# Charging Batteries From Tow Vehicle



## deepvee16 (Jan 27, 2014)

Last week was our first trip of the season. When we were getting situated in our camp space (before plugging in to shore power), I noticed the volt meter in the trailer was only reading 12.7 volts, and it was still hooked up to the (running) truck. My first inclination was that a fuse might be blown on the truck. So after getting unhooked and set up, I checked the output from the truck at the 7-pin connector. It read 14 volts. Hmmm, didn't know what to think. Then, hours later while on shore power, we wanted to use the TV, it had no power. Hmmm again. Sure enough an AC breaker was tripped for that circuit. When I reset the breaker, on came the charger fan and the voltage immediately jumped up to 13.8 where it should be when on AC.

I had always assumed that voltage from the tow truck would reach the batteries as long as the battery switch was in the on position. Guess not!


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## Oregon_Camper (Sep 13, 2004)

Your truck was probably charging the battery, but at a very low amps. Your voltage can be 14volts, but all trucks use a small gauge wire to power the trailer plug (7 way and 4 flat).

Guess is you are only getting 1-2 amps from truck to trailer....that won't charge anything fast. Plus, your RV is consuming power at the same time (unless you had 12v switch off).

Typically if your 12v switch is off, the only thing that still has power are the emergency brakes...and that is one way, meaning no power will come in from truck.


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## deepvee16 (Jan 27, 2014)

Yes with switch off, only the brakes and electric jack have power.

Something critical that I failed to mention is that once the AC breaker was reset, voltage in the trailer when towing is now at 14 volts. Sorry for that, because that was my point. With that AC breaker in the off position, the trailer gets nothing from the truck. Interesting, and good for us to know.


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## bob91yj (Jun 12, 2018)

I can't speak for Dodge/Ram trucks but GM products have a computer controlled 6 stage voltage regulator. It can vary from 11.8v to 14.7v depending on demand, lights/AC on, etc.


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