April 8, 2020

How to Disconnect the Converter Charger Automatically When Using an Inverter

When I installed my 2000 watt inverter I wired a shore power receptacle to the AC output of the inverter. This allows me to connect my 30 amp shore power cord to my inverter and power all of my AC circuits while boondocking. I have to “manually” disconnect (or shut off) my converter/charger when powered through my inverter.

I installed a relay to automatically shut-off my converter/charger when power is applied to my inverter. The process of switching to inverter power is now really simple and idiot proof. If you decide to connect your shore power cord directly to an inverter you install yourself, then keep reading. l’ll explain what I did and why.

The Type Of Inverter Makes a Difference

Some larger RVs come from the factory with built-in inverter/charger units which can sense power and switch between components automatically. Not in my case. I installed my 2000 watt inverter myself, which meant I had to install the automatic switching components myself as well.

If you plan to install a power inverter with dedicated electrical outlets (totally separate from the rest of your system) then this type of switching is not necessary. My installation, however, is a little different given the fact that my inverter powers all AC components in my RV through my main shore power cord. Let me explain.

Fooling My Electrical System

When running this way, the electrical system in my RV operates as if its power is coming from an RV park hookup. This, of course, is not the case given that my 2000 watt inverter is providing the AC power to my RV. Remember, my shore power cord is plugged into the inverter.

So where does the inverter draw power from? The battery bank in my RV provides DC power to the inverter which converts it to AC power. So you can say that I’m fooling my RV into thinking it’s on shore power. Is this OK you ask? The answer to that question is yes, but there is one critical step that can’t be missed. You must turn off or disable the converter/charger unit in your RV.

To understand why this is important, I must explain the function of my RV’s converter/charger unit.

What Does the Converter/Charger Do? Why Disable It?

The purpose of the converter/charger unit in an RV is to generate 12 volt DC power from 120 volt AC power (the reverse of what our inverter does). The converter/charger is intended to be used only when the RV is connected to a steady 120 volt AC power source. For most of us that means power from an RV park power pedestal or our on-board generator.

The converted DC power from the converter/charger is fed into the 12 volt distribution panel to power the 12 volt components in our RV. The charger component also monitors the state of charge of our RV battery bank and sends DC power to recharg them. This all comes from the converter/charger. And there is the problem. Did you figure it out?

The Problem

If the converter/charger is running while the inverter providing power to it, it will attempt to charge the batteries that are powering the inverter. You may want to read that again slowly if you’re scratching your head. The bottom line is that this condition cannot be allowed to happen. The converter/charger needs to be shut off when the inverter is running.

Maybe this diagram of my electrical system with the inverter installed will help illustrate the problem.

Diagram of my electrical system with the inverter hooked up without the automatic switch

In this diagram of my electrical system, notice that my inverter is receiving power from the battery bank. That same battery bank will be charged by the converter/charger unless the circuit breaker controlling power to it is switched off each time.

The Solution

As I suggested early on, my goal was to automatically switch off my converter/charger when my inverter is powered on. Here’s why.

With the converter/charger switched off, my inverter will provide AC power to all outlets and AC equipment that I choose to turn on. The battery bank will then provide DC power to both the inverter and all DC powered equipment. I simply want to take the converter/charger out of the loop, therefore, removing that dangerous loop.

Now to accomplish this, I needed an electronically controlled switch that could control power to the converter/charger. Under normal operating mode (when the inverter is not running) power to the converter/charger would be enabled (if connected to shore power or running my generator). When I turn the inverter on, 120 volt AC power from the inverter would activate the switch to disconnect power from converter/charger.

Electrical contactor to switch off converter charger

The solution was to use an electrical device called a contactor to accomplish this. Contactors are electromagnetic switches that are electrically controlled. They are often used to control high-powered equipment like motors or pumps and are readily available. I needed a contactor rated for 120 volts AC up to 15 amps (to match the circuit breaker controlling my converter/charger). I found this 120 VAC contactor on Amazon and ordered it.

The blue connections in this diagram illustrate where I connected the contactor and applied the 120 volt switching voltage (from the inverter output).

Diagram of my electrical system with the inverter hooked up with the contactor wired in

Note: A minor correction to the contactor location in the diagram. It is actually installed on the converter side of the circuit breaker.

I also added a light to indicate when the converter had power. This was simply a safeguard to confirm that the converter/charger is, in fact, off when I power up my inverter.

Materials I Used for This Project

120 VAC 30 Amp Contactor
Electrical Contactor : This contactor will automatically switch when a 120 VAC switching voltage is applied to the A1 and A2 contacts (from my inverter output). I wired the output of the converter’s 15 amp circuit breaker to the normally closed contacts (NC). Read more on Amazon

DIY Electrical Enclosure
DIY Electrical Enclosure : I mounted the contactor in this enclosure on a DIN rail and drilled access holes for the wires. Read more on Amazon
6" DIN Rail Kit
6” DIN Rail Kit : The contactor is designed to mount on DIN rails like these. I attached a single DIN rail with a hot glue gun to the bottom of the enclosure and mounted the contactor to it. Read more on Amazon
Wago Wire Connectors
Wago Wire Connectors : These are by far my favorite type of wire connectors. They are strong, accommodate multiple gauge wire (up to #12 AWG), and completely reusable. They are a little more expensive than standard twist connectors, but they are worth it. I get the multi-pack and now use them for everything. Read more on Amazon

Prototype and Testing

The next step was to setup a test circuit to verify that it would work the way I wanted. Doing this gave me the opportunity to figure out the correct setup for my RV prior to actually hooking it all up.

Testing the auto switching of the contactor

Finally, I installed the contactor in the enclosure box and hooked everything up in the RV before putting it to the test. Success!

What If Your RV Doesn’t Have a Stand Alone Converter/Charger

Not all RVs have a separate converter/charger unit. In larger class-A RVs the converter/charger function may be embedded into a larger piece of equipment that might also include an inverter, transfer switch. This type of unit is usually called an an inverter/charger, but many RV owners refer to them simply as their inverter when in fact, it does more than just convert DC power to AC power. If you have this type of setup in your RV, it is likely that automatic switching off of the converter/charger function is already built-in to your larger inverter/charger unit.

Most class-C motor homes, fifth wheels, and travel trailers have a converter/charger unit similar to mine. My RV also has a separate automatic transfer switch to switches between generator and shore power. Power inverters are often added later by RV owners looking to be more self-sufficient when camping off-grid without electrical hookups.


Often confused as Tito, Brian answers to both. Other known aliases are "Obi Wan Titobi" and "Brito". He thinks solar power is cool and enjoys being off-grid teaching others how to do the same.

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