The diodes in an automobile’s alternator serve the same function as a one-way check valve does in a hydraulic system. Diodes allow electrical current to flow in one direction only, and are the essential part that converts alternating current (A/C) to direct current (D/C).
As the alternator is running, it produces current that passes through the diodes to the battery connection on the alternator. The current then travels to the battery through the positive battery cable. As long as there is a good connection to the battery, the battery will be maintained in a fully charged condition and the electrical system will function properly. A small amount of current also flows through the circuit for the alternator light on the instrument panel to provide and indication of the alternator’s health to the driver. Properly functioning diodes also protect all of the sensitive electronic equipment in the vehicle from reverse current flow.