How can you tell if your car battery is dead?

This is easy - you can determine the functionality of the alternator in less than 5-minutes. With the engine off, using a volt meter switched to measure DC current, place the red lead against the positive battery terminal/post and the black lead against the negative battery terminal/post and test the voltage. A good battery should measure around 12-volts (a good battery has a range of slightly lower to slightly higher than 12-volts). If you get within this range you have a properly charged battery and can then move to the next step and test the alternator.

Turn the engine on and again place the red lead against the positive battery terminal/post and the black lead against the negative battery terminal/post and test the voltage. The alternator is designed to generate enough voltage to replace the 12-volts of power being used by the vehicle's electrical system and charge the battery at the same time. Therefore, a properly working alternator will push about 14-volts of electricity into the system (a properly functioning alternator will generate a range of slightly lower to slightly higher than 14-volts).

If the system is generating around 14-volts with the engine running, then you can reasonably conclude that the alternator is working properly.

If you you are having battery charging problems (will not hold a charge) and the system is pushing around 14-volts with the engine running, you can then reasonably conclude that the battery needs replacing. If the system is producing less than the normal range of about 14-volts, you may need to replace the alternator. Keep in mind that a good battery may go bad if it is repeatedly used in a system in which the alternator is not working properly. If the systems tests within the normal range of 14-volts while the engine is running, replace only the alternator first and see if the battery then holds a charge. If it doesn't, then you will need to replace the battery also.