The following types of special charges are usually required for proper maintenance of secondary batteries:
1. Boosting Charge:
Though batteries are usually of sufficient capacity to work for the whole day without requiring any intermediate charge during the day, but in some cases such as in battery operated vehicles, however, the batteries may require an extra charge known as “boosting charge” in the middle of the day.
The current for this boosting charge is usually determined by an empirical rule according to which the boosting charge current in amperes is numerically equal to battery discharge in ampere-hours up to that time when boosting charge is to be given. For example for an 80 ampere-hour battery when discharged to the extent of 25 ampere-hours, the boosting charge current will be 25 amperes. Due to this charging current there is no gassing.
2. Equalising Charge:
The equalising charge is a low rate charging and is given to the cells to ensure that every cell is brought into a fully charged condition at regular intervals. This charge is given until the voltage and specific gravity of the electrolyte remains constant for three successive hourly readings. This keeps the cell healthy for a longer period of time.
The usual practice of giving equalising charge is as follows:
(i) For batteries subjected to “trickle charge”—not required.
(ii) For “floating” or idle batteries-—once a month.
(iii) For batteries charged less frequently—once a fortnight.
(iv) For batteries charged every day—once a week.
3. Trickle Charge:
The trickle charge is extremely a low rate charge and is applied to standby batteries for compensating the open-circuit losses. The charge current should be of such a magnitude that there is no gassing and when it is allowed to flow continuously through the fully charged batteries, specific gravity remains constant at the specific value.
The batteries subjected to ‘trickle charge’ can be maintained always in a fully charged condition for several years. The service life of the battery also increases.