Which one of the following is NOT part of the auto's charging system?

The ignition coil is a component that plays a crucial role in the vehicle's ignition system, which is responsible for igniting the fuel-air mixture in the engine's cylinders to start the vehicle. It converts the battery's low voltage into the high voltage needed to produce a spark at the spark plugs.

In contrast, the components that are part of the auto’s charging system include the battery, voltage regulator, and alternator. The battery stores energy, the alternator generates electrical power while the engine runs, and the voltage regulator ensures that the voltage levels in the system remain stable. Therefore, the ignition coil is not involved in the charging system's functions, making it the correct answer in this context.