Dear Dave,
I cannot keep my coach battery charged. It is a new 100Ah AGM battery. It will not charge from a campground power post at all. The only way I can charge it is by running the RV engine or just by driving it. It will then fully charge and last a few days. I know how to use a multimeter. Is there anything you can suggest I try troubleshooting? Thank you very much. —Ron, 2018 Thor 28A (a Cruise America rebuild)
Dear Ron,
The first thing I would look at is the battery disconnect switch. Since this a Cruise America “rebuild,” it is hard to tell where it was installed. Many companies built a bare-bones unit for the volume Cruise America Rentals purchased. so things would be different than the traditional build. It looks like Cruise America bought a ton of the Thor Majestic 28A Class C units.
Where to start troubleshooting RV battery problem
Start by looking in the battery compartment which is typically in the lower curb side compartment just behind the passenger door. The battery disconnect switch is designed to keep any 12-volt component from draining the battery during storage, similar to removing the negative cable. However, it will also eliminate any charge power going to the battery. Here is a basic one that would most likely be in the compartment.
Some manufacturers have the switch located just inside the entrance door next to ceiling light switches. That switch would be a typical rocker switch like this one.
Check the distribution center next
Next, check for 12-volt power at the distribution center. Most likely your rig has an all-in-one version that has 120-volt circuit breakers on one side, and 12-volt fuses with the converter on the other. Your unit most likely has something similar to this PD4500 Series. Check the make and model number on the inside cover to verify.
Make sure the circuit breaker for the converter is on and then check for 12-volt power at the converter pos (+) and converter neg (-) terminals. This model has them in the lower right. You should see at least 13.2 volts here when the shoreline cord is plugged into a good 120-volt source. If not, there is an open circuit. Some models like this PD4500 have a battery disconnect pos (+) lug on the board that must be connected.
If you do have 13.2 volts at the converter output, there is an open circuit somewhere downstream to the batteries that is not allowing the charge to the batteries. Look inside the battery compartment and you will most likely see a rat’s nest of cable, wires, and connections.
This is a photo of a Coachmen Class C. Even though your rig only has one battery, it could still have these relays, fuses, and connections.
How the battery isolation management system works
The reason you are getting a charge when the engine is running is the unit has a battery isolation management (BIM) system. This allows a charge from the engine alternator or charging system to pass through the BIM and charge the house battery while you are driving. It also has a switch that provides a jump start from the house battery bank to the engine battery if it goes dead.
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RVDT2772
Coincidence. Our Coachmen Class C is one of the, I think, eight clones of the same Majestic 28A. Same issue. No charge from shore power or generator, just from the chassis motor. Had to replace the battery charge controller adjacent to the power distribution center.
“If you do have 13.2 volts at the converter output, there is a short somewhere downstream to the batteries”
Don’t you mean “open”?
Yes, sorry for the slip.
It could also be your transfer switch that controls if you are drawing power from the batteries or shore power. It is usually under the bed in those units and will have to remove a bed panel to access it. If someone is inside the unit to listen when you plug in the cord they should hear the switch click. If not that is probably it.