The voltage regulator supplies field current to the alternator and also controls the field current. It connects to the battery, and it has diodes that turn off the alternator’s field current when it senses that the battery is fully charged, to prevent overcharging. These are the diodes that can be damaged if the battery is switched in or out of the circuit, while the alternator is running, unless the battery switch is of the make-before- break type or has a separate set of contacts that also disconnect the field current. Connecting the battery with reverse polarity will also damage both these diodes and those inside the battery charger.
A high capacity marine alternator should be paired with an intelligent voltage regulator capable of implementing the 3-stage schedule of bulk, absorption, and float charging that deep cycle batteries require. It should have settings for each type of battery. It should sense and compensate for ambient temperature. It should sense and react to voltage measured at the battery, not at the alternator. many do, the voltage regulator should have a tach output to prevent the tach from zeroing once the batteries are fully charged. A delayed start function reduces engine wear by waiting until the engine is warmed up before starting to charge the batteries. An override switch that disables charging by disconnecting the field current can remove an engine load of 5 HP or more when full engine power is needed.
Somewhat less useful features of an intelligent voltage regulator can include current limiting to prevent the alternator from overheating, an equalization stage for occasional treatment of battery sulfation while connected to shore power or a genset, a bulk charge timer to prevent overcharging a damaged battery, diagnostic indicator lights, and the ability to modify the preprogrammed charging schedules.
WiringOne or sometimes two black wires from the voltage regulator connect to ground. A red wire connects to the positive (output) terminal of the alternator and should have a fuse of about 10 amps. A wire that is usually brown, and activates the voltage regulator, can connect to any circuit that is hot when the engine is turned on. A wire, usually blue, that supplies the regulator output attaches to the alternator’s field connection and energizes the field windings. This is the wire that can be routed through an override switch to deactivate the alternator when full engine power is needed. There might be other wires that connect to temperature sensors mounted on the alternator driving the engine tachometer.