Charging Trailer Batteries From Tow Vehicle

[HRTKD]

My 2017 F-350 diesel has dual alternators. That may lead some to think that my truck can output 300+ amps. The rating of the two alternators added together does exceed 300 amps. Based on my reading, this would be an easy error to make. The dual alternators do not run at the same time. Apparently they take turns, in an effort to avoid overheating the alternators.

On a trip out of the mountains where the temperature was near 10 degrees F at night a couple of times, I ran my tank heaters while on the road, so that I could dump in town (where it was much warmer). Despite the solar panel and the charge from the truck, my batteries were discharged to almost 50%. I learned my lesson. Tank heaters need shore power or the generator. Plus, the tanks were still nicely frozen.

There is a common complaint on the Ford-Trucks.com forum about the lack of strong charging from the truck. It's not just me.

Otherwise, my solar panel does a very nice job of keeping the batteries charged. I run the generator if the microwave or A/C is needed.

[speedyz4]

Originally Posted by nvestysly

Leading up to the summer of 2019 we realized there was a need for some sort of recharging methodology when boondocking and when traveling from stop to stop. Keep in mind that we don't intend to operate the air conditioner or any major electrical appliances. We do plan to use the Fantastic Vents, furnace, refrigerator, water pump, water heater and lights as you might expect in a dry camping location. Our usual mode of travel is a few nights here and there with several hours of travel from one stop to another.

We became aware of several aftermarket chargers that utilize the alternator of the tow vehicle, connected in a relatively direct line to the trailer, to more quickly charge the trailer batteries. One variety of car charger we considered started out about $300 but with the addition of extra cable length and a few other options increased to $500. In addition, that product only worked from the front of the vehicle so it was necessary to park the trailer and situate the front of the vehicle at the tongue of the trailer each time the trailer battery needed to be charged. This is a great product and certainly serves a purpose but not it's for us. We wanted something that worked from the front of the vehicle as well as from the rear while we are towing. Other charging systems were more generic, and could be made to work from the front and from the rear of the vehicle, but the cost of the brain was several hundred dollars with the need to add many more hundreds for cables and connectors. The advantage of the brain is you get multi-stage charging much like the multi-stage converter/charger already in our trailer.

After scouring the internet, finding all sorts of ideas from carry extra batteries to using jumper cables, it was decided to make our own charging system that can be used from the front of the tow vehicle (when the trailer is parked) and from the rear of the tow vehicle (while towing down the road).

The system described in Part 4 is a brute force solution that works for us. The total cost of the materials is under $400. The description of the system is not intended to be a "how-to" but more of an overview of what we implemented. Your needs may vary and this system may not be suitable on your vehicle and trailer.

I did the same system from my 2015 ram
100amp relay to my house batteries
Add a 2000watt inverter to run rv fridge
While in tow this pass summer
Works great keeping batteries top off

[thomasa98]

On an older rig with a slide-in camper I installed an isolation relay and fed a pair of T105 Trojans with #6 THHN cable. I ran the ground right to the alternator frame. National Electrical code rates #6 for 50A branch circuits. The alternator I used is a Delco 110A unit specd for some Cadillac or other. I had worried that charging into very low batteries would overload the alternator when I changed from #10 to #6 wire. The #10 acted as a current limiting resistor and eventually charged the batteries but not in one day's driving. Sure enough, after changing to #6 wire the rear bearing on the alternator failed. Changed that out and used some hi temp grease on the new one, found a bigger fan for the alternator and haven't had alternator troubles since. Now the trouble is the v-belts. They are good for about 10,000 miles tops. One drives the fan water pump and AC the other drives just the alternator -- that's the belt that fails. This is not directly applicable to the AS towing with modern TVs but I'm sure there are some lessons here. Consider some sort of current limiting resistor.

[kevinkatz]

Thanks for a great thread. This is something I have been researching especially since I am considering an electric only fridge. I tow with a BMW X5 diesel and the battery is in the back which would make the installation of an isolator even easier.
My concern is that with the finicky electronics of BMW I may cause alternator issues. Does anyone have guidance on this thought?
Also I am sure y’all have considered Redarc. What was your impression of that product?

[nvestysly]

Originally Posted by kevinkatz

Thanks for a great thread. This is something I have been researching especially since I am considering an electric only fridge. I tow with a BMW X5 diesel and the battery is in the back which would make the installation of an isolator even easier.
My concern is that with the finicky electronics of BMW I may cause alternator issues. Does anyone have guidance on this thought?
Also I am sure y’all have considered Redarc. What was your impression of that product?

OP here, I looked at Redarc and I'm still considering a DC-DC converter for the system we installed. Renogy has a new 50A DC-DC converter that also includes an MPPT input for solar. The Renogy product is very appealing in my opinion. Lower cost than the Redarc DC-DC converter and the added advantage of adding solar at a later date. Both seem to be good products, just depends on your needs/desires.

I haven't contacted Redarc tech support but their product has a great following and there are likely some retailers that can answer your questions about finicky BMW electronics. Renogy tech support was very helpful the day I called them.

The Renogy 50A DC-DC converter may eliminate the need for a battery isolator. I believe that feature is built in to the unit. However, I like the idea of an isolator regardless so it disables the line when the vehicle is turned off.

Renogy is currently sold out of the 50A DC-DC converter but the Home Depot web site indicates they have it available for shipment.

https://www.renogy.com/dcc50s-12v-50...ger-with-mppt/

[nvestysly]

OP back again...

The Renogy DCC50S, 12V 50A DC to DC Charger with MPPT, became available for shipment from Renogy in early January 2020. Renogy honored the Black Friday sale price for me because I contacted them on Black Friday but the unit was on back-order. So for $240 and free ground shipping the unit arrived in just a few days

The installation of the DC to DC charger was straightforward. The 2 gage wires I had previously installed from the tow vehicle to the trailer worked fine. I simply reconnected the trailer end of the positive cable to the Renogy unit, made a new wire from the Renogy to the batteries and made an additional ground cable to attach the Renogy unit to the common ground point.

Updates to the wiring diagram and the bill of materials are shown below.

I believe the brute-force, 2 gage cables I installed previously would have been fine for our continued use. We towed the trailer several times with the previous installation and did not experience any problems. However, nagging in the back of my mind was how to provide for a solar upgrade if and when we decide to take that approach. I also thought back to the comment made by the tech support agent at our battery manufacturer when they indicated the brute-force charging method was acceptable but they wouldn't recommend using that charging approach exclusively. As you might imagine, the battery manufacturer recommended a 4-stage charging solution.

Our conventional AC to DC converter/charger is 4-stage. When we're connected to shore power the batteries are charged with bulk, boost, float and equalization. Now, we've upgraded the charging system from the tow vehicle to include 4-stage charging too. In addition, we will be able to add solar input at a later date if we choose to.

The total installed cost increased from $388 for the brute-force approach to $690 for the 4-stage charging - a whopping 78% increase! Time will tell if this was the route to take. For now I'm comfortable with the decision.

[KevinFH]

Just purchased the Champion 3400 dual fuel and made the cable conversions to run it from the trailer propane. This will get us started. I know I want solar, but our first big trip (FL to AK) will be at hookup campgrounds, so solar will be later. Mybe I'll stop in OR and get it there.

[Jstsal69]

Did you look at the Redarc charging system? This equipment is used specifically for this purpose with the ability to add solar to the mix as well.

[MandN]

Nice idea, and install. We boondock almost exclusively, and the “drive” between spots always struck me as an excellent opportunity for recharging. Minimizing or eliminating generator use is a kindness to both yourself and neighbors. It’s amazing how loud the gens are in the quite expanse of outdoor spaces. We also charge at about 20-30 amps while driving via a different set up as we split the lithium solar install across airstream and tow vehicle described in another thread. If anyone is interested, can click on our name and easily find the thread.

[TouringDan]

I considered installing a Renogy 40 amp dc/dc charger but really didn’t like the idea of installing over 40 ft of #4 or #6 wire to go from the front of the TV to the rear of the Airstream where my lithium batteries are installed. As an alternative I installed a 1,000 watt inverter next to the Tundra battery, a remote switch on the dash and installed #16 wire to supply power to the Progressive Dynamics 45 amp converter to charge the batteries while I am driving. The total cost was about $250 and I no longer carry a Honda 1,000 watt generator and fuel in the bed of my Tundra.

Dan

[TouringDan]

The previous post describes installation of a 1,000 watt inverter in the Tundra and connecting this to the shore power connector installed at the front of the Airstream.
I wanted to provide some information on how this has worked the last 9 months. We have 400 watts of solar on the bed cap but this has been woefully inadequate to keep our batteries charged as we have camped mostly in the shade and generally travel with a kayak or two on top.
We have a 300 Ah lithium battery bank but our battery needs are very demanding since we have a compressor fridge and sometimes operate a 5k window AC unit. When we are ready to travel to the next camping spot our batteries may be at 40-50% SOC. No problem. Just turn on the inverter and connect to the shore power receptacle at the front of the Airstream. After about 3-4 hours the batteries are fully charged.
I also found another use for the Tundra/Zamp generator when we were camping and stationary. The 2,000 watt inverter in our Airstream stopped working a few days before the end of our last camping trip. No problem. Started the Tundra, turned on the inverter and had 120v power for limited use of the hair dryer and the toaster. I don’t think that idling for 10 minutes hurt anything.

Dan

[TeddyK]

Great thread! Thx to OP and all bringing knowledge. I’ve planned on doing something like this having seen a YouTube video about the Anderson feed. Only thing I’ll add is considering upgrading to a Mechman 370 amp alternator to my TV