Adding a DC-DC charger for my trailer

I have a 2017 1794 Tundra with 125K miles. The truck is completely stock except for LED headlights. These days, my main use for the truck is to pull my 16ft camper-converted cargo trailer from Texas to Montana in the summer. Depending upon my route, weather, etc, it's usually 3 or 4 days. I drive during the day, and then spend the night at a rest area or truck stop along the way.

The trailer has 400Ah of lithium batteries and a 4KW inverter. It also has a mini split for AC. The batteries will run the A/C all night without issue, but in the morning my batteries will be at 40-50% charge. There are solar panels on the roof, but they only provide about 8-10A, and that's on a bright sunny day. That's not enough to top the batteries off by the time I stop again. For when I'm camping in the boondocks, I use a portable generator. But I'd like to avoid using that when travelling.

So, I purchased a small DC-DC charger (Renogy, 20A) that I'd like to install in my truck. When in use, it'd provide 20 amps to the trailer, and that, with whatever the solar panels contribute, is enough to top the batteries off before stopping again the next night! My plan is to install the unit under the back seat (it has to be inside), and run heavy 6 gauge copper wires to and from the charger. I'd use Anderson connectors at the bumper to connect to the batteries in the trailer. Fuses where appropriate, obviously. The charger needs switched power to a signal input to turn on...so this prevents depleting the battery when the truck isn't running, and also allows me a way to add a switch to turn it off if, for instance, I'm climbing a long hill, or the batteries are close enough to full that I don't need the charger.

My only concern is making sure that's not too much draw on my alternator! Yes, it's only 20A, but it's 20A for 8-10 hours at a time. It's my understanding that my 1794, which is equipped with the towing package, came with a 150A alternator. Since I don't have any extra stereo or accessories drawing power during the day (and I rarely travel at night so even the lights wouldn't be on), it seems to me that this should not be any problem. But I wanted to see if anyone else had done anything similar, or knew of a reason why this solution wouldn't work?

Thanks in advance!

 

Your 7 pin trailer harness will have a DC power pin that provides 20 amps (I think) for that exact reason - to charge your batteries while you drive. That pin is only hot when the truck is running.

 

I knew there was a charging wire in the 7-Pin harness...but I never imagined it could deliver 20A! Just going by the size of the wires in the harness, I figured it was no more than a couple amps at best! But if you're right, then this is a much easier solution to my problem!

 

I would not worry about that use case. I have a 30 amp renogy in the bed of my truck to charge a 100ah lithium battery. One note on the Renogy chargers is most of them have a built in voltage cut off that will turn on the charger when the truck is running and as soon as you turn the truck off and the battery goes to rest voltage ~12.8 the charger cuts off so I would not even worry about a switch. I rely on the Renogy's own cut off and it has been flawless for me.

Also to @blenton point that would be the only pause in the system for me. How did you wire in the lithium battery and what happened to the towing power circuit? Is it still wired to charge your Lithium batteries or did you abandon all of the original trailer wiring when you put in your system? That circuit from the truck is actually fused at 40amps in the underhood fuse box, not sure what amount of amps it will actually provide to the trailer. However, that is still an absolute max of 60amps combined going to the trailer leaving 90amps of headroom for the rest of the truck's systems which it will not even think about. My truck runs the 30 amp charger, 30 amps of lights, and a moderate audio build on a regular basis and has never skipped a beat.

 

Thanks! Another thing I just thought about is this...the aux 12V wire in the trailer connector may well be able to provide the current, but I don't think I can use it to charge my batteries. Mostly because the lithium batteries, to fully charge, need a charging voltage above 14.5V. I believe the DC-DC charger steps that voltage up to the appropriate value when you set the dip switches for your battery type.

I knew about the voltage cut off on the Renogy charger, and of course I'd utilize that. My plan was to add a switch to that wire, so that I could turn it off if I didn't want the charger running for some reason.

 

Can you simply install the charger on the trailer side of the circuit, feeding it the 12v source through the trailer pin? That’s prolly what I would do - simple and the charger remains with the trailer instead of the truck.