Diagnosing the problem
You turn the key and hear the dreaded “click.” Sure, you may need a new battery, starter or alternator. But before you condemn either the battery or the charging system, follow these steps.
Turn on your dome light and turn the key again. If the dome light dims, focus your attention on the battery and its connections. If the dome light doesn't dim, the starter motor isn't drawing power. Have the starter checked by a professional.
Check the battery voltage. Scratch clean contact areas on the battery posts and test the battery voltage with a test meter. A fully charged battery should read 12.7 volts. At 50 percent charge, the voltage drops to 12.1 volts. A low reading can be the result of poor electrical connections, a problem with the charging system, or a battery that is at the end of its life.
You can't always see corroded battery and ground connections, so clean all of them (Photos 1 - 3). Start with the battery terminals. Note: Disconnect the negative cable first and reconnect it last. Then clean the connection between the battery negative cable and the engine.
Finally, clean the connection between the battery negative cable and the body. If the car starts, the problem is solved. If it doesn't, take the car to the shop and explain what tests you've done. Then have the battery and the starting and charging systems checked out.