Batteries supply a direct current (DC) output, and you may wish to verify this output. Vehicle batteries can provide over 100 amperes of current at 12 volts, which is sufficient to power the starter motor.
Often batteries will be of suspect quality, and you may desire to test their operation. Large-capacity batteries cannot be tested by simply hooking a voltmeter to them, since the voltmeter will not draw enough current to assure correct operation. Even a "dead" battery may still show 12 volts on a voltmeter. A battery load tester will give correct results.
Put on safety glasses and gloves. Automotive batteries contain sulphuric acid which is dangerous to eyes and skin.
Turn off the vehicle. If there are multiple batteries in the vehicle, disconnect each battery and test separately.
Connect the black lead of the battery load tester to the negative terminal of the battery.
Connect the red lead of the battery load tester to the positive terminal of the battery.
Press the load-test button on the meter and hold it for ten seconds. If the meter shows 12 volts or greater, or a pass indicator, the battery is fine.
Load testers will differ in their exact operating instructions. Read the owner's manual prior to testing. Oxidised or corroded battery terminals will not make good connection with the tester. Wiggle the tester clamps on the battery terminals to make a good electrical connection. If the battery load tester has a programmable load, set it for 50% of the cold cranking amp (CCA) rating of the battery.
Because explosive gasses may be present, always work with batteries in a well ventilated area.