Understanding how this system typically works, and performing a few simple tests, may help to ensure trouble-free RV RVing
By Brett Wolfe, F252125
Motorhome owners sometimes express confusion about batteries and what charges them. All RV manufacturers follow some conventions, but enough differences exist that one cannot categorically say all RVs are alike in this regard.
All motorhomes have two battery banks, which generally are referred to as house and chassis batteries. House batteries are — or should be — the deep-cycle type, and they supply the 12-volt-DC needs of the “home” portion of the motorhome, powering items such as the lighting, water pump, fans, furnace, refrigerator printed circuit (PC) board, water heater PC board, etc. The house bank generally consists of multiple batteries — either 12-volt-DC batteries wired in parallel or 6-volt-DC batteries wired in series (or pairs of batteries in series wired in parallel). Think of two 6-volt-DC batteries wired in series as a 12-volt-DC battery in two separate cases, since all house functions require 12-volt-DC power. Typically, the house battery starts the generator, but some generators use the chassis battery or have a separate battery for that purpose.
The chassis battery starts the engine. It is a starting battery, not a deep-cycle battery.
So, how do these batteries get recharged? On most motorhomes they can be recharged in two ways: from the alternator when the engine is running and from the converter, charger, or inverter/charger when 120-volt-AC is available from shore power or the generator.
With the engine running, most motorhomes charge both battery banks. This ordinarily is done through a battery isolator, so that the two battery banks are not left connected when the engine is off, which would run down both banks. The isolator can be diode-based, a simple solenoid (a large relay), or a smart relay. All do virtually the same thing, allowing both battery banks to be charged from the alternator, but separating them when the engine is not running.
You should periodically verify that the wiring and isolation device are working and that both battery banks are being charged by the alternator. Start by disconnecting from shore power/generator for several hours. At the batteries, check the voltage. If you have 6-volt-DC batteries, check from the positive terminal of the cable going to house to the negative cable going to house, not the positive/negative cable that just goes from one battery to the other. Do the same for the chassis battery bank. Voltages should be between 12.2 volts DC (50 percent discharged battery) and 12.7 volts DC (fully charged battery). Anything above 12.7 is surface charge and will quickly go away with even light loads. Note the voltages.
Now, start the engine. If your motorhome is equipped, go to high idle (same as you do to warm up the engine after 30 seconds at low idle). Go back and read the voltage measurements at the batteries. The reading for both banks should be higher than the prior one. If the batteries are not deeply discharged, your reading should be between 13.5 volts DC and 14.5 volts DC. If this is not the case, you will need to troubleshoot your charging/isolation system.
When charging from a 120-volt-AC source — shore power or the generator — the converter, charger, or inverter/charger should charge the house bank and, on many motorhomes, the chassis batteries as well. To verify, disconnect the motorhome from shore power and the generator for several hours. Read the voltage at the house and chassis batteries. Plug in to shore power or turn on the generator. Read the voltage again. The voltage at any battery receiving a charge should go from less than 13 volts DC to a reading of 13.2 volts DC to 14.2 volts DC. If the voltage does not increase after plugging in to shore power or operating the generator, that bank is not being charged. Some motorhomes are not wired to charge the chassis battery from shore power — a serious problem with all the parasitic loads being placed on today’s chassis battery. In my opinion, this should be corrected. A Xantrex Echo charger, a separate small smart-charger for the chassis bank, a solar panel, or even wire from the house positive to the chassis positive all work to keep the chassis battery charged. If using a wire from positive to positive, both ends must have a fuse, since touching any metal or ground terminal with either end would cause a dead short. Do not use a wire if one bank is deeply discharged and the other fully charged, as small wires are not designed to handle the rapid transfer of large amounts of current — this is for maintaining a charge only.
Your motorhome also may have a battery boost/combine switch. This allows the two banks to be combined, with the house battery assisting the chassis battery in starting the engine, or the chassis battery assisting the house battery when starting the generator. To verify whether this feature is functioning, disconnect from shore power/generator for several hours. Turn on some 12-volt-DC loads to slightly discharge the house battery. Read the voltage at each battery — all you are really looking for is that they are different. Now, with someone activating the boost switch, verify that both batteries are at exactly the same voltage. If not, you need to troubleshoot and determine whether the switch/wiring or the solenoid is faulty.
If the battery voltages are not the same with the boost switch activated, locate the combine solenoid. It should be near the batteries and have a large cable going to each battery bank. Recheck the voltage from each large lug to ground. With the solenoid working, they should be exactly the same. It also will have either one or two small wires to it. If there are two wires, one will be the signal wire and the other a ground. If there’s only one small wire, it will be the signal wire from the dash switch, and the solenoid grounds directly from metal-to-metal contact. With your voltmeter or test light, check for 12-volt-DC power/light at the small signal wire lug to ground. If present, but the voltage at the two large lugs is not the same, the batteries are not combined — the solenoid is defective. If you did not get 12-volt-DC power/light from the signal terminal with the boost switch activated, you need to check the dash switch and wiring. You will still need to verify that the solenoid is working. With a small wire (it will be carrying less than one amp), go from either large lug to the small signal wire lug (not the ground lug). If the solenoid is functioning, you should hear a loud “click” and the two large lugs should read the same voltage.
Taking time to learn a bit more about your motorhome’s charging system and ensuring that it is functioning properly could prevent you from ending up left in the dark or, worse, stranded and not able to start the engine when the time comes to leave for your next destination.