When the battery sends electrons out into the car, it is actually discharging itself. The battery is getting rid of its own power by this process.

Luckily, the 12 volt battery in the car is rechargeable. It is recharged by the alternator.

The alternator contains a magnet that is capable of receiving mechanical force from the engine and converting it back into electrical force, creating a new flow of electrons. The alternator directs and forces these electrons back to the battery so that the battery can re-charge itself. In this respect the alternator is functioning as a generator, or more accurately re-generating the flow of electricity, which is called a current.

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The introduction of these electrons back into the battery reverses the chemical reaction that occurred when the battery generated the electricity in the first place. This brings it back to square one and the whole process begins again.

When a battery goes dead the continuous circuit of electrical flow from the battery, out to the car and back through the alternator, has broken down. It can break down within the battery, in the alternator, or due to an electrical malfunction somewhere in the car.

The effect in any of these cases is that the chemical reaction ceases to occur in the battery. Sometimes you will be able to get the battery recharged if it goes dead. However for various reasons the battery may break down and no longer be rechargeable and must be replaced. This can be checked using equipment that your mechanic has.

An additional function of the alternator is as a "regulator", literally regulating the flow of electricity. It ensures that a steady current is sent to the various electrical components, and that this current does not get too strong. Too much current will damage the electrical components. It also regulates the amount of current going back into the battery, as overcharging the battery with too much current will destroy it as surely as letting it discharge.