Leveraging battery in trailer as dual battery setup


Adventurer- Toyota Nut
Guess I want to do something others think is crazy, but I have been struggling to leverage the battery in the front of my 06 Chaser trailer in a way to power my fridge while at camp, charge while underway, be charged with a solar blanket while at camp, and generally be a "remote" dual battery set up. Simple put, I want to retain the ARB 50 in the 100, and be able to disconnect the starting battery from the load circuit while overnight. All loads originate in the truck, second battery sits in the trailer.

The challenge is to charge the AGM in the tongue box while underway thru the use of a large set of cables w/ Anderson plug at the hitch and then a step up in changing voltage through a DC-DC charger, tho I haven't determined an efficient way of getting that current back to the truck without running a second set of cables. Wire size from the breaker to the fridge is 4AWG (red wire) and detailed in the Cruiser thread here.

Can I use the same wires to charge the trailer through a DC-DC charger that I then route back to the front of the truck and power my fuse box? Or will the DC-DC charger prevent that?

I imagine this will get only a few less opinions that an All Terrain tire thread!
chaser trailer wiring.jpeg


Well-known member
"I'm not fast (like Verkstad), but I do bad work"
I'm guessing you're using the DC-DC to compensate for the lackluster alternator output? if not, skip it and use properly sized cables--they don't break.

Your starter battery and the AGM have the same charging profile, so you can use an automatic charging relay like a Blue Sea ACR to connect both batteries once any charging source comes online.

The ACR connects both batteries, so I think your DC-DC would be on the motor side of the starter--not required to accept a charge. If the Solar starts generating power, both batteries will link and share the output (which is why you need similar batteries with the ACR, they need similar charging profiles), just wire the solar controller into the fuse panel that serves the truck. If the trailer has a fuse panel, the solar can connect to that, the two panels (truck & trailer) will connect with your single pair of heavy gauge wires.

Because both the truck and the trailer produce power, you'll want to fuse both ends, or use breakers, which make handy switches. you might consider adding another switch (or breaker, they are pretty cheap) at the Anderson connector coming off the truck--after the panel--so it is unpowered when the trailer is (or is being) disconnected.

To run the truck electronics off the trailer battery when the engine is off and the sun is gone, you'll want an either/or switch (one input, either/or output) that powers the truck panel directly from the trailer panel (or battery, if there isn't one).

One way to keep it simple is to put the ACR between the starter and the truck panel, but that means the truck, without the trailer, would not energize the truck panel unless you had a second connection, an ACR bypass--the E/O switch, which could be a switch before the ACR to keep the cabling short. Because all the added equipment connects to the battery, the truck electronics remain unchanged, and you don't have to reset the clocks or remember much.

Blue Sea does offer an ACR with a dash-mounted switch that will manually isolate or combine the batteries, the latter is for using your house to jump the starter, but it would work in your case... if you remember to switch it. The advantage of the manual switch is it can be part of connecting/disconnecting the trailer, and does not have to happen whenever the truck is turned off (your beer and meat are in less danger!).

So the setup is alternator => starting battery => Big breaker => Switch & ACR => Truck panel => Big breaker=> trailer Anderson=> Big Breaker=> trailer AGM or trailer panel => solar charge controller
--If you have solar on the truck too, connect that into the truck panel.

A helpful way to think of it is with the pressure analogy, as long as all your connections are not restricted to moving power one direction. Because the fridge is the most important component of the camping setup, you want to see that it is "pressurized" at all times, by the battery, alternator or solar--or all 3. The alternator and the solar add pressure to the system, so they can back-feed a panel and rely on the fuse to work in both directions, if they connect through a fuse or breaker.

Hope that makes sense-


I have thought about this too. How about running a wire back to the trailer to charge the battery, then moving the battery into the truck to recharge its battery. You would have to do some plug and playing to get it hooked up each time.


Grumpy Old Man
Unless you need a DC-DC charger I would go with a relay as llamalander described.

Here is how I would solve this:
  1. At the truck battery, appropriately sized fuse #1 on + wire going to back of truck; wires sized to current and length (round trip).
  2. Relay in back of truck near refrigerator, fuse #2 on truck battery side of relay.
  3. Fuse #3 or circuit breaker on trailer side of relay.
  4. On output of fuse #3: + wire going to trailer connector and wire going to refrigerator.
A relay will flow both directions when activated, DC-DC will not.

By opening fuse #3 (or opening relay) truck battery is disconnected from refrigerator and trailer battery.

You will need a fuse #4 at the trailer battery.

All fuses are on + wire. -- wires do not need to be fused.

If you go with a DC-DC charger you will need a jumper wire to bypass the charger to use the trailer battery to assist starter battery.


Adventurer- Toyota Nut
Thanks, all. I lost track of this thread with a big project at work. I appreciate all the ideas.

If I were to use this blue sea switch under the hood, starter battery as 1, trailer battery as 2, and combined load and charging at 3 (1+2) can someone explain why that would not work. I know I would have to remember to turn the switch, but I’m not that far gone yet.

Of course, I’d place a 60 or so amp fuse closest to each battery.



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