Thursday, February 2, 2023


Wire from converter to batteries—How long can it be?

Dear Dave,
My RV’s inverter is under the bed in the back and the batteries are about 20 feet upfront. I’m told this is too far away because of the voltage drop. I’ve looked at many RVs that are like mine. Most inverter specs say 5 feet max. Is this a problem? —Mike, 2019 Thor Windsport 27B

Dear Mike,
We just added lithium batteries and a new Progressive Dynamics converter to a 2015 Thor Challenger and had the same issue. The old WFCO converter was hidden under the bed and the cables ran to the batteries on the driver’s side compartment, which was more than 25 feet away.

We did some research and found that the wire going from the converter to the battery was an 8-gauge wire, which seemed to be heavy-duty enough for the journey. We took out the old WFCO and installed the new Progressive Dynamics that was designed for the lithium battery charging in the same position. All the wiring was there and the compartment that held the batteries was open, as it needed to be vented as the original batteries were lead-acid and would gas during charging.

4-gauge cable recommended

After hooking everything up and plugging it in, we found we only got a 13.8-volt charge to the batteries. I called Dennis at Progressive Dynamics and he did the calculations and recommended using at least a 4-gauge cable. Here is a photo of the rat’s nest of wires just for the new converter under the bed.

WFCO just conducted a seminar at the Florida RV SuperShow. They showed an example of a unit that had just purchased new lithium batteries and after six hours of charging they were only at 61 percent. The technician did some diagnostic testing and found the converter was putting out the proper voltage at the converter, but was much less at the batteries due to the distance and smaller-gauge wiring. A traditional lead-acid battery only requires 13.6-volt of charging power and 13.2 volts at maintenance charge. So the wire or cable is not much of an issue, it just takes longer to charge. In this case, it was 13.8 volts at the converter and 12.6 volts at the battery.

Converter versus inverter

You indicated this is an inverter. And you indicated your “inverter” is located under the bed, which I assume you mean converter, as they are typically in an outside compartment near the batteries.

Inverters and converters are two separate components. A converter is either plugged into a 120-volt outlet or direct-wired and provides a 12-volt DC charge to the house batteries. An inverter receives 12-volt DC power from the house batteries and provides 120-volt AC power to designated outlets. In the case of our 2015 Thor, a 1,000-watt inverter was installed in the compartment just forward of the batteries.

This only supplied power to the outside TV outlet, bedroom TV outlet, and refrigerator. Some inverters are large enough to also be a house battery charger; however, this was not.

Once we installed the new converter and the lithium batteries, we noticed that we were only getting 13.6 volts at the batteries and found the 8-gauge wire was about 25 feet long, maybe even longer, as there was about 10 feet coiled up under the bed. We moved the converter into the same compartment as the inverter and used a 6-foot, 6-gauge wire and got the desired 14.6-volts.

I would suggest you take a multimeter and test the voltage coming out of the converter and match it to the voltage at the battery. I would believe you will see at least a 1-volt drop. If you are charging lead-acid or AGM batteries it will not hurt it, it will just take a lot longer to charge. If you have lithium, it will only charge them to about 75 percent capacity.

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Read Dave’s answer.

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

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John D
7 days ago

You can, to save money buy another same length of 8 awg wire and combining it with the existing wires make a 5 awg wire. Which should give near the same effect as 4 awg. The even numbers are what you can buy off of the shelf.

8 days ago

Isn’t having a major electrical component literally in the bed base dangerous due to the potential electrical fire hazard? Not that the component would be more prone to start on fire there, but if it does, it’s right under sleeping occupants. I have read about fires started in these components, so it does happen.

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