The Truck Charging Trailer thread...

Grumpy_G

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. 

Mouseketab

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. 

Sharon_is_SAM

 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.

GatorEgg

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.

Grumpy_G

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. 

NMSunshine

Similar results for my 2011 V6 Tacoma with tow package and my recently acquired 2021 TAB 320ss. 

One additional observation that doesn't entirely make sense to me is with the tow vehicle connected and the battery OFF at the disconnect in the tub - the 12v fridge is getting (some) power directly from the tow vehicle bypassing the battery?

Yep. Battery OFF, Fridge set to 5,  tow vehicle UNplugged, fridge light and compressor are off.

Do not change anything else, just plug in tow vehicle and fridge powers up. Unplug it, fridge powers off.

I have a smartshunt installed on the battery and it confirms no current draw from the battery when it's off, so the disconnect in the tub really disconnects it, so the fridge is somehow connected directly to the tow vehicle charge wire when the battery is off?

Any idea if that is expected or is something wired weird?

Bill&Suze

Our tow vehicle is an EV (Kia EV9.) The camper is equipped with a BB 100 a/h lithium battery and solar, both of which I monitor with the Victron Connect app. The main draw on the battery is of course the 2 way fridge, there's very little else drawing from it and when the fridge compressor kicks on, its draw can be seen on the app. We were on a trip and had stopped for lunch and I thought to check on the battery with the Victron app and saw that it was supplying almost nothing to the camper, even though the fridge was on at setting 5 for the trip. I then disconnected the camper from the car and instantly saw the lithium battery begin supplying the usual voltage that the fridge draws. I could only conclude that the 12V car battery probably supplies what power the camper demands as the fridge cycles, along with powering running and brake lights. I don't know whether the car is also constantly charging the lithium battery. I would hope that rooftop solar would remain active while traveling to do that job.

Sharon_is_SAM

@NMSunshine and @Bill&Suze - The 7 pin provides power to the camper from the TV while running.  If your TV does not have an isolation relay, then the TV battery will power the camper when the ignition is off.  The risk is running down your TV battery while you stop for lunch somewhere and the 7 pin remains connected.

Our Toyota Sienna does not have an isolation relay, but our frig draws very little.

NMSunshine

@Bill&Suze Nice, I've heard good things about that EV9 from another friend.

Specific to the fridge behavior, from what I can tell based on the Victron  SmartSolar and SmartShunt readings with the Battery disconnect OFF and the tow vehicle connected, the tow vehicle is powering the fridge, and the solar panel is still charging the battery since it charges even when the battery disconnect is off. 

Note - I still don't know if this is normal. I know the previous owners so I know this wasn't something custom they did. 

Actual current etc varies based on a variety of factors... I straightened my camper plug connectors and went from 2.3A to 3A so there's a huge amount of variability. Fun...

My plan is to optimize as much as I can with the existing system before upgrading, make all my mistakes, fix whatever's broken from the previous owners and learn some before spending any money in the wrong places, and keep learning from the kind folks here...
 

NMSunshine

@Sharon_is_SAM thanks! I checked the current with the tow vehicle on and off, lights on and off, in the case of the 2011 Tacoma I have the vehicle has to be ON to transfer current to the trailer and the vehicle lights do not need to be on to provide 12v power to the trailer. Trailer lights follow the vehicle lights - Since I have seen comments from others regarding lights on/off in the TV to power the trailer.  Good comment about the relay though.

Sharon_is_SAM

I’ll borrow from @Mouseketab here:

” FYI, the easiest way to tell if you have a charge line without any meters. (This method does not tell you how efficient it is).
Turn off your T@B battery by disconnecting the negative, turning the battery cutoff, or removing the fuse at the battery.

Hook your camper to the 7-pin, crank the engine. Turn on an interior trailer light. If it comes on, you have a charge line. Turn off the engine, if the light goes out, you have an isolation relay. If the light stays on, you do not have an isolation relay.”

Bill&Suze

Thanks @Sharon_is_SAM, that's very helpful, I had read about this simple test had forgotten it.

SLJ

My Ford Maverick with the factory tow package and built in factory brake controller provides 7.5 amps (100 watts) to my T@B when idling. More than enough to run my 12 volt fridge and have 5 amps left for battery charging.

ScottG

This image might help. It shows the wires connected to the positive stud in the junction box under the front of a typical 320. Even if the battery is disconnected, power from the T/V can still pass via the trailer pigtail to the 12V distribution block in the power center (i.e. "converter").

Sharon_is_SAM

 “I don't know whether the car is also constantly charging the lithium battery. I would hope that rooftop solar would remain active while traveling to do that job.” @Bill&Suze - your solar will charge your lithium battery as long as there is sun.  I know the lithium batteries charge faster with a higher voltage (14+V), but I know our converters/chargers with a voltage of 13.6 on shorepower will charge them at least up to 80-85% and maybe higher if charged for a long period.  For us with no installed solar, I suspect our TV charge will be inadequate, so in transit, we will not run our frig.  Better to arrive at a campsite with a full battery.