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Tuesday, December 8, 2009

How to find drain on RV batteries

How can I test if I have a drain on my batteries that I am not aware of once I disconnect my land-line power and switch over to DC power?
Will, (Jacksonville, FL)

Will, there are a couple of worthwhile methods owners can choose for testing the battery system for a drain. Begin by making sure the shoreline cord is unplugged and that all 12-volt components are turned off. The easiest way to check for a battery drain is to use a clamp-on DC ammeter. This device clamps around one of the battery cables and reads directly in amps or milliamps depending on the size of the current drain. The nice thing about the DC ammeter is that no wiring needs to be disconnected.

Another method invokes the use of a volt, Ohm, multimeter (VOM). Most RVers will carry a VOM for measuring voltage, but many quality VOMs will have an “amp” scale for measuring DC current. Begin by setting the VOM to the 10-amp scale and then remove the negative cable from the battery. The result is an open DC circuit. Connect the red test lead from the VOM to the cable you just disconnected and the black test lead to the negative terminal on the battery itself. Now you have a complete circuit with the meter inserted in series with the negative cable. If a battery drain exists you will see a draw on the battery measured in amps or milliamps.There are a few parasitic drains on both battery systems that are considered normal if they fall within the acceptable standard of around 100mA or less (about one-tenth of one amp).

Next, make certain all 12-volt appliances, lamps, etc., are indeed turned off; don’t forget those lamps inside storage bays or other out-of-the-way locations! If the drain persists, go to the DC fuse panel and remove each fuse, one fuse at a time. If the drain disappears when a particular fuse is removed, the meter will indicate zero. When that happens, you’ll know that fuse is protecting the circuit which contains the component draining the battery. If your circuits are labeled somewhat nebulously, you may need to launch a more finite search. Here’s an example: Say the VOM indicated a fuse labeled, “Right Side,” indicating that circuit is situated on the right side of the RV. Now you have at least eliminated the left side and narrowed the search to just those 12-volt items on the right side of the coach. One by one, seek out every 12-volt item. Work from one end to the other in a systematic way.

In one troubleshooting case I finally found that the booster for the TV antenna was inadvertently left in the “on” position creating a small draw that eventually drained the battery. Turning it off, the measured drain on the meter fell to within the acceptable standard. It’s a matter of eliminating those “good” circuits in order to find the “faulty” circuit and component.


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If you are in doubt or do not feel comfortable about a procedure, do not continue. Simply call your local RV service facility and make an appointment with them. The advice, recommendations and procedures offered by the RV Doctor are solely those of Gary. They do not necessarily reflect the opinions, procedures and recommendations of our sponsors or advertisers.