Is your chassis battery charging when plugged into shore power?

This is our 3rd motorhome.  The previous two were Class As.  Our current one is a Class C.  Both of the other motorhomes would charge the chassis battery when plugged into shore power.  Our new RV does not.  But is it supposed to?  That's what I'll show, and discuss a few options.



Our first motorhome was a 1999 Fleetwood Bounder.

Under the hood was a Battery Management System (BMS).

Inside the box was a control circuit that would isolate the chassis battery from the house batteries when no charging source was available.  But if the engine was running, or generator running, or plugged into shore power, the large solenoid would engage, which would charge all the batteries.

Diagram of the BMS.

Our second motorhome was a 2015 Forest River Georgetown 328TS.

It too had a BMS, very similar to the other Class A.

Our current motorhome is a 2023 Thor Quantum KW29.

I expected a similar BMS with this RV too.  However, after being parked for a while, the engine started very slowly.  I put a meter on the chassis battery, and it was very drained.  However, the RV was plugged into shore power, and the house batteries were over 13V.  I dug through the manuals, couldn't find the answers I was looking for.  Is there something wrong with our new RV?

I decided I would take a look at the solenoid, to see if there was an issue.  On our RV, this is located under the radiator reservoir.  That's the tank with the yellow liquid in the top right of the picture.  It's only held in place with a couple of screws.  No need to drain it, or disconnect the tube.  It can be set aside intact.

First thing I noticed was the solenoid was not in a box, and was controlled by an external timer.

Here's How It Works

The unit operates as an isolator by sensing the level of voltage on the chassis 12 volt system. When this
voltage goes above 13.3 volts for approximately 12 seconds, as happens when the engine is running normally (normal alternator output voltage is approximately 14.4 volts), it will close the isolator relay providing charging current to the coach battery. When the ignition switch is turned off, the relay will open immediately.

If the voltage should fall below 12 volts for more than two seconds while the ignition is on, the relay will drop out to feed all the alternators available output to the chassis battery to keep the engine running. This might happen when the alternator is not able to supply sufficient current to all of the loads. When the chassis voltage goes above 13.3 volts again, the relay will again close in about two seconds to retry and charge the coach battery. The resultant flickering of lights would alert the driver of the system overload.

It ONLY engages the solenoid with the engine running. It does not charge the chassis battery when plugged into shore power, nor when running the generator.

Intelitec owner's manual

The RV also has a 100W solar panel on the roof.

And the house batteries stayed topped up, even when boon docking, but this does nothing for the chassis battery.

To fully recharge the battery, and desulfate the cells, I used a BatteryMinder charger.  While this works great, it's a bit big to mount permanently under the hood.  Any 12V charger will do for occasional charging.

I wanted a more permanent solution, but in a smaller package.  I used NOCO chargers at work to maintain batteries we used in helicopter communication systems.  They are very reliable.

The charger came with a small bracket that can be permanently attached to the firewall.  Unfortunately, there's little space available under the hood.  I used a piece of 3M VHB tape on the back of the bracket, and stuck it to the battery.  This worked very well, and is short enough that it won't hit when the hood is closed.

When in use, I route the short power cord through the gap between the headlight, and the fender.  When not in use, the short cord is tucked in beside the battery.

Here's the 3rd option.  On all of the RVs I've owned, there was a switch on the dash for an emergency start.  When the switch is pressed, it engages the solenoid.  This connects the chassis battery to the house batteries as long as the switch is held.  Great for a self jump start.

The dash switch is momentary.  You could swap it for a standard switch.  If left on, the solenoid would stay on, but could drain the chassis battery if it wasn't switched off when shore power goes out.  It's the least desirable in my opinion, but it is the least expensive.

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NOCO GENIUS2D, 2A Direct-Mount Onboard Car Battery Charger:

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3M Double Sided Tape:

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Last updated 02/28/24    All rights reserved.