Why trickle charge is done on board ship

Battery not used daily and kept idle for long-time will loose its charge So in order to keep it ready for use at any time. It is kept on trickle charge mode, the battery charging system is having two ways. Trickle charging and full charging. 

These type of batteries loses their charge over a period of time. So to keep them full charge there is a trickle charge. This trickle charge can't compensate for the losses of charge due to external load. It only can compensate for the losses of charge leakage. Trickle charge supply can be adjusted to provide continuous constant current. The current value is 50-100 mA per 100 ampere-hours of the battery capacity. 

When the battery has been discharged, then the trickle charge itself is not enough. to charge it completely. In that case, full charge is restored by switching it ON.