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 > Will fridge run off tow vehicle on batts on TT?

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ScottG

Bothell Wa.

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Posted: 03/26/20 09:31am Link  |  Quote  |  Print  |  Notify Moderator

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.


Scott, Grace and Wesly
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DrewE

Vermont

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Posted: 03/26/20 09:47am Link  |  Quote  |  Print  |  Notify Moderator

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.)





BFL13

Victoria, BC

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Posted: 03/26/20 10:12am Link  |  Quote  |  Print  |  Notify Moderator

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.


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wnjj

Cornelius, Oregon

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Posted: 03/26/20 10:55am Link  |  Quote  |  Print  |  Notify Moderator

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.

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