How to maximize the life of a 3-way fridge that requires a 12v connection to run.

[no_pun_intended]

Recently bought a 3.3 cu 2020 Dometic 3-way fridge to put in my trailer. Something that's different from the '83 fridge that I pulled out of the trailer is that newer fridges apparently require a constant source of 12 volt power to run in any mode, even 120v or propane. Not only that, but measuring across the 12v terminals with a multimeter, it looks like this thing is drawing 5 amps! (which seems really high to me. someone please correct me if I have measured that wrong).


My plan was to get a fridge I could run off of propane and spend several days away from a power connection. But if this fridge consumes 60 watts an hour, thats going to kill my battery practically before it can even get cold. I'm in a bit of a pinch here, but thankfully I haven't gone in on my battery setup yet so I believe I can work around this. I was originally planning on a single 80 or 100ah battery, as my only other 12v amp draw is the water pump. So really, my questions are as follows:

1. What is the best way to provide constant 12v power to the fridge while off the grid?
Let's say I want to keep it going for a week. Should I invest in a larger battery bank? A couple 100W solar panels?

2. Is there any way to just bypass the 12v "brain" of the fridge?
This would be preferable if possible. I understand that the brain controls things like power source, coolness settings, and even propane pressure and the pilot light. But the older fridges were able to be run off of propane without power, as I understand it. I am decently handy, and would love some guidance on if this thing can be run without 12v.

3. Please tell me I am wrong about this thing needing FIVE amps.
I just keep coming back to this. How could such a small appliance (essentially just a PCB and some LEDs) eat so much power. This high power draw is what worries me about trying to run it off solar; it seems like I'd need to spend close to $1000 dollars just to power a fridge that takes half a day to get cold and could kill my battery entirely on a cloudy day.

 

[Isaac-1]

5 amps sounds very wrong, unless it is running in 12V mode. Most people have 2 way, not 3 way refrigerators, 3 way runs the heating element on 12V power, the other modes just run the logic board on 12V power. My 8 year old Dometic 2 way refrigerator draws about 12 watts (or about 1 amp) of 12VDC power when running on propane or 120VAC, though when on 120VAC the converter generally keeps the battery fully charged and provides the 12 watts of DC. As to off grid, solar is your best option, propane consumption is very low, I have a 32 gallon propane tank and it can run the refrigerator in gas mode for months.

 

[SpencerPJ]

I agree, something sounds high, are you sure it's not in 12 vdc mode? Do you hear the burner light? I would guess it should be maybe 10ah a day, and I would also guess a very modest solar panel could keep that topped off.

 

[no_pun_intended]

I agree, something sounds high, are you sure it's not in 12 vdc mode? Do you hear the burner light? I would guess it should be maybe 10ah a day, and I would also guess a very modest solar panel could keep that topped off.

I am sure it's not running in 12v mode. I had the selector set on 120v, and the manual says that the 12 v mode consumes 14.5 amps.

I agree with you about a modest solar panel. In fact, since a full solar setup was not my original plan, I had kinda hoped I could hook this thing up to its own isolated battery with a small dedicated panel. Buc-ees sells little solar 6V chargers, and I was already able to get this thing running for a few hours off a couple 6V packs from lowes (wired in series to get 12V. But I need to get to the bottom of the amp draw issue before I set anything up. Not sure if there is a more accurate way to measure amp draw other than a multimeter across the terminals.

 

[Lou Schneider]

Not sure if there is a more accurate way to measure amp draw other than a multimeter across the terminals.

Maybe this is a nitpick, but to measure current the meter has to be in series with one of the terminals. Disconnect the cable and put the meter in line with it to bridge the gap. Across the terminals measures voltage.

 

[no_pun_intended]

Maybe this is a nitpick, but to measure current the meter has to be in series with one of the terminals. Disconnect the cable and put the meter in line with it to bridge the gap. Across the terminals measures voltage.

That's actually a very important distinction Lou, not a nitpick at all. I measured amps with the multimeter as a part of the circuit and did get the same 5 amp reading, although the fridge wont turn on with the multimeter in there (it does when I remove the meter from the circuit). I'm gonna set the fridge going off of 120v power with some 6v batteries wired up to be 12v, and just stick a 6v solar charger on them and see how long it lasts for. Really hoping I dont need to do a deep dive into solar to get this thing running steadily.

 

[Mark_K5LXP]

the fridge wont turn on with the multimeter in there (it does when I remove the meter from the circuit).

You're doing something wrong. The refrigerator should run with an ammeter in the circuit.

Mark B.
Albuquerque, NM

 

[IBTripping]

If you run the refrigerator on propane, the control board (brain) won't use many amps. You could recharge the batteries by using jumper cables connected to your vehicle. Charge should last a couple of days or more.

 

[Contributor]

You're doing something wrong. The refrigerator should run with an ammeter in the circuit.

Mark B.
Albuquerque, NM

Mark is right, the multimeter has different connections for the leads for amps vs volts or ohms. Look closely at the meter, you probably are using the wrong inputs for the leads.

Also rotate the dial to point to 5A setting

here is the HF model but it's the same for all meters

 

[Isaac-1]

Note most better multimiters have a 10A setting not a 5A like the cheap (sometimes free with purchase and a coupon) Harbor Freight model

 

[Mark_K5LXP]

Huh, the free HF meters I have are all 10A (I have about a dozen of them). I've never seen the 5A version before. Maybe since the leads themselves are pushing it for 10A, rather than spend the money on better ones just change the scale of the meter.

Mark B.
Albuquerque, NM

 

[Gary RV_Wizard]

As others have pointed out, the amp draw to operate the control board alone should be less than 3A @ 12v. In 120vac mode it is only the circuit board using power to monitor the thermostat; in LP Gas mode it's circuit board plus holding the gas valve open. Neither should reach anywhere near 5A.

You should be able to run the fridge for a week with the typical size 12v battery, assuming minimal use of other 12v items (lights, etc).

 

[no_pun_intended]

Thanks everyone for helping me figure out how to use the multimeter to read amps correctly. I got it hooked up as described by Pedro and Mark this morning, and the brain is drawing 0.09 amps while the unit gets power from 120 VAC. Phew!

Pictured below is how I have it set up now, gonna see if it can run in perpetuity like this. If not, a modest solar setup is totally doable.

 

[Gary RV_Wizard]

Let's call it 0.1 amps, so 2.4 amp-hours (AH) per day. A typical 12v automotive battery has a capacity around 75-80 AH, so would power the fridge for 30+ days. A smaller motorcycle or lawn tractor battery might be only 9-12 AH and last only 4-5 days.
You still need the battery with solar to smooth out the power and carry through the night, but you can use a smaller AH battery if you have solar charging to replenish it. A 30-40 watt panel should suffice.

 

[Mark_K5LXP]

Just an observation that pops out at me, you're measuring the 12V draw running on 120VAC. Running on propane you'd have to add the power draw of the gas solenoid. I know on my absorption fridge it draws 230mA running on gas, so I would measure yours running on gas to get an accurate number.

So are the lantern batteries just for a mockup or did you intend on using primary batteries as a power source? Have you considered the option of operating or charging from the tow vehicle?

Mark B.
Albuquerque, NM

 

[no_pun_intended]

Just an observation that pops out at me, you're measuring the 12V draw running on 120VAC. Running on propane you'd have to add the power draw of the gas solenoid. I know on my absorption fridge it draws 230mA running on gas, so I would measure yours running on gas to get an accurate number.

So are the lantern batteries just for a mockup or did you intend on using primary batteries as a power source? Have you considered the option of operating or charging from the tow vehicle?

Mark B.
Albuquerque, NM

TV charging is out because my truck is old enough to only have a 4 pin connector. I'd rather not redo that whole mess and I'm fine with that.

The 6v setup was a mock up at first, but I'm gonna pursue that as the main plan now, since I like the idea of a self-sustaining power source for the fridge. Might double up on the batteries and do another set in parallel with another charger on it just to be safe. Worst case scenario I just have to run a few wires from my main 12v battery (80ah).

That's a good point about the propane amperage being higher, and I'll have to wait a few days until I can get it installed and connected. I'm sure it will be well under 1 amp and totally manageable. I'll try to put an update in this thread with what I work out in the end. Thanks so much for your help!

 

[Mark_K5LXP]

My truck is old and has a flat four trailer connection. But I didn't run my popup through that. To charge the popup battery (a U1) I had a small inverter and I'd connect to the popup with a 120V extension cord. I could also connect to the truck 12V battery directly, which for light loads (say your fridge) it could easily keep up with. I've been doing it this way since my tent camping days back in the 1980's and still do when operating my portable radio equipment.

Mark B.
Albuquerque, NM

 

[Ray-IN]

Absorption refrigerators(some makes) made in the last 20 years or so have 12V heaters to get rid of accumulated moisture around door seals. Check your wiring diagrams. I see no reason the heaters could not be put on a manual switch.
LP regulators are often wired for 12V too.

 

[Old_Crow]

TV charging is out because my truck is old enough to only have a 4 pin connector. I'd rather not redo that whole mess and I'm fine with that.

The 6v setup was a mock up at first, but I'm gonna pursue that as the main plan now, since I like the idea of a self-sustaining power source for the fridge. Might double up on the batteries and do another set in parallel with another charger on it just to be safe. Worst case scenario I just have to run a few wires from my main 12v battery (80ah).

How about using a motorcycle battery? I just don't know about the long term reliability of the dry cells, plus you'd have a bit more reserve capacity with the m/c battery.

 

[Mark_K5LXP]

Motorcycle batteries will not last long as storage batteries. If you go down the storage battery path you'll discover there's economy factors to consider in the form of size and chemistry. You can throw something at it and see how it works out but it can be more systematic than that, by considering the anticipated power, duration, and budget requirements. If your power usage really is minimal, that's why I suggested the tow vehicle. It already contains a modest battery and a means to charge it, plus run your stuff. For most of my camping that involved just a few lights or charging a phone or whatever I usually didn't need any storage in the tent/popup at all. For long term/frequent usage a house battery in the camper is a way to go but for "once in a while" the tow vehicle fills the bill and saves the overhead of buying and managing a house battery.

Mark B.
Albuquerque, NM