Anyone can learn how to use a modern 12-volt battery charger in a matter of minutes. Most battery chargers are actually designed to charge both 6-volt and 12-volt batteries and the operator has to select the proper voltage setting before connecting the charger. Modern cars all have 12-volt electrical systems but there are still 6-volt lead acid batteries around that need charging.
There are four types of chargers: fast chargers; slow chargers; combination fast/slow chargers; and trickle chargers. The standard 6/12-volt chargers are combination fast/slow chargers.
Make sure the battery charger is turned off before plugging it in to a 110-volt outlet.
Set the voltage selector switch to 12 volts if it's a dual voltage charger.
Select the desired charge rate. Typical choices for 12-volt batteries are two amperes, 10 amperes and 40 amperes. The charge rate determines how long it will take to fully charge a battery with a given AH (Ampere-Hour) Rating or CCA (Cold Cranking Amperes) rating. For example, a battery with 315 to 550 CCA having a 50 per cent charge would take 20 to 35 hours at two amperes, four to seven hours at 10 amperes, and 45 minutes to two hours at 40 amperes.
Attach the red, positive (+) battery charger to the positive battery terminal. The positive battery terminal can be identified by the letters "POS," a plus sign (+) or by the positive battery post's significantly larger diameter, as compared with the negative post.
Attach the black, negative battery charger cable to a clean, unpainted spot on the car's frame. It may be attached directly to the negative battery post, but it's a good habit to use the car's frame as a ground when working with car batteries. It's dangerous to attach directly to the negative post when jump starting a car because sparking can cause a battery to explode.
Rotate the timer clockwise to the desired charge time. It's possible to destroy a battery by overcharging. That's why the maximum time on most battery charger timers is three hours. Most chargers do provide a "Hold" setting on the timer control which deactivates the timer motor and allows you to charge for periods exceeding three hours.
Watch the battery charger's ammeter. The amount of current accepted by the battery will decrease as the charge progresses. A full charge is indicated by a zero or near zero reading on the ammeter.
Turn the timer knob completely counterclockwise to the "Off" position.
Disconnect the charger cables. Disconnect the ground cable first, then the positive cable.
Use caution when working with batteries. Lead-Acid batteries contain an electrolyte solution of Sulfuric Acid and water. This solution can burn your skin, burn holes in your clothing, and even blind you if it get in your eyes. Wear safety goggles at all times when working with batteries. During normal operation, these batteries produce hydrogen, an inflammable, highly explosive gas. These batteries can explode so use caution when connecting and disconnecting them so as not to cause any sparks. Always be sure that the charger is turned off before connecting or disconnecting it.