No matter what your car's battery looks like, all wet-cell batteries (those that use cells filled with acid — currently used on all vehicles except hybrids) work the same way. Car batteries produce electricity through a chemical reaction between the acid fluid and the lead plates inside the cells. They can be recharged by applying an appropriate voltage to the outside terminals, thereby reversing the chemical reaction inside the cells. For a detailed explanation, see our article on battery theory.
Car batteries are sized according to the current requirements (power needed to run all electric accessories) of the vehicle and the space allowed for its mounting. The larger the battery is, the greater the amount of lead there is. The greater the amount of lead and volume of acid, the greater is its capacity, or rating. A typical car's battery might have a rating of 600 cold-cranking amperes (CCA), for instance. The cold cranking ampere (CCA) rating refers to the number of amperes a battery can support for 30 seconds at a temperature of 0°F until the battery voltage drops to a level below which it won't turn the engine with the starter. Thus, a 12V battery that carries a rating of 600 CCA tells us that the battery will provide 600 amperes for 30 seconds at 0°F before the voltage falls to 7.20V or below.
Car batteries are recharged either by the car's alternator/voltage regulator circuitry or by means of an external charger. The time required to recharge a battery is a function of its CCA because the same amount of energy must be put back into the cells as was depleted. Therefore, the greater the amount of current (amps) a charger produces, the shorter the time it will take to fully charge the battery.
Inexpensive car battery chargers are usually "trickle-type," which plug into household receptacles and produce 2 amps of current. These must be left connected to the battery for long periods of time to fully recharge. They have the advantage, however, of not creating too much internal heat in the battery. At the opposite end of the charger range are the "fast chargers," devices that produce 100, 200 or more amperes to charge a dead battery in an hour or less. These can produce tremendous internal heat in the battery and should only be used by trained technicians.
Most household car battery chargers have an output range of 2, 6, 10 and, sometimes, 50 amps at 12 volts (6 volt chargers are getting hard to find, since by the mid 60s all cars changed to 12 volts). The user can select the output depending upon whether a slow, overnight, charge is desired or a relatively fast (50 amp) charge is needed to help start the car within a few hours. Intermediate settings allow the user to charge the battery in a safe manner over a number of hours and to provide a voltage source for testing various devices (lights, horns, radio, etc.) for experienced users.
Most chargers have a gauge on the front panel. The gauge shows the condition of the battery and the progress of the charging operation and the faces typically are color-coded in red and green. Some show the actual amperage being produced and many chargers shut themselves off once the battery reaches a charged condition or have a gauge reading of "charge complete" or "OK."
High quality car batterty chargers have built-in safety systems and indicators that give very useful information. A "fault" indicator says that the battery might be shorted or a cell is incapable of charging, or that something is not operating correctly in the charger itself. Some chargers have a "change polarity" indicator that tells the user that he/she has connected the positive and negative leads to the wrong battery terminals. This protects the charger and battery from damage.
These are the important things to consider when charging a car battery: