The alternator in a car is responsible for recharging your battery while you drive, as well as powering other components of the car’s electrical system. If your car battery is flat but you haven’t left any lights or accessories on, there is a chance that your alternator is failing or has failed and is no longer charging your battery. The Car Doctor diagnose and repair many alternators each year and testing the alternator is quite a simple process. The cost to repair or replace your alternator varies from vehicle to vehicle, but it is a relatively simple task and can be completed quite promptly.