For charging a battery, a dc supply is required. D.C. supply can be obtained from:
(1) D.C. motor generator set
(2) Rectifier
(3) Battery charger
The various methods of charging the battery are :
(1) Constant current method
(2) Constant voltage method
(3) Modified constant voltage method
(1) Constant Current Method
Here the charging current is kept constant throughout the charging period of the battery. As the voltage of the battery increases gradually during charging, a variable resistance R is used in series.
A number of batteries can be connected in series for charging, with the same charging current.
This method has the following disadvantages:
* It takes a longer time as compared to other methods of charging.
* The battery charger must have a high voltage rating if a number of batteries are to be charged simultaneously.
* Circuit is to be interrupted while removing or connecting any battery with the charging circuit.
(2) Constant Voltage Method
In this method, the charging voltage is kept constant during the charging period.
The charging current is more initially but decreases as the voltage of the battery increases.
A number of batteries with the same voltage rating can be charged by connecting them in parallel. Due to the parallel connection of batteries, the circuit need not be interrupted at any time. This method is faster compared to the constant current method.
(3) Modified Constant Voltage Method
The charging voltage is kept constant as in the earlier method. A suitable resistance is connected in series with the battery to safeguard against the high initial charging current.
When the charging current reduces sufficiently, the resistance is removed from the circuit.
This method is also faster as compared to the constant current charging. If a number of batteries are to be charged simultaneously, they are connected in parallel, so that the circuit need not be interrupted at any time.
Types of Charging:
There are four types of charging:
(1) Initial charging
(2) Normal charging
(3) Float charging
(4) Trickle charging
(1) Initial Charging
A new battery is without electrolytes, the charging of such a battery is called initial charging. First, the electrolyte is prepared appropriately and poured into the battery container.
After a few hours, the battery is connected to the charging circuit. Any one of the charging methods described above can be used.
The voltage and specific gravity of the battery are measured after every two hours. When there is no change in the battery voltage and specific gravity for two consecutive readings, the battery is said to be fully charged.
The specific gravity of the electrolyte is normally kept at about 1.26.
(2) Normal Charging
When after normal use a battery is recharged, it is called normal or routine charging. Any charging method can be used for this. If there has been a loss of water, distilled water should be added.
(3) Float Charging
In some applications eg. batteries used in railway compartments, the battery is charged and discharged simultaneously.
This is called float charging. In a railway compartment, the batteries are connected with the line as well as with the lights and fans.
When the train is moving the batteries are lightly loaded or not loaded at all. But when the train is stationary, the batteries supply the entire load.
Float charging is preferred for old batteries.
(4) Trickle Charging
To keep the battery in the fully charged condition, it is kept on trickle charge. Here, the battery is connected across the bus bars of required potential difference when not in use.
The rate of charging is very low, which is just sufficient to compensate for the losses due to open circuit or leakage action. The charging current may be as low as 0.2% of the normal charging current.
However, this will keep the battery in fully charged condition at all times. If the battery is kept idle, it will slowly deteriorate and not give satisfactory service.
To prevent deterioration and achieve satisfactory performance, the battery must be kept in fully charged condition even when not in use.