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

The ignition coil is indeed not part of the automobile's charging system. The charging system primarily consists of the alternator, battery, and voltage regulator.

The alternator is responsible for generating electrical power while the engine runs, recharging the battery and powering the vehicle's electrical systems. The battery stores electrical energy and supplies it when needed, particularly during engine start-up. The voltage regulator plays a crucial role in maintaining the correct voltage output from the alternator to prevent overcharging or undercharging the battery.

In contrast, the ignition coil's main function is to transform low voltage from the battery into high voltage to ignite the fuel-air mixture in the engine's cylinders. This function is essential to the ignition system, but it does not directly relate to the charging system's purpose of maintaining electrical power within the vehicle. Thus, identifying the ignition coil as not part of the charging system is accurate.