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Old 06-24-2017, 06:42 PM   #1
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Default Fridge on DC (battery power)

I have a 2001 Roadtrek 190 that uses the standard Dometic, 3 way power fridge. It works fine on gas and AC settings, but poorly on DC (battery). I also have a solar panel with 2 AMG house batteries.

My question concerns the DC setting and battery switch. My Roadtrek only has a converter, not an inverter. AC is provided by shore power or generator.

What component will provide DC power with both the engine on and the engine off and with the battery switch on and off as described below?

Engine on-Battery switch off-the alternator should provide the DC voltage for the fridge.

Engine on-Battery switch on-alternator and house batteries ?

Engine off-Battery switch off-car battery will provide DC power but run down quickly

Engine off-Battery switch on-2 house batteries and car battery?

Main question:On the DC setting, do I need to turn on the battery switch when I drive to power the fridge or should it be off?

Thanks in advance.
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Old 06-24-2017, 07:18 PM   #2
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Normally, if driving with frig on DC, you would need the 12v power switch on. You would then be running the frig and everything else in the coach of the alternator, plus you will be charging all the batteries. If the 12v powre switch is off, the frig won't usually run on DC if the engine is on or off.

It is pretty common that the frigs work better on gas an AC than DC,

You don't ever really want run the frig on DC without the engine on, for more than a few minutes as it pulls a lot of amps. Batteries and two panels won't be enough to carry it long periods. Put it on gas whenever you are not driving or on shore power.
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Old 06-26-2017, 03:52 PM   #3
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I'm still kinda new to this but in my 2004 Roadtrek 190 Popular, I have found that running the fridge on batteries is sort of running it in "maintenance mode" and I only use it when driving long distances or when parked in the sun (I've got a 160w solar panel). My understanding is that while on battery, the thermostat is overridden and the thing runs at optimum cooling level which is another reason why the batteries take such a hit and can be quickly drained under many conditions.
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Old 06-26-2017, 11:47 PM   #4
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Quote:
Originally Posted by ManWonder View Post
I'm still kinda new to this but in my 2004 Roadtrek 190 Popular, I have found that running the fridge on batteries is sort of running it in "maintenance mode" and I only use it when driving long distances or when parked in the sun (I've got a 160w solar panel). My understanding is that while on battery, the thermostat is overridden and the thing runs at optimum cooling level which is another reason why the batteries take such a hit and can be quickly drained under many conditions.
This is exactly what I do also.

Over the weekend, I got the refrigerator cold with the AC and then went for a one hour drive with the refrigerator on DC. It was not cooling and the temperature went down quickly.

I came home and cleaned the ground terminal and connector on the black wire. I then turned on the engine, and put the refrigerator on DC and checked for 12V on the red wire and ground and got 12+ volts.

Two questions:

1. Just because I have 12 volts, does that mean the unit is good or do I have to check the ohm value?

2. When checking the ohm value, can I just disconnect the car battery and then do a test with a multimeter?

(In order to totally disconnect the red positive wire, I have to pull the refrigerator. I certainly don't want to do that unless I can definitely confirm the heating element is bad.)
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Old 06-27-2017, 08:21 PM   #5
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After reading the Dometic troubleshooting manual, I found the following formula for calculating ohm values.

volts divided by (watts divided by volts) equals ohms


I did some testing today and disconnected my car battery and measured the ohm value of the 12V heating element. The original unit was rated at 125 watts which would be 1.15 ohms at 12 volts. Ohm values are plus or minus 10%.

My reading was 1.6 to 1.7 ohms.

But with the engine running, the actual voltage is approximately 14.77 volts. If I use that voltage and assume I have the original unit, 125 watts, then the value should be 1.7 ohms. So it appears that the 12V heating element is functioning at the actual voltage being put out by the alternator.

My next step will be to get it cold on the gas or AC setting, then take a drive with the refrigerator on DC and see if it stays cold.

Perhaps, just cleaning the ground spade and connecting lug may have solved the problem. We will see.
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Old 06-28-2017, 01:32 PM   #6
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The best long term option is to just replace that 3 way refrigerator with a new compressor refrigerator that is designed to run efficiently on 12 volt power. They aren't that expensive and you won't have to deal with propane for the refrigerator anymore.
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Old 06-28-2017, 02:16 PM   #7
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The best long term option is to just replace that 3 way refrigerator with a new compressor refrigerator that is designed to run efficiently on 12 volt power. They aren't that expensive and you won't have to deal with propane for the refrigerator anymore.

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Old 06-28-2017, 04:47 PM   #8
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Think reading through the manual is a great step- and doing some testing.

and esp on an older rig, brightening electrical connections can really help.


like others, I have found that propane ( which also runs my water heater, furnace and burners) will cool best, followed by AC.
I can dependably get my fridge to 60 or 65 below ambient temps on these, esp if the van is parked in the shade.

and usually start the fridge a day or 2 before rolling- esp if it's 115 out

we have a indoor outdoor wireless thermometer, with the "outdoor" sensor in the fridge - this allows us to monitor temps

The dc can really only maintain temps when rolling and if we stop for more than say 30 minutes I switch to propane.

test the door seals with a dollar bill

I have a small fan internal to the fridge- you can get a camco which runs off batteries- I wired a fan into the 12 volt after buying the camco on amazon and seeing how well it worked

I also have 3 fans on the external fins- one was part of a kit with a temp sensor the previous owner had on there, I added 2 more on a switch.

we never have trouble making ice- freezing.
we don;t use the freezer much for food. I have a bunch of the blue freeze packs which I rotate from the freezer to the fridge- this helps stabilize temps.

and we are conscious of asking the other if they need anything before opening the door.


over the past couple of years we have gone from "fridge trouble" to pretty good function.

a comp fridge would be nice, but our electrical capacity isn;t sufficient.
right now we know we can go 5 or 6 days off grid with our old tech

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Old 06-28-2017, 05:48 PM   #9
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Mike, thanks for the post, seems I learn something every time I read through these inputs. You mention about wiring an internal fan to the 12V - can you talk more about it - sounds like a great way to improve the cooling. Ron
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Old 06-28-2017, 06:09 PM   #10
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Xplorer Steve

Cool RV 3 way Refrigerator mods and maintenance

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