Friday, June 2, 2023


Shore power goes off, so do interior 12-volts; battery fluid low. What should I do?

Dear Dave,
Lost shore power and the interior 12-volts would not work. When the shore power came back on, the 12-volt lights worked. I have a converter. Checked the water in the battery and it was very low. Stumped!!! —Larry, 2017 37-foot Keystone Cougar 5th wheel

Dear Larry,
First, let’s look at how your interior power systems work. You have 120-volt AC power that runs items like the refrigerator, outlets, roof air conditioner, and other components. Power comes from the shoreline cord either through the campground post or a generator to the distribution center, which has circuit breakers for those items. You also have a 12-volt DC system that powers the interior lights, roof vents, water pump and other items. These get power from the deep cycle house batteries, which also go to the distribution panel and have 12-volt automotive-style fuses.

When you are plugged into shoreline power, the converter is supplied with 120-volt power and provides a 12-volt DC charge to your batteries. The converter is either in the distribution center, like the photo here that is behind the vented area on the right, or could be a stand-alone model that is mounted on the floor under cabinets or the bed pedestal. The converter supplied 13.6 volts when the batteries are low and dropped to a maintenance charge of 13.2 volts when the batteries reach 12.6 volts.

I do have a few questions. First, you indicated you checked the water level of the battery and it was very low. Do you only have one battery? If so, this is most likely not enough storage power for a 5th wheel as they generally have two. Second, how often have you checked the water level and did you use distilled water to bring the level back up over the plates and just below the opening? When being charged, flooded lead acid (FLA) batteries will gas and lose fluid, so they need to be checked periodically depending on usage.

What probably happened

Most likely your house battery is shot. However, if you are plugged into a campground source, the converter will provide a continual charge of 13.6 volts as it senses the battery needs a charge, so everything works as long as you are plugged in. This is not an ideal situation as the converter is now working at maximum capacity and heat and will not last as long. When the shoreline power was interrupted, the converter shut off, which means there was no charging power going to the battery. If the battery is not good, all the 12-volt functions will quit working. It’s almost like you have no battery there at all. When the shoreline power was restored, the converter kicked in and supplied the power for the lights.

And if you checked the fluid level and it was very low, I would suspect you have not checked it periodically, which means the battery ran dry for a time while the converter tried to charge it, which means the battery is no good.

How do I check this?

First, make sure you have properly filled the fluid level with distilled water and that all the connections are clean and tight. Then leave the unit plugged in for at least 8 hours to let the converter charge the battery. While doing this, use a multimeter to check what voltage is going to the battery. It should be at least 13.2 volts, and I would believe you will see 13.6 volts. I recommend letting it sit overnight charging. Then in the morning, unplug the shoreline cord, let the unit sit for about one hour to let the surface charge dissipate or “settle,” and then use the multimeter to check the voltage of the battery. If it is not at 12.6 volts, the battery is no good.

Do I need a new battery?

If you find the battery does not hold a charge, I would suggest a new battery. Even if you are connected to a campground source all the time, you found that you can’t rely on continuous power. A good house battery is needed for a backup, or the occasional times you may dry camp without hookups. Plus, as stated earlier, having a dead battery or no battery in the system makes the converter work at maximum capacity and it will not last. That doesn’t mean you have to get the top-of-the-line or most expensive battery if you are going to be plugged into a campground source most of the time. Just make sure you check the fluid level periodically.


I sent the rough draft of this post to Larry this morning and got the following reply from him:

“Thanks for your reply. I unhooked the battery and tested it and got 9.35 volts. I tested the cables while off the battery and got 15.16 volts. This is after filling cells and rehooking the cables for 24 hrs. Sounds like the battery is gone but the converter is still working. I thought Camping World put in a maintenance-free battery when we bought this camper… ha ha, joke’s on me. I should have checked it.”

My response:

Larry, If you are getting 15.16 volts it most likely is a multi-stage charger that does a high voltage bulk charge to break up sulfation then goes down to the equalize and float charge. If your battery is low on fluid, that 15.16 volts will burn the battery up fast. When you put in a new battery, check to make sure your charger doesn’t stay at 15.16 volts all the time.

Here is the response from Larry:

“Put in a new battery and checked the voltage coming from charger… mid-14’s.”

I would imagine the new battery had a surface charge and the converter has gone to an equalizing charge. It should eventually go to a float charge of 13.2 volts as, mid-14 is still too high. This could take over 8 hours. If it doesn’t go down to 13.2 volts, the converter could be bad.

 You might also enjoy this from Dave 

6-volt or 12-volt batteries for RVs—Which is better?

Dear Dave,
I keep reading about using two 6-volt batteries in series for RVs. Why? It seems like it would be simpler to have one 12-volt or two 12-volt batteries in parallel. Sorry, still a newbie trying to figure all this stuff out. —Richard, 2023 Geo Pro 19FDS

Read Dave’s answer.

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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Tom H.
28 days ago

Great article on batteries and 12VDC. So many RVers have no clue how the electrical systems work. I also appreciate the follow up conversations.

Bill Byerly
28 days ago

Thanks again Dave for all the enlightening information and help you put out there!

23 days ago
Reply to  Bill Byerly


Thomas D
28 days ago

I dont believe I’ve ever seen a deep cycle maintenance free battery. AGM and starting batterys yes but? My camper has 2deep cycle marine batterys. Has caps to remove and add water.

28 days ago

His first mistake was ASSUMING Camping World installed a maintenance free battery. They put in the lowest end battery they could find. Not even a true deep cycle battery, but an RV/ marine battery.

Thomas D
28 days ago
Reply to  Bob

Would you expect more?

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