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Automatic power transfer for a teardrop trailer

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New Member
Hi, I hope I will be able to explain this well enough to get an answer. I need help!
I am building a teardrop trailer and all of the lights, radio and fan etc. will be 12-volt dc. They will run directly from the batteries through a fuse box. But I also would like to plug into 120-volt ac power when it is available. I will have a converter to convert the 120V ac to 12V dc.
I would like to have the power switch from the batteries, to the converted power automatically when I plug into the 120-volt power source.
So I am thinking that as soon as I plug into 120-volt power, this will be converted to 12 volts, and this 12-volt power will operate a solenoid or relay that will do the main switching from the batteries to the converted power by opening a circuit between the batteries and the fuse box, and closing a circuit between the converter and the fuse box.
I would appreciate it if someone could point me to a switch, or relay or solenoid or whatever that would make this work. I have been searching but most of the switches etc. that I find don't explain how they operate, and since I am not versed in the terms for any of this, I don't really know what I am looking at.

Thanks for any help!


Well-Known Member
Most Helpful Member
I'd just use a relay with a 120V coil. That avoids complexities with isolating 12V feeds to the relay coil, just connect the coil to the incoming power.

Use a type that has two or more high-current change-over contacts and connect all sets of contacts in parallel so they share the overall load.

The common contact goes to your 12V equipment.
The normally closed contact (made without coil power) comes from the battery.
The normally open contact (made with coil power) comes from the 120V > 12V power unit.

This is an example of a common style of relay, that has three sets of 10A changeover contacts, so capable or 30A with them all in parallel.

You can easily get bases for that type of relay which have screw terminals, so are easy to connect, something like this:

There are many other types and styles of relay that could do the job.

relay https://uk.rs-online.com/web/p/non-latching-relays/8841515/
socket https://uk.rs-online.com/web/p/relay-sockets/8841310/

dr pepper

Well-Known Member
Most Helpful Member
If you want something ready made try a chandlers, you see stuff like that on boats.


Active Member
I think you may want to do one more thing when the 120V AC power is available, that is charge the battery. There are battery chargers that can remain connected to the battery when there is no power to operate the charger. I think it is important to know what size of 12V battery to use for all the lights and appliances. One way to solve the problem is only run the lights and appliances off the battery. This way when the 120V is connected the battery is charging and the lights and appliances will work also. The only problem I see with using this method is selecting th battery charger. If you can list the current (amps) or watts that all the lights and appliances require I would be happy to help you decide the battery charger you need.


Well-Known Member
I have a teardrop camper for 12 years it is my favorite camper trailer. I have a 12 volt car battery to run 12 volt lights. 12 years ago they are the old lights battery use to last 9 days camping now it last months with LED lights. I have 9 Harbor Freight solar panels I hook up to charge the battery. I have 3 different 12v to 120v. All the inverters use power from the battery even if they are turned OFF as long as they are still connected to the battery. My 350 watt inverter will run my car battery dead in 2 days turned off. After learning this the hard way on a camping trip I connect each converter to the battery with a house on/off light switch so they all stay off until I need to use one. If I need less than 150 watts inverter I turn on the 150w switch so inverter is connected to the battery, there is no need to every turn off the switch on the inverter it stays on all the time. If I turn on the switch to the 150w invert the 150w outlet has 120v. If I turn on the 300w inverter the 300w outlet has power. If i turn on the 350w inverter then the 350w outlet has power. I put a neon light across each 120v outlet so I know it is on because I don't ever want to forget and leave it on if it is not being used it will run the battery dead. I think inverters waste about as much power and they make battery does not last long if you use inverter very much. 12 yrs go we had 15w 120v lights 4 lights could be used 3 hours every night after dark for 9 days with no solar. Now we have LED stick on the all lights they run a whole year for 2AA batteries. I stopped using car battery for lights 2 years ago but last week I bought 120v LED lights $1 each at Dollar Tree store they hardly use any power I think you can probably leave them all 24 hrs every day for 2 months car battery will still work. I experiments with my solar panels last week too I have a 10a controller for the 135 watts of solar panels they produce 7.5 watts total sun will charge the battery every day. Controller keeps battery charged automatic on solar panels. If you want 120v inverters to come on automatic mount a micro switch next to the plug so when you plug it into the 120v outlet the plug pushes the switch that turns on the 12v to 120v inverter. You should still have a light to tell you when it is on/off. Use this solar controller not the 12v to 120v inverter. I have not tested my solar control yet I hope it does not waste power like the inverters.

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