Ammeter to Voltmeter...who does it?

toolmanmike said
You just answered your question as to why your alternator is constantly charging, The coach battery (or batteries) are shot. If your motorhome doesn't have a battery isolator you need to get one. I had chassis/coach batteries in 4 different tool trucks in 28 years of the tool business. @ trucks had isolators and 2 used a constant duty relay to charge the coach batteries. The isolator separates the 2 electrical systems and only charges the batteries that need the charge. Your engine start/chassis battery doesn't need much charge unless you are using the lights, heater, wipers, etc. If you use the camper for any length of time those batteries will need charged. With no isolator your system will charge one set and overcharge the other set. I had many battery and alternator failures because of overcharging in the trucks without a isolator.

Sorry but MOST of this is not true. A "battery isolator" is nothing more than a pair of diodes. They do not magically figure out which batteries "only need charged." IN FACT back in my parts days there were a couple of VERY DANGEROUS failures because of isolators. The diode supplying the main battery runs hotter because that battery sees more draw and use. So that diode tends to fail, and it sets up the following condition:

If the main diode fails, the main battery starts to drop and that is where the regulator sensing is connected. So the regulator ramps up charging voltage to a higher and higher level, trying to bring up the main battery. Since there is no longer a closed loop so to speak, that can get fairly high. Meanwhile, the poor secondary battery is over there bubbling furiously with overcharge

In my opinion you are WAY safer and better off with a constant duty solenoid. It ties the batteries together and tries to equalize their voltage, and can also be used as a "jumper" if you leave the lights on eg and run the main down. It can fail, but will leave the alternator properly connected to the main

There is no doubt that a bad battery such as one with a low/ dead cell can screw things up