When a rundown battery is brought into the shop, you should recharge it immediately. There are several methods for charging batteries; only direct current is used with each method. If only alternating current is available, a rectifier or motor generator must be used to convert to direct current. The two principal methods of charging are (1) constant current and (2) constant voltage (constant potential).

Constant current charging is be used on a single battery or a number of batteries in series. Constant voltage charging is used with batteries connected in parallel. (A parallel circuit has more than one path between the two source terminals; a series circuit is a one-path circuit). You should know both methods, although the latter is most often used.

CONSTANT CURRENT CHARGING.- With the constant current method, the battery is connected to a charging device that supplies a steady flow of current. The charging device has a rectifier (a gas-filled bulb or a series of chemical disks); thus, the alternating current is changed into direct current. A rheostat (resistor for regulating current) of some kind is usually built into the charger so that you can adjust the amount of current flow to the battery. Once the rheostat is set, the amount of current remains constant. The usual charging rate is 1 amp per positive cell. Thus a 21-plate battery (which has 10 positive plates per cell) should have a charging rate no greater than 10 amps. When using this method of charging a battery, you should check the battery frequently, particularly near the end of the charging period. When the battery is gassing freely and the specific gravity remains constant for 2 hours, you can assume that the battery will take no more charge.

The primary disadvantage of constant current charging is that THE CHARGING CURRENT REMAINS AT A STEADY VALUE UNLESS YOU CHANGE IT. A battery charged with too high current rate would overheat and damage the plates, making the battery useless. Do NOT allow the battery temperature to exceed 110 while charging.

CONSTANT VOLTAGE CHARGING.- Constant voltage charging, also known as constant potential charging, is usually done with a motor generator set. The motor drives a generator (similar to a generator on a vehicle); this generator produces current to charge the battery. The voltage in this type of system is usually held constant. With a constant voltage, the charging rate to a low battery will be high. But as the battery approaches full charge, the opposing voltage of the battery goes up so it more strongly opposes the charging current. This opposition to the charging current indicates that a smaller charge is needed. As the battery approaches full charge, the charging voltage decreases. This condition decreases the ability to maintain a charging current to the battery. As a result, the charging current tapers off to a very low value by the time the battery is fully charged. This principle of operation is the same as that of the voltage regulator on a vehicle.

CHARGING PRACTICES.- It is easy to connect the battery to the charger, turn the charging current on, and, after a normal charging period, turn the charging current off and remove the battery. Certain precautions however are necessary both BEFORE and DURING the charging period. These practices are as follows:

1. Clean and inspect the battery thoroughly before placing it on charge. Use a solution of baking soda and water for cleaning; and inspect for cracks or breaks in the container.

CAUTION

Do not permit the soda and water solution to enter the cells. To do so would neutralize the acid within the electrolyte.

2. Connect the battery to the charger. Be sure the battery terminals are connected properly; connect positive post to positive (+) terminal and the negative post to negative (-) terminal. The positive terminals of both battery and charger are marked; those unmarked are negative. The positive post of the battery is, in most cases, slightly larger than the negative post. Ensure all connections are tight.
3. See that the vent holes are clear and open. DO NOT REMOVE BATTERY CARS DURING CHARGING. This prevents acid from spraying onto the top of the battery and keeps dirt out of the cells.
4. Check the electrolyte level before charging begins and during charging. Add distilled water if the level of electrolyte is below the top of the plate.
5. Keep the charging room well ventilated. DO NOT SMOKE NEAR BATTERIES BEING CHARGED. Batteries on charge release hydrogen gas. A small spark may cause an explosion.
6. Take frequent hydrometer readings of each cell and record them. You can expect the specific gravity to rise during the charge. If it does not rise, remove the battery and dispose of it as per local hazardous material disposal instruction.
7. Keep close watch for excessive gassing, especially at the very beginning of the charge when using the constant voltage method. Reduce the charging current if excessive gassing occurs. Some gassing is normal and aids in remixing the electrolyte.
8. Do not remove a battery until it has been completely charged.