By Mike Sokol
Welcome to my J.A.M. (Just Ask Mike) Session, a weekly column where I answer your basic electrical questions. If you’re a newbie who’s never plugged in a shore power cord (or ask – what’s a shore power cord?), or wonder why your daughter’s hair dryer keeps tripping the circuit breaker, this column is for you. Send your questions to Mike Sokol at mike (at) rvtravel.com with the subject line – JAM.
Dear J.A.M.
I have a question for Mike that I have not seen discussed anywhere. A teardrop owner wants to know if it is acceptable to hook up their trailer to AC shore power and remain hitched and connected via the 7-pin trailer connector. I guess this is for a fast getaway in the morning when on the road – no need to hitch up or to remember to connect the 7-pin plug.
Thanks for directing this question to him! —Sharon
Dear Sharon,
That’s an interesting question and I can see where it could be confusing. So let’s investigate the two different connections and see if there’s any possible interaction between the 120-volt shore power and 12-volt DC systems.
I’m sure that if you’re discussing shore power for a teardrop trailer, then it’s likely a 20- or 30-amp cord and plug, and not 50-amp (even though that won’t make any difference for this discussion). This AC connection will likely power some kind of converter or inverter/charger whose job it is to keep your RV’s house battery fully charged while you’re plugged into shore power. So there’s an interconnection between the AC and DC systems, but the converter or inverter/charger keeps them separated internally. That is, there’s a transformer of some sort and perhaps an internal transfer switch in the converter/inverter to keep the AC and DC power systems isolated. So no problem there.
What about the turn signals and running lights? Well, that’s a connection between your tow vehicle and RV lighting system exclusively without any AC power from the campground, and there is no real interconnection. That is, all of the power for the brake lights, running lights and turn signals should come from the tow vehicle’s electrical system. So no problem there, either.
Finally, some tow vehicles and RVs have a separate battery charging connection between the tow vehicle’s and RV’s 12-volt DC electrical systems that will charge your RV battery while you’re driving on the road. Just be aware that this isn’t really a fast charger like you may have in your inverter or converter. It’s more of a maintenance charger limited to maybe 8 or 10 amps of current.
If this charging circuit was installed properly there should be some sort of diode or relay-based battery isolator to keep your RV from discharging your car (or truck) battery. Again, there should be no problem leaving the 7-pin trailer plug connected overnight even if there’s a battery charger interconnect.
In a nutshell, leaving your 7-pin trailer plug connected to the tow vehicle while your RV is plugged into 120-volt AC shore power shouldn’t cause any problems at all. Yes, then you’ll be able to make an even quicker getaway first thing in the morning.
OK, everyone. Remember that electricity is a useful and powerful force, so we all need to pay attention to safety precautions while using it.
Let’s play safe out there….
Mike Sokol is an electrical and professional sound expert with 40+ years in the industry. His excellent book RV Electrical Safety is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.
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I have a 2016 Forest River M/H. I have a “Toad Charge” for my 2 vehicles, one for my Subaru & the other for my Jeep. They are connected to the M/H starting battery that has a 15 amp fuse, there is no current unless the M/H is running & is diode protected for only charging the toad! I love this set up as I use a brake buddy on both vehicles (its the law) & on the Jeep I also have an ARB fridge freezer that needs 12vdc to operate. Installation was fairly easy & its a one time deal as everything stays where you put them!
If there is no isolator on the vehicle battery, wouldn’t the inverter also charge the vehicle battery if both the inverter was connected to power, and the trailer is connected to the tow vehicle ?
Yes, and that’s why I said there should be a properly installed battery isolation system between the tow vehicle and RV battery systems. Charging circuits are very sensitive to battery chemistry and size, so intermixing your RV and Vehicle batteries, especially during charging, is a bad idea.
Just as long your vehicle isn’t running. Just make sure you unplug the shore power before starting the vehicle or does it matter?
With a battery isolator it shouldn’t matter if the 7-pin connector is hooked up during starting. In fact, some relay battery isolators have a manual connect button or lever that lets you jump the batteries in parallel momentarily if the engine battery is dead, but I’ve only seen them in class-c RVs.