How do you know whether you need a new battery or new alternator?

Answer by  kennethjgoodson (4903)

Find someone to hook up jumper cables and start your car. If it dies after you turn on the lights your alternator is bad. If it doesn't it's your battery.

Answer by  SSport (53)

You can test your battery with a simple volt meter. If the volt meter reads anything under 12.6 volts, it usually means it is bad and therefore not holding a charge. To test your alternator, you can usually bring it to any local automotive shop. They will usually do this for free.

Answer by  mike33 (61)

A digital multi meter will tell you the voltage across your battery. If it is below 12 volts, the battery is likely the culprit. If the battery has sufficient voltage, remove the alternator and take it to an auto parts store for testing.

Answer by  bug57 (1048)

First, replace the battery. Most auto parts stores can perform a charging system test free of charge. If you have a multimeter, however, then set it to Volts DC and put each probe on the corresponding battery terminal. You should see around 14.4 volts while the car is running, if the alternator is working fine.