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The Truck Charging Trailer thread...

Grumpy_GGrumpy_G Member Posts: 447
in Battery/Electrical & Solar
Recently the question if the tow vehicle charges the trailer battery came up a few times, and members had varying experiences. As background, a proper 7-pin trailer connector does have 12V on pin 4 which is intended to charge the battery for the breakaway system. Keep in mind that all sorts of non-travel trailers have electric brakes, and they have a small battery (typically 5Ah) for the breakaway. Depending on vehicle manufacturer the 12V line is more or less limited in terms of current it can deliver. 

I decided to do some experimentation to get some data points. Note that the data is for my specific truck, and other makes and models will behave differently. All voltage and current measurements are taken at the batteries. Trailer battery is a run-of-the-mill flooded lead-acid group 24. 

- Truck: 2013 Ram 2500 - Voltage at the truck battery was 14.3V when idling regardless of load from the trailer.

- Trailer unplugged idle: 12.8V and 0.2A current draw
TV and radio in standby draw a little current

- Trailer unplugged with fridge running on 12V: 12.3V and 9.1A 
This comes out to about 110 Watts and is ballpark what the 12V heating element consumes.

- Trailer connected to truck, fridge off: 13.5V and 3A current into battery, changing to 13.7V and 2.3A after 10 minutes
The battery is getting a low current charge. As voltage rises the current drops as expected. There is a voltage drop between the truck battery and the trailer battery. Back of the napkin calculation shows about 0.2 Ohms calculated resistance. 

- Trailer connected to truck, fridge on 12V: 12.7V and 0.7A current draw. 
Note that the current changed direction, instead of charging the battery it now draws a small amount. The tow vehicle supplies the remaining 8.4A to the fridge. 

In summary, the truck does provide a low current charge to the trailer battery unless the fridge is running on 12V in which case there is a small battery drain. Again, your combination of tow vehicle and trailer batteries may behave differently. 

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    MouseketabMouseketab Member Posts: 1,230
    I knew I needed more charging capability on my new fancy F-150. The new vehicles do amazing things EXCEPT charge your camper battery. so I added a DC-DC charger to the truck. 
    Carol
    MOUSE-KE-T@B
    2007 Dutchmen T@B Clamshell #2741
    2022 nuCamp T@B 320 CS-S
    2021 F-150 502A Lariat SuperCrew, 3.5 EcoBoost 4x2
    Harvest, AL
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    Sharon_is_SAMSharon_is_SAM Administrator Posts: 9,496
     Newbies should still confirm placement of a charge wire.  There have been more than a few owners who found that they were not charging their TaB battery because the wrong 4 pin to 7 pin adapter was wired, etc.  Also important to check if a brake wire was placed - preferably ground and brake wire 10 gauge.  Although it should be obvious if a brake controller is hard wired, we went home without a brake wire when we told the installer that we planned to use a RF controller.  Had to return for the brake wire.
    Sharon / 2017 T@B CSS / 2015 Toyota Sienna Minivan / Westlake, Ohio
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    GatorEggGatorEgg Member Posts: 452
    Thanks Grumpy_G.  Your numbers confirm mine, but on my 2019 Tacoma.  My 2003 Tacoma which I wired myself would run everything including refrigerator/freezer while on the road.  But moving to a new 2019 Tacoma, not so.  After going around in circles with Toyota corporate on trailer charging problem I did my own diagnosis.  I’m retired Navy Electronics tech so it wasn’t difficult. Approx around 2095-6 Toyota and other manufacturers starting putting “smart” alternators in their vehicles.  So we now have this situation.  There are ways around it.  A dedicated heavy gage direct wire from vehicle battery to trailer battery.  In essence creating on big battery bank.   And offerings of a dc-dc converter from folks like Redarc or Renogy.
    2022 TAB 400 Boondock, 2019 Toyota Tacoma Sport 4x4
    2018 TAB 320 Boondock (previous)
    Odessa, Fl.  

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    Grumpy_GGrumpy_G Member Posts: 447
    Out of curiosity I ran the trailer battery down and then hooked up the truck. The battery took over 9A of charge current which is similar to a typical household car battery charger. In comparison the converter got up to 15A. 
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