How to Test an Auto Battery
There is no single car problem that strands drivers as often as a dead battery. A battery often dies suddenly. It can be fine one day and ready for recycling the next. Drivers think very little about them while they're working, so when a battery reaches the end of its life, it's often a surprise. There are other components that can cause starting problems, however, that present like a dead battery, so it's always wise to test the unit first before replacing. There are a few unscientific tests to help you narrow the problem down, but the only way to test the actual performance of the battery is to use a voltmeter.
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Instructions
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Pry the plastic covers from the battery cells to check the water level. Maintenance-free batteries are sealed and never need water, but many batteries should be checked regularly for water levels, particularly in the summer. Use only distilled water if the cells are low and need to be filled. Be very careful not to overfill. This may solve battery performance problems without any further testing.
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Make sure jumper cables and a car with a good battery are available, then try turning the lights on in the car with the suspect battery. The battery can be charged and working, but still be bad if it is unable to hold that charge. Leave the lights on for five to 10 minutes, then try starting the car. If the starter turns over slowly, or doesn't turn over at all, your battery is likely failing.
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Test the voltage output of the battery with your voltmeter. A voltmeter is an always handy, inexpensive tool for any car owner's garage. Pull the protective positive battery post cover (red) away and attach the red terminal from the voltmeter. Attach the negative (black) terminal to the negative battery post of the battery. A proper voltage reading for a battery at rest should be between 12.5 and 12.8 volts.
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Reconnect the battery and start the car. Attach the voltmeter to the running car and verify that the voltage hasn't dipped below 9.5 to 10. If it hasn't, the battery is good and you may need to look at the other components if a problem persists.
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Tips & Warnings
Your local auto parts store likely has a load tester. This will put a strain on the battery to gauge its performance in real world conditions. Testing is often free.
Always wear gloves and protective eyewear when working with a car battery.
