3 or 4 watt lamp (the porthole should be with isolated legs). It will work just like a vehicle. When you connect the batteries to charge them, voltage will reach one leg of the lamp, and by the regulator and the rotor it will make mass and it will turn on, when it begins to charge that mass will become positive and it will turn off the lamp, like a car.
What happens is an equipment that adapted an alternator to maintain the batteries ... It is not a vehicle
From the positive of the batteries take out a cable, and place a button so that the other end reaches the excitation of the alternator, when they run the engine that moves the alternator, press that button once and it is already excited. And if you want to make it more automatic, instead of the button it puts a warning light, an ox eye with a