They must never be left inside equipment when they are exhausted, as they are liable to leak very corrosive chemicals which can cause expensive damage to the equipment. On board, they are only likely to be found in flashlights. Alkaline manganese batteries These are the premium 'longer life' batteries such as 'Duracell'. They too have a nominal voltage of 1.5 volts per cell. If the battery is getting low, then by turning the equipment off for a while and letting the battery rest the useful life can be prolonged.
Continuous discharge reduces the capacity of the battery.
Mercury batteries Mercury cells have a nominal voltage of about 1.4 volts per cell. Because of the environmental problems associated with the disposal of mercury batteries, they are used less frequently now. Silver oxide batteries These are familiar as the little round silver coloured batteries found in watches and calculators. The nominal voltage is around 1.5 volts per cell. Apart from calculators and watches they can be found as back-up batteries for memory circuits in some equipment.