Utilizing Trailer 7-Pin Power

[FJR Colorado]

I recently upgraded to a large 75L dual zone Dometic fridge. Nice unit and I really want to run half as a freezer. Problem is, my system is having trouble keeping up with the new increased power demand.

I have a single 100W Renogy panel, Rover MPPT controller, and Renogy 100Ah battery. All runs stand-alone with no alternator power. First thing I will do is add a 2nd 100W panel. That will help.

I'm thinking maybe I really need to finally incorporate alternator power. Problem is, everything is in the bed of a truck. I really don't want to run heavy gauge cables and buy a DC-DC charger or other means of achieving "dual battery" charging.

I already have vehicle power readily available in the bed via the 7-pin trailer feed. I realize this wouldn't do much if anything to help charge the aux battery--correct?

I had another thought though... That trailer power should easily be able to power the fridge when the truck is running. What if I set up a relay by-pass?

In other words, the fridge is set to run off the solar aux battery full-time. However, when the trailer circuit powers as the engine starts, it initiates a relay which re-directs the fridge to tap into the vehicle power.

As soon as the engine stops, the relay bypass is de-powered and hence automatically defaults the fridge back onto the solar aux power.

Anybody have experience on this? Suggest a relay by-pass unit?

(I work in high-end comm infrastructure and these devices are very common on things like graphics engines.... if the device loses power, it bypasses the device so you don't lose signal entirely... you just can't do graphics...)

 

[FJR Colorado]

Right. A typical ”RV 7 pin” will flow several amps, but they dont do much for charging. Especially if its a low battery and its on OEM wiring. Despite I say its daft plan to not use alternator charging, no reason your relay scheme on its own wont work fine.

Of course, the idea is that the thirsty fridge never sucks aux battery power when there is trailer harness power available by the bucket....

I think I should use 2x 5-pin Bosch type relays. One for the positive and one for the negative side.

Something like this...




Of course, fuses where appropriate.

 

[FJR Colorado]

Thats needless complexity. Just switch the positive. Only one of those relays is needed.

That was my original thought. Then I figured for another $12 and a few more wires, I can get total isolation.

You think no great virtue in isolating the ground as well?

 

[67cj5]

If you want to power the fridge via the 7 pin, and use the battery power when stopped, you need to have the AC lead and the DC lead plugged in to the fridge at the same time and Then you could try powering a Small 150-200w 120/240v AC inverter from the 7 pin and have the DC lead going to the AUX battery So when the vehicle shuts off and the 7 pin goes dead the AUX DC lead then powers the fridge and the next time you start the truck it will switch back over to the 7 pin / AC supply,

That would be the most simple and cheapest way around it,

Hope that helps.

 

[FJR Colorado]

I set this relay system up today and it works great. Defaults to the aux battery which is fed by solar. As soon as 7-pin 12v from the alternator is present, it shunts over to vehicle power. Turn off vehicle and automatically defaults to aux/solar once again.

Simple Bosch type 5-pin relay. Just used one as Verkstad stated.

 

[broncobowsher]

Skip the relay. Providing the 12V charge wire on the 7-pin is switched with ignition (some GM stuff 10 years ago wasn't switched, or even hooked up as it left the factory). There is no fighting or anything else going on. Your auxiliary battery can benefit from a charge coming from both the alternator and the solar at the same time, and power the fridge also at the same time. Putting a relay according to your plans would only help charge the auxiliary battery by removing the fridge load, but you would be giving away the charge potential of the alternator. I'll take the power of an alternator charge over a solar charge any time the engine is running.