Saturday, December 2, 2023


RV’s batteries charge with solar but drain overnight when plugged in. Why?

Dear Dave,
It seems my two 6v in-house batteries only charge through the solar controller. The battery voltage is fine during the day but they drain during the night while parked with no use. That occurs whether it’s plugged into shore power at 30 or 50 amps. I also plug into standard house power 115v at home to maintain the batteries, but it doesn’t work. Help. —Reno, 2022 Thor Magnitude BT36

Dear Reno,
According to the Thor website, your Magnitude Super C has an 1800-watt inverter; however, it does not give any specifications on whether it provides a charge to the batteries or not. We are working on updating a 2015 Thor Challenger that also has an 1800-watt Xantrex inverter, but it does not charge the batteries. Rather, it had a WFCO 8955 deck-mounted converter/charger under the bed. According to one of the video walkarounds, your unit should have the Firefly Multiplex System called VegaTouch Mira. It is a panel in the hallway providing readings for all your holding tanks, LP, and battery condition state of charge.

When plugged into shoreline power

When you are plugged into shoreline power, either the converter or the inverter should be providing a charge to the batteries. Your two 6-volt batteries should be connected in series, which means positive to negative, that provides a 12-volt bank for the lights, roof vents, and other 12-volt appliances. The inverter will take 12-volt power from this battery bank and supply 120-volt power to the residential refrigerator. This is not a very efficient system, as your 6-volt batteries are probably only a group 27, which means about 110 Ah. But since they are lead acid, they can only be drawn down 50%, and, as you have noticed, will not power the unit overnight.

When you are plugged into shoreline power, 120-volt power should be coming into the distribution center and a dedicated line goes to the inverter which has a “pass through” feature. This senses 120-volt power coming in and will let that power pass through to the items powered by the inverter rather than drawing 12-volt battery power.

So why are you not getting a charge when plugged into shoreline power? The power cord is wired directly to an Automatic Transfer Switch (ATS), which senses either shoreline power coming in or power from the generator and automatically switches either way. This could be stuck in the generator position. Have you tried running the generator to get power?

One other question: When you are plugged into shoreline power, do you have 120-volt power to other items such as the microwave, roof air conditioners, and 120-volt components that are not powered by the inverter? If yes, then your ATS is switching properly and we need to look further into the system.

Power comes to the distribution center

With the shoreline cord plugged in, 120-volt power then comes to the distribution center, which has the 120-volt circuit breakers for all the operations. There is a main breaker for the entire 120-volt system and individual breakers for the components. There should be one listed for the converter and/or the inverter. That will send 120-volt power to the converter, which should charge the batteries at 13.6 volts until the batteries get fully charged at 12.6 volts, then drop to a maintenance charge of 13.2 volts constantly. Below is the distribution center in our 2015 Thor. It’s hard to see, but the second circuit breaker on the right is a 15 amp labeled “conv”.

This circuit breaker has a 120-volt Romex wire going to the WFCO converter under the bed. Or, I should say it “had” this converter under the bed as we swapped it out for the Progressive Dynamics converter for the lithium upgrade.

Start by verifying you have 120-volt power coming to the distribution center through the shoreline cord and the ATS. You can do this by verifying the microwave or roof air conditioner is working. If not, the ATS switch is bad or the main breakers are off.

If you have verified 120-volt power to the distribution center, verify the circuit breaker for the converter or inverter is not tripped. It may look good, but turn it off and on, then verify there is a crisp snap and it stays in the on position.

Use a multimeter

Knowing that you have 120-volt power coming to the distribution center and the circuit breakers are on, use a multimeter to check voltage and charging at the batteries. With the unit unplugged and the solar charge controller off, check the voltage at the batteries. They should be 12.6 volts fully charged. Plug the shoreline cord into a campground source and if the batteries are low, you should see 13.6 volts, or 13.2-volts if they are fully charged. This will verify that the converter or inverter is actually providing a charge when plugged into shoreline power or running the generator.

If you do not see the higher voltage when plugged in, use a non-contact voltage tester to verify there is power going to the converter or inverter. You can also do this with a multimeter, but a non-contact tester is much easier. We don’t need to verify the actual voltage at this stage, just that 120-volt power is getting there. Just touch the white Romex wire going into the converter or inverter and press the button. If there is power, it will light up and beep.

If you do not have power here, then you have a defective circuit breaker or loose or disconnected wire either at the circuit breaker or somewhere down the line. But if you do have power coming to the converter or inverter, then the next step is to check the 12-volt power coming out of the converter with a multimeter. If there is no power, check the fuses, which you can see in a previous photo. If they are good and you still do not have at least 13.2 volts, the converter is bad.

Verify volts

And if you do have 12-volt power coming out of the converter, verify that it is at least 13.2 volts, as you could have an overloaded power consumption condition. You have a very large Super C, which has a bunch of 120-volt components. When you are plugged into a residential 115v outlet at home, which you indicated, there may not be enough power to go around. Plus, if that outlet you are plugged into is not a dedicated one for the rig, it is most likely “ganged” to several other outlets in the garage, and could have appliances like a refrigerator, freezer, or air compressor drawing power. In that case, there is not enough power to allow the converter to charge the batteries.

When I worked at Winnebago in the early 1990s, we introduced a unit called the Elante, which was a highline unit. Even if it was plugged into a 30-amp service, the batteries would drain as the converter could not keep up. Engineers had to redesign the system so both roof air conditioners could not be run at the same time. They developed an energy management system with Intellitec. You should be able to use a multimeter to determine what voltage is coming out of the converter.

Hopefully, it’s just a circuit breaker or fuse on the converter. However, if you are plugged into a good 120-volt source and have power to the converter with the fuses good, then your converter is no good.

 You might also enjoy this from Dave 

My batteries don’t charge when connected to shoreline power. How can I test my converter?

Dear Dave,
My converter does not appear to be working. My batteries are not being charged when plugged into shoreline power. How can I test/check to verify an inoperative converter on my Airstream? —Jay, 2020 Airstream Classic 33

Read Dave’s answer.

Dave Solberg is a leading expert in the RV industry and the author of the “RV Handbook.”

Read more from Dave here


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Dave Solberg
Dave Solberg
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. He has been in the RV Industry since 1983 and conducts over 15 seminars at RV shows throughout the country.



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Bob P (@guest_257297)
1 month ago

In the first letter when you told them to remove the battery and store in the garage you should also tell them not to store on the concrete floor but sit it on a board as the concrete will cause it to discharge, I don’t know why but I’ve had a battery discharge fully sitting on concrete.

Bob p (@guest_228090)
8 months ago

Or it could be the name on the RV.

Neal Davis (@guest_227209)
8 months ago

Thank you for the discussion, Dave! Eventually you and Mike Sokol will raise my understanding of electricity, particularly as it pertains to RVs. One day I may even know enough to ask a question. 😎

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