Your battery is what provides power to your car when the engine is not running. It is what allows you to start your car, and it’s also what lets you operate systems that run off of electricity, like the power windows, sunroof, or radio, when the car is not turned on. Once the engine is running, the alternator takes over for the battery in supplying power to these systems. Batteries recharge themselves through use, but may fail as a result of broken terminals, corrosion or low electrolyte levels. Eventually all batteries lose their ability to charge and need replacing.