Power up!
You may not be aware of this, but your car's battery loses some of its importance when the engine is running. Sure, battery power is crucial to turn the starter motor and power the engine's control system, but once the engine is running, it basically becomes a passenger in the car – riding along until the next time you want to start the engine.
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This is because a running engine's electrical needs are satisfied by the power coming from the alternator – an engine-mounted device which is tasked with charging the battery and powering the car's electrical system. Simplifying the technicalities, it can be said that the alternator uses some of the engine's power to generate the electrons which keeps the whole car alive.
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But if the alternator doesn't work properly...
Alternator malfunctions can come in many forms, with the most common cause being a broken or slipping drive belt. If the drive belt isn't working the way it should, the alternator cannot deliver the electrical power needed to keep the car working. In this case, the electrical system will draw its power from the battery, and if this carries on long enough, the battery will “die” - it will become depleted of its charge. This problem can be remedied by the fitment of a new drive belt, and can be avoided altogether if the drive belt is regularly inspected and renewed before it fails.
An internal malfunction in the alternator is another possible scenario, where it would take power from the engine, but fail to convert it into electricity. This is usually due to a defective internal component such a voltage regulator or rectifier, or worn-out electrical contacts inside the alternator. In this case, the alternator will have to be professionally repaired – this job isn't always easy or simple, so it's not advisable to tackle it yourself.
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Warning signs
The little “battery” warning light in your car's instrument cluster is the tell-tale for most electrical malfunctions. If it illuminates while driving, it indicates that the electrical system isn't charging the battery, and that the engine will soon come to a stop. If you're capable of fixing your own car, check the condition of the drive belt and replace it if needed. But if the belt is intact, it points to a bigger problem, which may necessitate a visit to the auto electrician. Keeping the electrical system in good order will keep a healthy battery from dying while you're driving.
