Have just installed a 140 watt panel on the roof of my van and between the panel and the controller I have put in a switch on the live side to turn the panel off when the engine is running and the alternator is charging the leisure batteries. My idea is that if the batteries are low and it is bright light the van alternator would "sense" the 13.8 volt the panel puts out and cut back as it thinks the batteries are fully charged leading to a slow charge. Am I wrong here or is there a better charger/ controller that can control both inputs without cutting back the biggest one from the engine .