Depends how they wire the things, it may get charged from the house battery but the system must have its own independant power source capable of supplying 15 mins of brakes ................ I have discovered that if the battery goes flat, just remove battery put it on a bench charger till recharged then put it back in its spot, and check that it is getting a charge from the tug when hooked up... If it goes flat the units charger isn't capable of bringing it up to speed, its only a trickle job to keep it topped up............. I have wired mine so it gets 12v from the van to stay charged up as van is constantly at full charge from solar.