ask "why NiCd (as mentioned upthread re A350 etc) and not NiMH?"
Wikipedia says in part "A NiMH battery can have two to three times the capacity of an equivalent size NiCd, and their energy density approaches that of a lithium-ion cell.
The typical specific energy for ... larger NiMH cells is about 75 W·h/kg (270 kJ/kg). This is significantly better than the typical 40–60 W·h/kg for NiCd, and similar to the 100-160 W·h/kg for lithium-ion batteries. NiMH has a volumetric energy density of about 300 W·h/L (1080 MJ/m³), significantly better than NiCd at 50–150 W·h/L, and about the same as lithium-ion at 250-360 W·h/L.
Useful discharge capacity is a decreasing function of the discharge rate, but up to a rate of around 1×C (full discharge in one hour), it does not differ significantly from the nominal capacity.
The significant disadvantage of NiMH batteries is the high rate of self-discharge; NiMH batteries lose up to 20% of their charge on the first day and up to 4% per day of storage after that. In 2005, a low self-discharge (LSD) variant was developed. LSD NiMH batteries significantly lower self-discharge, but at the cost of lowering capacity by about 20%."