During charging, the chemical action takes place which is exactly opposite to that of discharging. Thus current in opposite direction at the time of discharge, is passed through the battery. For this the voltage applied is in excess of the voltage of the battery or cell. The battery voltage acts in opposite direction to that of the applied voltage and hence called back e.m.f.
Indications of Fully Charged Battery
The various indications of the fully charged cells are,
1. Specific gravity: The specific gravity of the fully charged cell increases up to 1.28 from about 1.18.
2. Gassing: When the cell is fully charged, it starts liberating the gas freely. In lead acid battery the hydrogen is liberated at cathode while oxygen at the anode. Gassing is a good usb extension cable indication of fully charged battery. Some acid particles may go out with the gases hence the charging room must be kept well ventilated.
3. Voltage: The voltage of the fully charged cell is about 2.7 V.
4. Colour: The colour of the plates changes for fully charged cell. Colour of the positive plate changes to dark chocolate brown while that of negative plate changes to grey colour. But as plates are immersed in the electrolyte, this indication is not clearly visible.
usb extension cable is used and later on lower charging rate is preferred.
Constant Voltage Method
In this method, the constant voltage is applied across usb extension cable the cells, connecting them in parallel. The charging current varies according to the state of the charge of each battery. The batteries to be charged are connected in 6 or 12 volt units across the positive and negative bus bars i.e. mains supply. When the battery is first connected, a high charging current flows but as the terminal voltage of the battery increases, the charging current reduces automatically. At the end of the full charge, the voltage of the battery is equal to the voltage of the bus bars and no current flows. The charging time required is much less in this method.
When battery is required to be charged from a.c. supply, the rectifier method is used. The rectifier converts a.c. supply to a.c. Generally bridge rectifier is used for this purpose. The step down transformer lowers the a.c. supply voltage as per the requirement. The bridge rectifier converts this low a.c. voltage to D.C. this is used to charge the battery.