So you have a camper shell… or are wanting to start all over with wiring up your camper? This guide/thread should help. Technical explanations and even terms like “current” “voltage” and “watts” are not used unless absolutely necessary.
If you have questions or comments, please post them in this other thread instead of here, to keep this one cleaner and easier to read.
Charging your camper from the truck?
The wiring that comes with your truck for a trailer connection is too thin to carry enough power back to your camper batteries to do much good. That includes wiring that your camper installer may have put in when you picked up your brand new camper. This needs to be upgraded to at least 6AWG wire. Copper wire is expensive these days! The wire should also be rated to handle automotive (read: greasy and hot/cold). I use 2AWG welding cables in my builds.
This wire goes from the engine bay, under the truck and back to the truck’s box or flatbed tray to a heavy duty “Anderson Power Pole” connector. Be careful when you route the wires/cables to make sure they don’t rub or interfere with anything. When going through metal surfaces, ensure you protect the cable from any sharp edges.
You need two cables, one for the positive wire, and another for the ground. Using a “frame” ground is not recommended. The positive wire leaving the battery has to immediately go to a fuse/breaker to protect the wire (and you) in case it gets shorted somehow. I use 100A Blue Sea 285 breakers. Its nice to have a breaker (vs a fuse) that you can use as a “switch” to disable the alternator charging function or to remove power during maintenance work.
You will also want to make a new hole or two in the wooden “floor pack” part of your camper and route the wires into the battery box of the camper. I used a ½” drill, and sealed it with a urethane caulk. Butyl rubber tape works well as a sealer too.
Just before these hefty cables connect to your camper batter(ies), you will want to add another 100A breaker to the positive wire. This way the cable is protected on both ends. From there a short run from the breaker to the positive battery wire completes the basic circuit.
You may wonder why a fuse or circuit breaker is needed at each end of the main battery charging positive cable. Isn't one good enough? Good question! (and thanks to Jon R for suggesting that I answer this question here)
That heavy duty wire can carry a lot of juice. And it is getting that juice (power, aka current and voltage) from BOTH batteries. Meaning that if any part of that wire touches a metal part of the camper or truck, all the available power in those batteries is going to come out of that wire and create a LOT of sparks and heat and more than likely burn down your camper & truck. And if you are unlucky enough to be part of the circuit that the current goes through, it could kill you. It's only 12V you say? Yes, and people have been known to weld with 12V out on 4x4 trails. Do you really want to experiment with that unexpectedly?
Having just one fuse/breaker at one end still leaves the other battery connected to that wire. So, it just makes sense to have one at either end, as close to the batteries as possible.
OK, two breakers, got it.... (I hope!) Now the camper/truck batteries are linked, unless you trip one of the breakers manually, and the truck will charge the camper battery if it running and the camper will charge the truck battery if you have solar and the sun is shining. Cool, eh? (yes, I’m Canadian!)
If you do not want to charge the truck from the camper (via solar for example) and the charging function will only ever flow from truck to camper, then you might want to add a DC-DC charger into the mix. I say might, because an older truck with a “dumb” alternator will now happily charge your camper batteries. My truck is a 2006 Chevy 1 ton, and I can saw 90A going over the wires (I have a gauge for that) into the AGM batteries in my first FWC. That’s lots.. like over 1200W! That usually ramped down very fast (10 minutes) to more like 40A for the duration of the charging time.
BUT, now that I have LiFePo4 batteries, they have a different reaction to charge current, and will take whatever my alternator can through at them. I was afraid that my alternator would burn up. So I added a DC-DC charger to LIMIT the charging current to a nice stable 30A. I’ve used both the Renogy and Victron brands. The Victron is smaller, but runs quite hot. I added a cooling fan to keep it happy.
On the other hand, if you have a truck with a “Smart” alternator (almost all have those now) they tend to “manage” themselves and reduce how much power they put out, in order to reduce the load on the engine, and thus improve fuel economy. As smart alternator will not charge your camper batteries much at all with the stock trailer plug sized wires, and likely not much more with 2g wire. In this case, a DC-DC charger will make sure you do get a decent amount (30A) of charge whenever the truck is running.
The DC-DC charger goes between the 100A breaker in the camper and the camper battery. It will likely not fit into an already full camper battery box, so you need to find a well ventilated spot for it nearby. The closer the better.
If you want to charge the truck from the camper, then you can’t use the DC-DC charger wired like this. Instead, you will want to use an Automatic Charge Relay (ACR). I use the Blue Sea ML-7622. It is robust enough I could actually start my truck with the camper batteries if necessary. This relay can sit anywhere in the circuit between the truck and camper, as it automatically senses the presence of enough power on either side of the ACR to connect the two systems together. Meaning, if the sun is shining on your solar panels, you are charging both the camper and the truck. Or, if driving, the alternator is also charging both the truck and the camper.
More later ...