Nearly every motorhome owner will at sometime experience a charging problem with their house battery. Note that even if you have two (or more) house batteries we will use the single term battery in this article. It is important to be able to do some basic troubleshooting yourself to fix the problem. Your will need a multimeter (sometimes called a VOM or Volt Ohm Meter). A basic multimeter from Harbor Freight can do what you need for $5.99. Or you can spend a lot more on a more versatile multimeter.
Batteries are vital to RVers. If you camp without electrical hookups it is important that you keep your batteries in good condition and that they get charged. The easiest and fastest way to charge your battery is to drive. Your alternator will charge both your engine battery and your house battery. Alternator power flows normally through an isolator or a separator. The job of these devices is to charge both house battery and the engine battery, yet to keep the batteries separate or isolated from each other so you will not deplete your engine battery if you run your house battery too low (and vise versa). Shown at the right is a battery separator. However this article will not discus how to test a separator although the process is similar. In addition to charging both batteries from the alternator, the battery separator allows both house and vehicle batteries to be charged when you are plugged into shore power. Unlike an isolator, a separator will draw some current while idle. [Roadtrek changed from battery isolators to battery separators in the 2006/2007/2008 time period. This article addresses isolators only.]
The isolator will be mounted under the hood, close to the alternator. Location will vary from an obvious spot on the firewall to practically hidden under the engine battery. If you have trouble finding the isolator follow the big electrical cable from the alternator. It will lead to the center binding post of the isolator. If you don't know what your alternator looks like, follow the big wire from the positive (+) terminal of your engine battery. It will lead to one of the terminals of the isolator. The isolator on a Chevy will have 4 posts (an additional smaller post), while the isolator on a Dodge will have 3 posts. The isolator (Dodge shown) in the photo has the middle wire going to the alternator, the top wire going to the engine battery, the bottom wire to the house battery.
Now check the voltage on the post going to the house battery. That is the
Of course you may find that the necessary voltage is present at the battery for charging, but your battery is not getting charged. In that case, have your battery tested free at an auto parts store. You may need a new battery. But if you should discover in the middle of a trip that your engine battery is not being charged because the isolator is bad, you can get out a wrench and move the engine battery wire to the same terminal as the alternator wire and have a temporary solution to the problem.
This test was to see if the isolator is allowing your batteries to be charged. Isolators can also fail by connecting the engine and the house battery. This allows your engine battery to be depleted by the use of your RV appliances. And unmatched voltages between the two batteries will result on the higher voltage battery trying to charge the lower voltage battery which can shorten the life on both batteries. By measuring the voltages on the 3 isolator terminals with the engine off, you can see if the voltage from either battery is showing up on the alternator terminal (which should be zero with the engine off).
Even if you will never test your own isolator, you need to know about it. Most mechanics work on cars not motorhomes, and make not expect an isolator in the system. More than one RV owner has had their “bad alternator” replaced when the problem was a bad isolator!