Batteries are one of the energy sources available onboard vessels which are used in case of blackout and emergency situations on board a ship. These batteries are used for low voltage dc system like bridge navigational instruments and thus need to be kept charged to be used in case of any need of temporary power.
Also, arrangement should be available on board to charge the batteries again after the use. Moreover, the arrangement should be such that the batteries are able to be fully charged as they gradually lose charge over the time.
The batteries can be charged with the help of dc power supply; however presently there are no ships working on dc supply system and thus it is required to change the ac power into dc to charge the batteries.
A simple circuit used for battery charging is shown below:-
For converting ac into dc several components are required as shown in the circuit diagram above. First of all the ac is stepped down to the required voltage and then the AC is converted to DC with the help of rectifier system which changes sinusoidal wave of ac to dc system.
The only problem in the above circuit is that there is no arrangement provided for maintaining the charge, and the usage of same circuit will lead to overcharging and reduction of the battery life. In order to avoid this, a slight modification is done in the same circuit and an arrangement is provided to maintain the charges at the terminals. Also an arrangement to connect automatically to low voltage dc system is provided in case of a power failure.
In normal circumstances the battery is charged using the full charge circuit and once the battery is fully charged, the charges on the battery are maintained by the trickle charge circuit.
As it can be seen in the diagram, the batteries are in standby mode with the charging switches C closed and the load switches L open. The positions of these switches are held with the help of an electromagnetic coil against the spring tension. The electromagnetic coil gets its supply from the main power source available on the ship. As soon as there is a loss of main power, the electromagnetic coil loses its power and the batteries are connected to load switch L which gets disconnected from the charging switch C.
Once the power is available from the main system, the batteries are connected back to the charging circuit again manually. Also there is a test switch provided to test the system as a part of the routine checks.
Marine Electrical Equipment & Practice By McGeorge