* Alternator failure: This is the most frequent cause. The alternator is responsible for recharging the battery while the engine is running. If it fails, the battery will continue to power the car's electrical system, eventually draining completely. Symptoms often include a warning light on the dashboard (often a battery symbol).
* Alternator belt broken or slipping: The alternator is driven by a belt. If this belt breaks or slips significantly, the alternator won't spin and therefore won't charge the battery. You might hear squealing noises from the engine bay.
* Faulty voltage regulator: The voltage regulator controls the output of the alternator. A malfunctioning regulator can either overcharge (damaging the battery) or undercharge (draining the battery).
* Faulty wiring: Problems with the wiring between the alternator, battery, and other electrical components can prevent the battery from being charged. This could involve corroded terminals, loose connections, or damaged wiring.
* High parasitic draw: Even with the engine off, some electrical components draw a small amount of current (parasitic draw). An excessively high parasitic draw can drain the battery, even while the engine is running, if the alternator can't keep up. This could be caused by a faulty component that remains powered even when the car is off, or a short circuit.
* Very weak battery: While less likely to cause a complete die while running, a severely degraded battery may not be able to accept a charge effectively from the alternator, especially under high electrical load. The battery may simply not have enough capacity to sustain the demand.
In summary, while a completely dead battery *could* theoretically prevent the engine from running at all, a battery dying *while* the engine is running nearly always indicates a problem with the charging system. Getting it checked by a mechanic is crucial to avoid further damage and ensure safe operation.
