Will fridge run off tow vehicle on batts on TT?

arcsum68 wrote:

wopachop wrote:
Curious what brand paint will you spray on the metal? Ive got some frame rust around my battery box that has needed attention.

Had not really thought that far in advance, I have a few cans of black laying around, thought I would use that. If it starts rusting I will spray it with a rust inhibiting paint, I tend to stay on top of those things. I call it attention to detail, my friends call it something else.

POR-15 (paint over rust) does a phenomenal job at covering and stopping rust. It’s expensive but a little goes quite a ways.

The OP has a 2-way, but of interest to some, the 3-way's 12v element depends a lot on how much voltage it is getting for how many watts it can do.

It needs a minimum wattage to get any fridge cooling done. The usual RV with the fridge wires long and thin already makes for a voltage drop between fridge and 12v source such as the RV's battery. Add the long thin wires using the 7-pin and now your voltage drop is worse.

When I had my jury rig fridge cooling using the 12v element powered from an adjustable voltage converter plugged into shore power, I found there was a big voltage drop between the fridge's 12v connection behind the fridge to the wires from the battery and the connection to the circuit board where the 12v element goes on the terminals. Thin wires there too plus the path through the circuit board to the terminals the 12v element connects to. The element's own wires are thin.

That meant the 12v element still needed the converter's voltage to be higher to get any cooling done. The element needed about 200w to do a proper cool down like the 325w AC element would. If it was only getting 150w it would not do much at all. I had to set 15.2v on the converter (15.4v is the high limit for DC on the fridge)

So imagine how low the wattage must be on the 12v element doing it normally from the RV battery at 13.6v from converter or alternator.
That is why they keep saying the 12v element is just to maintain the fridge at a low temperature once it is already cooled down.

(I had the Kill-A-Watt meter to show the different wattages the converter was pulling to run the element at different voltage settings)

With some solar on the roof and a controller with adjustable voltage, you might do better with a 3-way on 12v by cranking up the controller voltage set points but keep it below 15.4v.

ajriding wrote:

* just in case someone reading is a very newbie... propane fridges, or 2-way or 3-way fridges heat the gases to make the fridge cool. sounds backwards, but is how it works, so a DC heating element makes the fridge cool, or when on propane, the hot flame makes it cool.
The 3rd way is a heating element that is AC household current, which if you run a generator this will work fine while driving. The DC and AC heating element probably are the same in one and run off either.

The DC and AC elements are not the same element on three-way fridges, because the operating voltages are so widely different. A 300W 120V element has a nominal resistance of 48 ohms; a 300W 12V element has a nominal resistance of 0.48 ohms. To use the same element, the fridge would have to either step down the 120V power to 12V, or step up the 12V to 120V, or convert both to some common voltage, as well as having some circuitry to switch the element between the different supplies. It's simpler and cheaper to just have separate elements. (Very often, too, the 12V element is less powerful than the 120V element to keep the current consumption a little lower.)

wolfe10 wrote:

ajriding wrote:

Driving with propane lines open is not legal always, but many have done this since the beginning of RVing.

Yes, with the exception of a few tunnels, etc, please provide a link to support this statement.

X2. It is totally legal except as indicated by AJ.

ajriding wrote:
Are you talking about running it off the battery to heat the element, or off the battery to run your circuit board while you run it off propane?

Answered on page 1.

ajriding wrote:

Are you talking about running it off the battery to heat the element, or off the battery to run your circuit board while you run it off propane?

I was referring to powering the circuit board to run off of propane. *Keep in mind you asked for that answer but also asked for it to not be discussed.

ajriding wrote:

Driving with propane lines open is not legal always, but many have done this since the beginning of RVing.

Yes, with the exception of a few tunnels, etc, please provide a link to support this statement.

There are some vehicles that the engine runs on propane.
Show the law that states that it is not legal to have propane on and in use while driving.

You have answers you need (just pre cool, or use ice for that short trip) but im curious now...
Are you talking about running it off the battery to heat the element, or off the battery to run your circuit board while you run it off propane?
Older fridges do not need power to run off propane. Not sure what you have.
Newer ones need power to run propane which is regulated by the circuit board powered by DC, and yes, this circuit board should run off the TV's power through the trailer plug, it does not need much power.
Running a DC heater element off the TV is not a good idea, if it were able to pull enough amps through the long wire run from alternator back to trailer fridge, then it would either run your TV battery down or do some hefty damage to the alternator making it run more and run hot. I once burnt out a 130 amp alternator trying to run a fridge like this on a class-c. I learnd much since then.


* just in case someone reading is a very newbie... propane fridges, or 2-way or 3-way fridges heat the gases to make the fridge cool. sounds backwards, but is how it works, so a DC heating element makes the fridge cool, or when on propane, the hot flame makes it cool.
The 3rd way is a heating element that is AC household current, which if you run a generator this will work fine while driving. The DC and AC heating element probably are the same in one and run off either.

Driving with propane lines open is not legal always, but many have done this since the beginning of RVing. It has been discussed to death extensively with everyone's opinions and discussed more after that.
It is possible to run the fridge on propane and drive. Dont get caught. Dont tell us you did this.

opnspaces wrote:
I find it hard to believe that the tow vehicle differentiates between the trailer battery and the truck battery. My guess is the two batteries wires are just tied together at some point and the computer sees the two as one big battery.

One easy way to find out. plug the 7 way umbilical into the truck and start the truck up. Then go into the trailer and see if the interior lights come on. If the lights come on then the refrigerator should work on gas.

It is connected in the way you imply but the truck battery is connected to the alternator with a short run of heavy gauge wire that can handle substantial amps.

The trailer wiring is a longer run of small gauge wire with typically poor connections, so sending the amps to the fridge becomes problematic.

PS: most fridges pull somewhere around 150-250watts...that's 13-22amps at 12v.