Lesson 2/Learning Event 2
Whenever the gravity of a battery drops below 1.225 (1.135 in tropical climates), it is best to
recharge it with a battery charger under controlled conditions. The two principal ways of charging
are constant current and constant potential (voltage). The method you use will depend on the
equipment and facilities available.
Constant-current charging is usually done only at installations where a regular battery shop is set up.
Electrical power is furnished by the 110-volt current supplied by the high lines. Since the 110-volt
supply is usually AC, which will not charge batteries, it must be changed to DC. This is usually
done by rectifiers.
After the incoming 110-volt power has been changed to DC, it must not be connected directly to a
single 12-volt battery or the battery will be ruined. A resistance must be placed in the circuit to
limit the current. This can be done by connecting a number of lamps in parallel. A 110-volt, 100-
watt lamp consumes about 1 ampere of current, so four bulbs will permit 4 amperes to flow. A
variable resistor of the proper value can be used instead of lamps and adjusted to give the desired
Several batteries are generally connected in series when charging by the constant-current method, so
the power that is consumed by the current-limiting resistance is reduced. When the batteries are
connected, their combined voltage should never be higher than the charging voltage. Allow 2.5
volts per cell when you add up the battery voltage. All batteries in the same series should be in
about the same condition. Batteries being charged by the constant-current method must be
checked often, particularly in the final stages, to avoid overcharging.
Battery chargers supplied for use in the field are the constant-potential type. A charger usually
consists of a DC generator powered by a gasoline engine. They are made with 7- and 15-volt
outputs or with a 15- and 28-volt output.
When using the battery charger to charge one 12-volt battery, connect its 15-volt output or load
terminals to the battery terminals. Connections must be positive-to-positive and negative-to-
negative. If more than one battery is to be charged from the 15-volt output, they must be
connected in parallel. If you connect the batteries in series, their voltage will be higher than the
charging voltage and the charger could not force current through the batteries.