Wednesday, February 8, 2023


Ask Dave: My engine battery is dead in 3 days. How do I find the parasitic drain?

Dave Solberg is a leading expert in the RV industry and author of the “RV Handbook” as well as the Managing Editor of the RV Repair Club. Today he discusses dead RV batteries and parasitic battery drain.

Dear Dave,
How about a hint on how to find a parasitic battery drain on the engine battery. I installed a knife switch to totally disconnect the battery from the RV. The engine battery will drain in about 3 days, low enough to not start the engine, if the RV is not being used. It’s a totally new AGM battery. —Tom

Dear Tom,
There are several things that can drain an engine battery, starting with the engine computer or ECU. That has all the settings for oxygen sensors, fuel injection, and much more. Those need to be recorded and kept so you don’t have to go through a reset process every time you start the engine. The computer monitors your driving patterns and records the settings.

What happens if you don’t charge the batteries when in storage

Every year my dad would store his Class A without plugging in or charging the batteries. Then every spring we would have to jump start the unit and go through a series of different rpm and speed resets. The last unit he had was on a Workhorse Chassis with a Vortec engine. We would jump it to start and let it idle for 15 minutes as it would not go any higher rpm pushing the pedal, but would instead stall. After about 15 minutes, the rpm would rise slightly and that was the time we had to drive it at a certain rpm or speed for about another 15 miles, then one more step until it would run properly. All this was recalibrating the settings for all the engine components.

Another drain is the dash radio, which needs power to keep those important preset stations from getting lost. Ever notice how you have to reset those stations any time the battery went dead or you had to replace the battery? Any function that has a preset such as seat position, GPS favorites, and such needs power to maintain the preset.

Use a multimeter

You can help identify the severity of the drain and even find some using a multimeter. Set the meter on the amp setting and remove the negative cable. Place the positive probe on the negative cable/post on the battery and the negative probe to the unhooked negative cable. If there is a drain, it will flow through and register on the meter. The photo here shows this procedure on a house battery. However, it’s the same for an engine battery.

You can pull automotive fuses from the engine fuse block on items you might suspect are draining power and see if the reading goes down. Installing a knife switch does nothing more than removing the negative cable. You will likely notice some of the functions not working well in the spring.

Identify the drain and get a solar charger to maintain battery

My suggestion is to identify the drain you have and see if there is something that could be disconnected without disconnecting the battery or losing important information. Then get a solar charger that provides a maintenance charge such as this one:


This can be mounted just about anywhere and will connect to either the battery or a 12-volt charge port—formerly known as a cigarette lighter hole. Some RVs still have them.

My next question is… How are you maintaining your house batteries during storage? I would suggest doing the same parasitic drain verification for these, as well. Then take steps to maintain them, as low voltage means they can sulfate and even freeze. That’s a question that has been covered several times in previous posts.

Read more from Dave here


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Bob Weinfurt
1 year ago

So you don’t have to worry about the amperage, instead of a meter, hook a 12 volt (incandescent bulb) testlight inline with the negative cable to check for current draw. Normal parasitic draw won’t light it but something left on or excessive parasitic draw will. Removing the culprit will dim or extinguish the light.

1 year ago

Some inexpensive meters cannot handle more than a few amps when using them as described and should blow the fuse in the meter if you go over the meters amp rating. My Fluxe 87 can be used up to 10 amps but the meter cost close to $600.00 and even with that rating I use a Fluke AC/DC clamp on adapter. If you are not sure your meter can handle the current draw then purchase a cheap DC clamp on amp meter.

1 year ago

Never check or work around batteries with rings or watches (as noted in this photo), or jewelry of any kind! If in doubt, Check with Mike Sokol for photo’s of the possible results!

RV Staff
1 year ago
Reply to  DW/ND

Thanks for the reminder, DW/ND. Here’s a link to Mike Sokol’s article (with graphic content) that you’re referring to: Have a great day. 🙂 –Diane

1 year ago

The original question included the fact that he “installed a battery disconnect that completely disconnects the battery from the motorhome” so it is unclear whether the battery is still discharging with the battery disconnect open or he wants to be able to leave the battery disconnect closed without having the battery discharge.

Bob p
1 year ago

The first thing I would check for is an interior light not turning off, I once had a glove compartment light that wasn’t turning off and it would drain the battery in 3 days.

1 year ago

When in storage, disconnect the negative cable to all batteries and they won’t drain as much as when cables are connected. I’ve been doing that with a car battery that sits for 6 months not being used. There has been always enough juice to start the car once the cable has been reattached. Never had to jump the car battery.

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