Yet Another Supplemental Charging System for Camper Batteries

Managing your camper power consumption can be a bit challenging if you’re generally not plugged into shore power, especially with hungry loads like the compressor refrigerators we’ve had in our last two campers. Keeping the batteries charged can become interesting, as in “may you live in interesting times.” Over the years, having owned four different campers, I’ve come up with three different power options. Looking back it makes me think of the fable about the clueless young frog in a pot of nice cool water on a stove.

All of my campers have had solar power and one or more group-31 batteries; two have had propane refrigerators, two with compressor units. Shortly after I got my second camper and found it was too heavy for my Chevy 3500 dually, I traded up for a 2008 F-450, 19-inch tires, dual alternators, and zero issues with payload. Lots of DC power, and lots of room underneath for extra batteries, plus a 1,500 watt modified-sine inverter. I built a frame to hold two group-31 AGM batteries, and a weatherproof box for an inverter mounted 3 feet away up under the truck bed. The inverter could be controlled from the cab of the truck, and fed a duplex ground-fault outlet mounted on the side of the truck hitch. The camper got plugged into the inverter, everything ran on AC as we went down the road, securing propane for a long ferry ride was a non-issue, and we basically doubled our dry camping time sans generator.

That said, camper number three with a compressor refrigerator needed something more to handle those times we might be parked for a week or more for a canoe trip, lousy weather, no inverter support, no generator, no shore power. To handle that I installed an Efoy 1600 Methanol Fuel Cell—search online for “truck camper fuel cell” and you’ll find a write-up about that installation. It worked well, basically an automatic, 65-watt battery charger, but the fuel was (and still is) expensive and difficult to obtain—most of the RV installations are in Europe or Canada, and they aren’t yet too popular with the boating crowd in this country. Given unit cost, its operating hours lifetime, plus fuel and its Hazmat shipping charges, you’re looking at around $1.80 per hour of run time—almost enough to consider warm beer at the end of a river trip. I think I still have the only truck camper Efoy installation in this country. Can’t imagine why, although in hindsight by that time the water on the stove was definitely getting hotter.

Efoy 1600 Fuel Cell with fuel.

Come 2016 and our fourth camper, a Northstar 12STC. It has 160 watts of solar, two group-31 AGMs, a propane-converted, manual-start Honda EU2000i generator, a WFCO converter-charger with typically anemic charge rate, and another compressor refrigerator. The old truck inverter system went away when I finally traded the F-450 for a 2016 F-350 dually with a 65 gallon aftermarket diesel fuel tank. There’s now just no room between its frame rails for auxiliary batteries. I’ve reinstalled the Efoy since it still has another 1,500 hours or so of operating life. As it turns out, Efoy plus solar is still not enough juice—more about that in a moment. I’ve got Torklift Talon camper tie-downs, and on this truck those are incompatible with a Torklift “Hidden Power” setup. Back to the drawing board and some pondering.

Our camper batteries are monitored by a TriMetric battery monitor, which among lots of other things, tells me remaining battery capacity—a most useful boondocking feature—and the power draw of any load. The Danfoss compressor in the refrigerator is by far the largest of those. Running on DC its starting surge can hit 8 amps, which isn’t an issue if the battery bank voltage is 12.7 volts or higher. Running current on DC is just under 6 amps. However, the overall resistance of this refrigerator’s DC power wiring, assorted connections (or whatever may be in play) is enough that the compressor’s non-adjustable low voltage cutoff will typically activate around 12.6 volts indicated, generally (grumble) about 0300 every morning. The compressor will fail to start, low voltage timeout begins, and the cycle repeats. This stresses the compressor and its controller, resultant warm food in the refrigerator definitely stresses my long-suffering better half, and that always gets my full attention. So, first order of business after discovering that power issue—keep stuff cold, then deal with any additional battery charging consequences.

After looking at what would be involved in rewiring or perhaps repairing/replacing the refrigerator controller I chickened out and just mounted a small, 150 watt pure sine inverter in the refrigerator compartment, with a short run of nice heavy wire to the batteries. When the refrigerator is plugged into the inverter it automatically chooses AC if the inverter is powered. This nicely resolves the low voltage cutoff, warm food, warm beer and cook-stress issues, but aggravates refrigerator power usage even though the inverter is the most efficient small pure sine model I could find. I now see up to a 10 amp refrigerator starting surge, and around 7 amps steady-state. With a typical 50 percent cooling duty cycle plus other normal loads that gets me close to the 20 hour battery discharge rate. With our usual driving, hiking and camping activities, all this equates to around a 20 to 25 percent drop in battery capacity every day, all of which needs to be restored— somehow. Forget a few days parked in cloudy weather—we have to run the generator, and the WFCO converter makes that rather less useful than it should be.

Inverter used to power the refrigerator on the road, a Go Power 150 watt pure sine wave model.

The truck 7-pin connector is good for about 4 to 5 amps steady-state down the road. The solar panel can yield around 10 amps under optimal conditions, but with clouds, rain and/or snow the contribution might well be closer to 1 or 2 amps, or nothing. The generator can’t be run unless parked and in addition may annoy the wildlife, two or four-legged, depending on location or time of day. With the new refrigerator inverter the Efoy can—just—keep the battery bank from going below 75 percent, if running continuously. Add some furnace time or perhaps the vent fan overnight in hot weather, and it can’t quite keep up. For long trips where we know we’ll encounter that scenario we try to plan ahead, if need be shutting down the camper refrigerator and putting critical items (think milk, OJ, favorite foods, wine, beer) in an Engle MT45F freezer/cooler, carried along in the truck. We can move it into the camper or connect it through the removable camper pass-through window and sealing boot. It draws about 3 amps DC peak, less with typical duty cycle, and the Efoy with or without solar assist can easily cope with that load for a couple of weeks or more on one 10 liter fuel container, but as noted at a cost.

I can’t really add more fixed solar panels because I need roof space to move about for loading and unloading a 17-foot canoe. Larger fuel cells—methanol or propane run $6,000 to $15,000 respectively—nope. As a certified (or perhaps certifiable) geezer I just don’t want to mess with portable solar panels or perhaps a small wind generator, got other things to do while I still can do.

What’s needed, says that old fellow still sitting in the pot on the stove, is another supplemental DC charging circuit aka the F-450 setup, which could automatically supply up to perhaps 50 amps or so for a quick charge or at least compensate for lack of sunshine going down the road. At least we’d always arrive with full batteries and could use the truck engine rather than generator if needed. Whatever I built would need to work independently of all other charging sources, either while parked or driving, and still allow full system monitoring by the TriMetric power meter.

So, finally, here’s what I’ve built this time around. A big tip of the hat to Cal Willis, another TCA contributor, who built himself a very nice high current charging circuit back in 2016. Lot’s of useful info there. As he noted, the charging isolator is a key element in the success of any design. You want one that has essentially zero voltage drop when it automatically turns on, so you only have to worry about wire resistance back to the camper batteries as a limiting factor for the necessary full-charge 14.4 volts. To that end I bought another Hellroaring BIC 95150B Battery Isolator, same type/model I selected back in 2008 for my F-450 aux battery setup. It’s made in Montana, specifically designed for this sort of aux battery application. Hellroaring’s website has complete specs, directions, nice diagrams, and a helpful staff should you have questions or need to discuss potential installation issues. (Note: I have no company connection other than as a twice-happy customer).

HellRoaring Auxiliary Battery Diagram (courtesy of HellRoaring Inc.)
Battery isolator, 8 AWG wire and other components used in this alternator charging circuit modification.

Starting truck front, the 6.7L Powerstroke diesel fills just about all the available space under the hood—forget about mounting anything else on the firewall. As before I’ve taken advantage of an unused stud on the negative terminal cable clamp of the passenger side starting battery. I machined a brass mounting adapter, tapped 6 x 1.00 millimeter on the underside for the stud and necked down to fit in the stud recess on the clamp. Midway up, high enough to clear the clamp body, there’s an integral “shelf” to support the mounting plate holding the isolator and a 50 amp resettable circuit breaker. The portion of the adapter above the support is threaded 3/8-16 for a lock nut to clamp the plate in place, and the very top is tapped 8-32 for the isolator’s fused ground wire.

The mounting plate needed to fit in the available space and not be crunched when closing the hood turned out to be a 4×8-inch piece of 0.050-inch 6061-T6 aluminum, which I made from scrap found during an archeological dig in my shop kultch bucket. I wire brushed it after drilling component holes and bending two edges for stiffening; this removed burrs plus oxide and roughened the surface for primer plus three coats of protective Plasti Dip. The 3M rubber-based spray-on equivalent works nicely as well—used that on the F-450 setup. Those coatings last longer than just paint when exposed to the fumes and acid vapors from the battery. The positive connection to the isolator goes to a stud on the same battery’s positive connector, and the output back to the camper goes via a 50 amp circuit breaker there on the plate. When it comes time to replace the battery, all that’s required is a socket extension to loosen the cable clamp nuts and the whole isolator assembly tips off with the battery wires.

Unused negative stud used for mounting.
Mounting Plate

All of the (copper) wire lug terminals in the system are crimped and then sealed with heat-shrink tubing. As it turned out bending up a simple 4×12-inch 16 AWG steel “L” bracket and screwing it to the factory hitch via a 1/4-20 drilled/tapped hole was all that was needed for the truck half of the charging connector. The connector parts are weather resistant, designed for trolling motor use, and came with nice 6 AWG pigtails and in-line crimp connectors. The positive terminal pigtail has an inline crimped connection to the wire from the isolator, which also got heat-shrink. The connector negative (black) pigtail terminal is grounded to the frame via the connector bracket fastener. The entire length of the red 8 AWG charging wire from the engine compartment back is encased in split plastic wire loom tubing similar to that of the other truck wire runs, with care taken to keep all the wiring well separated from hot exhaust treatment components. Think lots of tie-wraps and a couple of sore knuckles.

Connector for 8 AWG wiring with rubber heat shrink tubing.

In this non-basement model camper, wiring (both positive and negative leads) run from the camper battery box goes alongside and is tie-wrapped to supports for other wiring underneath. It’s also in loom once underneath the camper overhang where exposed to road conditions. I mounted a second 50 amp circuit breaker for the charging line in the camper battery box, with a short lead over to the nearest positive terminal; the black ground wire went to the load side of the TriMetric shunt. That shunt connection location puts it in parallel with the existing 10 AWG ground wire in the camper 7-pin pigtail so camper lights and such still work safely/properly, and the meter still tracks charge/discharge properly. In all, I ended up with 30 feet of 8 AWG charging wire, truck plus camper run, for a total resistance of 0.019 ohms. For currents of 10 amps or less I should see 14.2 or more volts or more at the battery. As in the old F-450 installation, this should allow topping off at 14.4 volts full charge as charging current decreases.

Completed battery isolator set-up under the hood.
Rear socket for charging circuit.

The true test will be our next road trip sometime this Fall, charging when the batteries are low, but for now while parked, truck engine and camper running, no solar input, and batteries starting at 99 percent, I’m now seeing about 2 amps of charging current at 14.2 volts where before that amount was a net loss under the same conditions. If parked where we can’t run the generator, the truck engine is quiet and remote start provides enough time to put a few amps back in the can when needed. All in all, this old frog is optimistic that the power issues for this camper are now manageable.

About Duncan Crawford 6 Articles
Duncan Crawford is a long-retired project engineer and engineering services manager, with a background in physics, nuclear engineering, and technical management. Unfortunately his long suffering, more sensible wife retired four years after he did, leaving him alone during the day without adult supervision to develop a severe tool and new technology addiction, not to mention time to fiddle with their current truck camper. His favorite mantra is “never saw a new tool or technology I didn’t like.” His most often-used phrase is, “yes, dear,” as he slinks back into the shop carrying his latest wanna-add camper widget.


  1. Hi, Clinton

    Thanks for the feedback. The BIC isolator setup uses 8 ga wiring, camper and truck factory wiring is 10 ga; resistance is calculated/measured for 8ga. There’s a picture of the wire used but I managed to delete the size in one or another of the article edits. Lots more re the BIC on the referenced website– it’s a fairly unique product. As for the refrigerator voltage drop– sigh. This particular rig is S/N#2 of the model, wiring since refined/improved, but for whatever reason (length, flaky connections, whatever) I get the voltage drop/short cycling, hence this project. I could have run new heavy wire directly to the fridge controller, with a bit of ugliness, but my fear was that the controller had been damaged after a year+ of short-cycling and didn’t want to mess with a replacement. Non-adjustable cutoff, but zero issues running on AC. As noted, I chickened out. Our next trip, after the holidays, will be the true test of the setup as to charge voltage and the like with depleted batteries. That said, when I did basically the same setup on my F450, batteries under truck in that case, the isolator kept them nicely charged as monitored in the cab by the inverter they fed. Now the camper batteries will see independently controlled charging from the isolator, truck and solar so I’m at least optimistic about having cold beer at the end of the day. I’ll provide feedback once we’ve had some decent road time.

    As for portable solar, the pigtail on the camper can now be used for that were I to buy another mating connector and make an adapter for a portable panel. I’d consider that for Quartzite but usually we just don’t stay anywhere long enough to bother with a portable panel.

    On another note, the Honda we carry lives in a storage compartment in the rear of the camper, a large door opening upward and held by two latches. Gen has to be out on a slide so it hangs outside the body of the camper. Can’t do all that unless parked, and even then the camper converter won’t do much for the batteries– we just use the gen for microwave and A/C. Given 25 ft of truck plus camper I’ve been hesitant to add a front hitch and become a true land yacht. Between the camper overhang plus a front carrier we’d be looking at about another $300 for most ferries we’ve used in the past.


  2. The author might know something I don’t or I might not know even the little I think I do. I have never fiddled with DC to DC converters… But, I think his calculations for voltage drop might be optimistic. Was his .019 ohms calculated or measured? I come up with a .03 ohms (30′ of 10AWG) and a voltage drop of .6 volts at 10 amps – 14.4 drops to 13.8. I also would imagine that the truck’s charging system will drop well below 14.4 volts when its two batteries are satisfied, without a care about how happy the camper battery is. I think his aim should be to add more solar – portable if need be.

    • Now that I have had coffee. I couldn’t see a way to edit my earlier comment. First question I should have asked is why would a 12 volt device have a cut-out voltage at 12.6? That is nearly a full battery. Or, is it a 24 volt fridge? I would think cut-out should be more in the 11 volt direction. Not knowing the Danfoss fridge, is there a way to change this? I would then think, that if that is possible the best way to power the fridge is direct to battery without suffering even more loss in an inverter when changing to AC. I am interested in how many actual amps and actual voltage gets to the camper battery when the trucks batteries are full using this isolator.

  3. Granted my brain is not as big as the authors but disagree on not charging with a generator while truck is moving. During summer months traveling the lower forty-eight my propane frig cannot cool down. My Honda 2000 is mounted on the front of my truck and I just used my TC power cable routed from the camper, to in between the cab and the bed, turned along side the passenger side to the generator. A few zip ties and one has all the power needed to cool the frig. Refill the gen tank during truck refueling.

    Granted I travel solo and access to the passenger door is limited but is a simple and effective way to cool the beer.

  4. Wow. Great timing with this article from my view point. I’m going to install a couple of Group 31 AGM 125AH batteries in our camper this spring before we hit the road for an extended trip. I’ll copy your installation to enhance the charging from the truck alternator. We also have a 6.7.
    One other thing I’d like to do is to use a couple of newish 65AH batteries that the previous owner installed just before we bought the camper in August. I’d install them in the wheel well as a back up battery bank. I need to work on the wiring layout over the winter.

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