Optional for Single-Rectifiers
When the single rectifier is installed in a dual battery setup, the auxiliary battery is connected to the anode post while the main battery is connected to the cathode post. In some applications, the auxiliary battery may be connected in a towed vehicle like a trailer. If the trailer is uncoupled from the vehicle, the auxiliary battery is being removed from the anode post. Because the alternator output is common with the anode post, depending on the nature of the alternator’s regulation system, if there’s no battery present on the anode post, the alternator may fail to excite at start-up in which case no charging would occur.
The excite circuit is enabled when we attach an additional wire (purple) which [ideally] gets connected to the starter solenoid. When the purple wire gets triggered with 12v (or 24 volts on a 24 volt system), it enables the excite circuit to pass battery voltage/current from the cathode [main battery] to the anode [alternator] for a period of sixty seconds. This insures the alternator’s regulator has the requisite excitation current to start generating power. If the sixty second window is missed, simply turn the vehicle off, and then initiate the start sequence and the excite circuit will again stay energized for sixty seconds.
If the starter in not easily accessible, then ignition switched system voltage is the next alternative.