The L12B is used as a power supply for liferaft and lifejacket lights. The cell consists of a moulded polythene case with four activation holes each sealed by moulded-in buttons. Inside, the case contains silver chloride and magnesium plates, which are connected internally to socket terminations at the top of the case.
A cord is threaded through each of the buttons so that they are removed in the correct sequence when the cord is pulled. The unit is especially designed so that when suspended in an upright position, its performance in fresh water is substantially the same as in salt water.