The equipment for charging batteries, instructions for building and wiring charging benches have already been given. What we shall now discuss is the actual charging. The charge a battery receives on the charging bench is called a "bench charge."
Battery charging in the service station may be divided into two general classes:
1. Charging batteries which have run down, but which are otherwise in good condition, and which do not require repairs.
2. Charging batteries during or after the repair process.
The second class of charging is really a part of the repair process and will-be described in the chapter on "Rebuilding the Battery." Charging a battery always consists of sending a direct current through it, the current entering the battery at the positive terminal and leaving it at the negative terminal, the charging current, of course, passing through the battery in the opposite direction to the current which the battery produces when discharging. When a battery discharges chemical changes take place by means of which electrical energy is produced. When a battery is on charge, the charging current causes chemical changes which are the reverse of those which take place on discharge and which put the active materials and electrolyte in such a condition that the battery serves as a source of electricity when replaced in the car.
Batteries are charged not only in a repair shop but also in garages which board automobiles, and in car dealers' shops. No matter where a battery is charged, however, the same steps must be taken and the same precautions observed.
When a Bench Charge is Necessary:
(a) When a battery runs down on account of the generator on the car not having a sufficient output, or on account of considerable night driving being done, or on account of frequent use of the starting motor, or on account of neglect on the part of the car owner.
(b) Batteries used on cars or trucks without a generator, or batteries used for Radio work should, of course, be given a bench charge at regular intervals.
(c) When the specific gravity readings of all cells are below 1.200, and these readings are within 50 points of each other.