Best Way to Charge Trailer Battery From Tow Vehicle

Updated 08/27/2019 | Published 08/26/2019

Question:

I have a trailer less than 1 year old that is not receiving a charge from the tow vehicle. I have tested the 7-pin on the vehicle with the engine running and test roughly 14 volts. Testing the 7-pin cord on the trailer, with the trailer plugged into shore power, I test at 13.7 volts. In camp, the trailer sat at roughly 12.6 volts, but while towing for several hours the voltage did not increase, nor does it display a higher voltage when the vehicle is connected and idling. As far as I can tell there are no loose or corroded connections, ground connections appear to tight and not have corrosion issues.

asked by: Alex

Expert Reply:

The 12V circuit from a tow vehicle's 7-Way connector does a nice job of providing a maintenance charge for the trailer battery but it's not going to send enough power to actually fully charge it.

You have ruled out the tow vehicle as being an issue on your setup and you're fairly certain all the wiring is sound on the trailer. What you are experiencing is simply the limitations of this particular setup. The distance between the battery/alternator on the tow vehicle and the trailer's battery is just too great to allow for efficient charging. There is inevitably going to be some drop-off in amperage by the time the power reaches the trailer battery.

There are some things you can do to try and improve this with your current setup, such as using larger gauge (thicker) wiring to try and reduce resistance or simply moving the battery on the trailer to try and shorten up its distance from the tow vehicle.

Your best solution though is to use a BCDC charger such as the excellent Redarc # 331-BCDC1225D. This will actually boost the power/charge that you get from the tow vehicle's battery/alternator so that the trailer's battery can be fully charged from it during travel. A nice thing about this particular unit is that you also have the option to add a solar panel down the line for even more efficient charging.

I have attached an install video of this charger along with the written manual just so you can get a better idea as to how it all works.

expert reply by: Chris R