Sunday, January 29, 2023


Ask Dave: Will my engine battery provide a boost to run my slide room?

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 Battery Isolation Management systems.

Dear Dave,
If I run the engine of our Class C while operating the Schwintek slide, is it the same as a battery booster? We have a Forest River Sunseeker 3050S. Thanks. —Bob

Dear Bob,
To answer this correctly, we need to know if your Class C has a Battery Isolation Management (BIM) system, which is a solenoid that is between the engine charging system and the house battery(s). A fully charged house battery would register 12.6 volts. When you connect your shoreline power or start up an onboard generator, the converter will provide a 13.6-volt charge to low batteries and then taper off to a maintenance charge of 13.2 volts. This is the level of voltage that a fully charged house battery will not accept.

If you have a BIM, the engine alternator should provide a charge back to the house battery(s) and typically it will be somewhere in the 14-volt range. This should help “boost” the house battery(s) enough to operate the Schwintek slide motors. What typically happens is the house batteries are sulfated and even though they read 12.6 volts initially, when a load is applied, they drop dramatically and the slide either stops or doesn’t move.

How to determine your battery system

Here is how you can determine what your system has: Take a multimeter and check the standing voltage of your battery(s) without being plugged in or with the engine running. It should be 12.6 volts fully charged or less. Plug the unit in and the voltage should go up to 13.6 volts or higher. Unplug the unit and start the engine and check the voltage, it should go up to 14 volts or more. The solenoid is typically located in the same compartment as the house battery(s). This system typically has a switch located on the dash that provides a boost from the house battery(s) to the engine battery. If there is a switch like this on your dash but you are not getting a charge when the engine is running, it could be a fuse on the BIM. Check your owner’s manual for specifics.

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1 year ago

Great explanation and tip to determine your system. On my mh, the bedroom slide will not operate if engine is running, the heavy wall slide does operate when engine is running.

1 year ago

Just a FYI on some motorhomes the slides and leveling jacks will not work if engine is running. It is a safety feature to prevent use when driving.

1 year ago
Reply to  Terry

I believe that Fleetwood recommend having the engine running to operate the slide.

Jeff Craig
1 year ago
Reply to  Terry

My Forest River Georgetown has a slide lockout when the engine is running, but we need the engine running to level in anything less than two minutes, even with new house batteries.

1 year ago
Reply to  Terry

My Forester Class C has a interlock that prevents slides or jacks from working with the ignition key on,

Glen Cowgill
1 year ago

“This is the level of voltage that a fully charged house battery will not accept.”
Dave, 13.2V is the minimum voltage to overcome the internal resistance of the battery. Most vehicle charging systems are limited to 14.2V with several exceptions. Some of the older Ford charging systems allowed up to 15V while charging. Some slide systems recommend have the engine running while using the slide system such as the slides in my Fleetwood motor home.

25 days ago
Reply to  Glen Cowgill

My Gulfstream BT manual says to have the engine running and ebrake set in order to operate the schwintek slide. If the ebrake is not set the system won’t operate.

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