Without such isolation, your house battery and SLI battery will seek to maintain the same charge level-and if that’s too low to start your rig, you’re left in the lurch. ![]() At the same time, the isolator allows your house batteries to charge up by drawing on your chassis or tow vehicle alternator. It used to be there were just two kinds of isolators now there are three. The simplest battery isolator is nothing more than an electromechanical relay. Somewhat comparable to a vehicle starter relay, the solenoid is a big electrically operated switch. ![]() When your vehicle is running, current throws the switch closed to allow current to flow from your vehicle charge system back to your RV. Turn the ignition system off, the relay switch opens, preventing current to flow from either the chassis to the RV, or vice versa. The solenoid isolator is relatively inexpensive-it’ll set you back less than $20. You simply need to find a source of power from a circuit that’s only hot when the ignition system is “on,” and run it to the relay. Another wire connects the relay to the positive post of the SLI battery, and another runs back to the RV. The solenoid doesn’t use any power in itself, other than a tiny amount when the ignition system is switched on. These isolators are relatively trouble free, but like any mechanical device are subject to occasional failure. Solid-state battery isolators utilize electrical diodes, which act like a one-way valve. Current can flow from the vehicle alternator back to the RV, but not from the SLI battery. This keeps the SLI battery charged up, and the house battery too, without either one of them “seeing” each other. This prevents either battery from drawing down the other. Solid-state isolators are not 100 percent efficient-they tend to add a little resistance to the circuit, reducing the effective amount of voltage to the batteries by over half a volt. This explanation works only IF the invertor charging system and BIG BOY/BIRD systems are working properly.That may not sound like a lot, but to quickly charge batteries, more voltage is important. ![]() Upon disconnection from all charging, the batteries should show 12.6v to 12.7v. As the batteries become charged, the invertor will switch to absorption charging showing a reduced voltage, and finally switch to float charging where the batteries should show 13.1v to 13.4v on the charge-status equipment. The inverter will go through three stages of charging - bulk, absorption and float - starting with bulk charging showing over 14v on the charge-status equipment. When stopped traveling and after plugging into shore power or with the generator running, the inverter will, after a time, charge and maintain the coach and chassis batteries to 13.1v to 13.4v. Fully charged and while stopped at a rest area, for example, the chassis batteries should show 12.6-12.7v without being plugged into shore power, without the generator running, and without the coach engine running. But you are correct, after a few hours of leaving MI, the chassis batteries should have been showing fully charged during the whole trip from MI to Laredo. I don't know the length of time elapsed between these two posts. In Tim's second post: "We just drove from Mi the Laredo so the chassie batteries should be fully charged." In Tim's first post: "The chassie batteries are 12.4 I'm hooked to a 50 amp service." I realize you were concerned about the noise, and you should be able to hear the big boy cycle, but the louder it becomes the more likely you have a good corrosion build up inside the Big Boy. The ground wire on the BIRD was being temperamental. Once i fixed that all operation was normal. I did have a case where the house batteries were not being charged while driving and found my BIRD to be malfunctioning. When ignition is on and engine running, then Chassis batteries have priority and if house batteries drop to a certain voltage the big boy will close charging both battery banks. When on shore power the House batteries have priority and when the Chasis batteries drop below a certain voltage the big boy will close and charge both banks. Also it is normal for the big boy to cycle between battery banks. Once I cleaned mine the noise from the Big Boy was much less. Thats assuming you have a corrosion build up. Disassembly and cleaning should bring it back to normal operation. Big Boy will make more noise as the corrosion on the inside becomes greater.
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