Lesson 2/Learning Event 2
To charge 12-volt batteries from the 28-volt output of a charger, connect two batteries in series,
then connect them to the 28-volt load terminals. To charge more than two batteries, you must
connect them in series-parallel. The batteries in each series group should be in about the same
When a battery is first placed on a constant-potential charging system, its voltage is much lower
than the charging voltage, so a high current will flow. As the battery is charged its voltage
increases, but the charging voltage remains constant. This causes the current to decrease as the
battery is charged and to taper off to a very slow rate. Less attention is required near the end, but
care must be taken to prevent overheating the battery at first. The constant-potential battery
charger is generally equipped with a variable resistor so the operator has some control over the
Batteries should be charged only in the open air or a well-ventilated room because of the gases they
expel. Before attempting to charge a battery, give it a thorough cleaning and inspection. Do not
waste your time on batteries that are cracked or damaged in any way that will make them
unserviceable. Place good batteries on a board or wooden rack. Never stack them on top of each
other or on the ground or a concrete floor.
Check and record the specific gravity of each cell of all the batteries to be charged. Add water to
bring the electrolyte to the proper level. Arrange and connect the batteries for charging. All
batteries connected in one series group should have about the same specific gravity readings. Make
sure all the vent plugs are secure and the vents are not plugged, then begin the charge.
Watch closely for overheating or excessive gassing for the first few minutes of charge. If either
occurs, reduce the charging rate. For best results, the battery electrolyte should not exceed 110F
temperature. The battery should gas very little when it is first placed on charge, but steady gassing
later on when the battery nears full charge is normal.