Does Your camper spend a lot of time connected to your truck through a 7 pin connector? If It does, there’s a potential that you could completely kill the battery in your truck. In this article we will talk about battery combiner and why you might need one. Let’s say you accidentally leave something on in your RV overnight, maybe an inverter or the refrigerator. You could wake up in the morning and find that you have a completely dead camper and truck batteries. Here I’m going to explain how to check if this potential problem exists and how to fix it.
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Potential Issues If you Don’t Have A Battery Combiner
Recently I received an email from a couple that had this very problem. They woke up in the morning to a dead battery in their camper and their truck. At its best, this situation could put a damper on your camping plans, and at its worst it could leave you stranded somewhere. It’s also going to damage your truck’s battery and shorten its lifespan. Whether you have a truck camper, a fifth wheel, or a travel trailer the 7-pin connection is pretty universal between the camper and the truck. Your truck and camper are going to have their own independent batteries and charging systems. The truck relies on the battery to start its engine and run all of its accessories. And in your camper, there’s a battery that powers your fans, your lights, and possibly an inverter. Ideally, the truck and RV batteries would be completely separate, each having their own charging source. However, in most cases, the two systems are connected through the 7-pin cable that goes between the truck and the camper. This can become a bigger issue for people with truck campers because most of their time they’re plugged in in the bed of the truck. The 7-pin connection provides brake lights, directionals, marker lights, and power for your camper. Since I’m talking about truck campers, travel trailers, and 5th wheels, I’m going to limit this discussion to the three major truck brands, such as Ford, GM, and RAM. However, the same principles are going to be applied to any vehicle with a 7-pin trailer connection.
What We’ve Learned About 7-pin Connector
The 7-pin connector uses one of the pins to provide power from the vehicle’s battery to the trailer or camper. This provides some charging power to your RV batteries, but it’s pretty limited. It’s only going to be able to maintain the batteries or just charge them very slowly. But for the sake of simplicity, the main goal here is to prevent accidentally draining the battery in your truck. We have a 2018 RAM 3500. And the power through the 7-pin connection is always on. The only way I can shut it off is to remove the fuse. On some trucks from Ford and Chevy, the power turns off and on with the ignition. That helps prevent this issue in the first place. I asked people in Facebook groups how their trucks are set up, and I got a wide range of answers. From my research and the responses of the group, I’ve learned the following.
- RAM trucks are always on
- Ford, Chevy, and GMC all vary depending on their year model and trim level.
How To Test Your Truck
So the best way to determine if your truck is always on is to just test it. The first thing you’re going to need is a multimeter. If you don’t have one, you can use a test light. Some trucks shut off the power, but only after a delay. To have an accurate test, make sure your truck has been off for at least 15 minutes before checking for power.
The multimeter I got is just the cheapie from Home Depot years ago. I want to turn this to the DC range right around the 20 V mark. I need to climb under the truck and show you how to test the 7-pin connector.
This diagram on the 7-pin connector cover tells you what each of the pins does. What you’re looking for is to make sure that you don’t have power coming through the 7 pin connector. My truck batteries have been sitting all day and they’re at 12.69 V . My truck stays always on and this is the problem, especially because I have lithium batteries in our truck camper. If I had this still connected all the time that would mean that my lithium batteries are trying to charge my truck batteries all the time. That’s some of the issues when you have two different types of batteries. Whether it’s lead-acid and AGM or AGM and lithium, you want to have dissimilar batteries disconnected. If you have a truck that’s always on, you can solve the problem in a few different ways.
- The first method is to just unplug the connector when you’re not driving. This is a free and easy method to disconnect the camper from the truck. However, it’s very easy to forget and it’s somewhat of a hassle depending on your setup.
- The second option is to install a switch somewhere in the truck or the camper. This would give you manual control and make it much easier to access and remember to do it.
- The third option is to install an automatic switch known as a combiner or an isolator. These devices automatically turn on when there’s charging voltage coming from the truck and turn off when that charging voltage drops below a certain value.
Our Choice Of A Battery Combiner
I chose to go with the third option for simplicity and so that I didn’t have to think about it. There’s a couple of different types of these devices. The first one is a battery combiner and the other one is a battery isolator. A combiner is basically an automatic relay. It’s just an on and off switch that does all of the work for you. An isolator uses diodes to separate the batteries. There’s usually about a one-half volt drop, which doesn’t sound like much, but it makes a big difference when you’re charging your house batteries. There are also FET-based battery isolators that work similarly to diode battery isolators. However, FET isolators have almost no voltage loss, but they are larger and more than double the price of the simpler battery combiner.
After a ton of research, I chose a Victron 120 Amp Cyrix-ct combiner. They are very reliable, accurate, and small, which was great considering where I installed ours. I’ve had mine installed for over a year and a half now. I just tested it while making this video and it’s still working exactly as it should.
Installing Battery Combiner
The installation of this device or a device like it is going to depend on your camper. We have a nuCamp Cirrus 920 truck camper. If you’re curious about how it held up after 2 years, you can read about it here. I was able to get the wiring diagrams from nuCamp as well as ask a few questions about the best way to access the wire that I needed. This wire was right inside our side compartment. Here I have all my water and sewer controls as well as the main battery disconnect. After talking to nuCamp, I found out that the wire I needed to install the combiner on was located right behind this battery switch. I can’t show you how to install a combiner on your particular RV. But what I can do is to show you how I have mine set up. This will give you an idea of how to connect it.
The manual is going to give you all the information you need. Basically, you’re taking battery #1, which is going to come from the truck, and connecting it to battery #2 on the battery combiner. When I originally did this a year and a half ago, this cable had some extra slack in it, and I was able to cut it. I put these ring terminals on and connected one side to battery #1, which goes to the truck. This is going to be the source. The other cable on the other side leads to the battery in the camper, and I connected that to #2. The only thing I had left to do was connect one of the wires to ground. Since this aluminum diamond plate is attached to the frame, it also goes to ground. It gives me a grounding point that goes both to the camper and the truck. This is how it completes the circuit. So the computer inside of the battery combiner monitors the voltage on both sides. It determines when to turn it off and on. So when the truck is sitting at rest, the battery is around 12.6 V, and when charging it’s somewhere between 13 and 13.6 V. The battery combiner senses the voltage change and turns the relay on. That allows some of the power to come in through the battery combiner and then out to the camper battery. 
An Alternative Option
There is also another option, such as the Start Assist Plug. If the truck’s battery was completely dead, I could attach 12 V to the prong. It would turn the combiner back on and let power feed from the combiner back to the truck battery. I have no interest in doing that. It would take a while to transfer enough power from my camper batteries through this wire to recharge my truck’s batteries. If I had a problem, I would take the camper’s battery, and use my jumper cables to start the truck. As you can tell this is really small and fits inside our little compartment. I’m sure that your camper has a fairly similar wiring setup that is possibly easier than ours. You can always reach out to the manufacturer and ask them for some wiring diagrams and some advice on how to access this particular wire. Also, Facebook groups are a really handy source, if you’re looking for information like this. I hope this information prevents a few dead truck batteries and gives you some peace of mind knowing that you won’t accidentally kill the battery in your truck. We also made a video on this topic, you can watch it down below. Contact us if you have any questions!