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Old 06-03-2014, 08:04 PM   #1
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Operation of 3-way fridge

My rig has a 3 way fridge - AC/DC/LP. In the past, I've had AC/LP without any complaints.

According to my Owner's Manual, when driving, the alternator charges the house battery. When plugged in, the house battery is charging via the PD9200.

So I ask you, if you are traveling (driving) would you just leave it in DC and save your LP? When you get to your destination, and can plug in, would you just leave it in DC, or switch to AC? Why?

My first instinct, if I were going someplace with no hookup, would be to switch to LP and save my battery for other uses. Or would you keep it on DC and run your genset each morning to charge back up?
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Old 06-03-2014, 08:35 PM   #2
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Most RV fridges are really only 2 way,LP and AC, controlled by the 12 v battery that runs controller, interior light, LP ignition, etc. They used to make 3 way fridges, but a 12 v heating element is very wasteful of your battery power and I think discontinued. What make, model refrigerator do you have?
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Old 06-03-2014, 08:50 PM   #3
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It's a brand new Dometic RML 8330:

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Old 06-03-2014, 09:27 PM   #4
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The Dometic web site doesn't show a manual (or refrigerator) RML 8330, but the video and other web sites talk about it. I did find out it's a German import. It is a 3 way fridge, working (in order of auto selection) 120 v AC, 12 v DC, or LP gas. This is a European safety issue, they prohibit LP use in a moving vehicle. As I mentioned before, a 12 v DC heating element is very consuming of your battery power. It should only be used when the engine alternator or bank of solar cells can provide energy. The refrigerator shuts down 12 v and switches to LP if voltage reaches 10.5 v. Unfortunately depleting a battery to 10.5 v very often will shorten battery life. Batteries shouldn't be drawn below 11.9 v, which is at a 50% discharge. This chart shows battery state of charge by voltage:



So, I'd do as you suggest, drive in 12 v mode. When hooked to shore power I'd put it on 120 v. power because the heating element is more robust than the 12 v element and it will cool and recover faster. Unless you have a really tiny LP tank, the flame is pretty small and it won't consume all that much LP in gas mode.
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Old 06-03-2014, 09:36 PM   #5
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Yes, I understand all that about batteries.

The manual states that you can experience flame-outs above 3200 ft elevation, so this refrigerator needs to be checked when traveling at those elevations. I hadn't heard of that before.

Running on LP last week, it hardly used any gas over 3 days, nor any appreciable battery power. It will be interesting to test consumption on DC only, as I suspect it would be 2-3 amps like a chest type compressor refer/fridge I have, or do you think more?. I guess what I don't know for sure is is it more efficient (when plugged in)to just run on AC or leave on DC while the charger continues to charge the battery.
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Old 06-03-2014, 09:55 PM   #6
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Are you sure about the 3,200 ft. elevation? That wouldn't get you across Kansas! Perhaps 3,200 meters, which is 10,500 feet. At that elevation I would expect problems with O2, but not at 3,200 ft. My brother used to race tune motorcycles and he didn't bother until they got over 5,500 ft.
As I said, the 120 v heating element is probably bigger than the 12 v. one. Stick a remote thermometer in the fridge and monitor how fast it cools down after being open to see which works faster.
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Old 06-03-2014, 09:58 PM   #7
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Yes. There is the one reference - but it says "could" and not "would". And then goes on to say this is normal operation and not a failure (warranty claim). I suppose there are lots of reasons why your fridge to shut off in LP mode - particularly when driving. I've never experienced something like that. I was at 5400 feet last weekend for 3 days and had no issues.
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Old 06-03-2014, 10:07 PM   #8
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I really don't think it will be an issue, I still think it was lost in translation from German (metric) manual. I've used all my gas appliances, (oven/stove, furnace, water heater, and refrigerator at 9,500 ft. and not given it a thought or noticed anything different. Happy Camping!
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Old 06-04-2014, 12:09 AM   #9
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My rig (2005) has a Dometic 3 way fridge like what you describe. I run it in 12v when on the road. I actually like that the LP isn't on while driving. I know that many drive with the LP running because that is the only option, but there is a greater measure of safety not needing to run LP and having that whole system turned off.

If you've got it, may as well use it.
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Old 06-04-2014, 06:44 AM   #10
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Just my 2 cents. We run dc on the road and ac in campsites. If we had no ac at campsites we would run LP. Have yet to use the LP feature. Maybe I ought to test that option someday and make sure it works.
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Old 06-04-2014, 12:18 PM   #11
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I am not sure if it still true but the 12 volt used to be reasonably OK for maintaining temperature but was woefully inadequate in raising temp in a reasonable amount of time. In other words, the 12 v was good on the road but not a good option when parked and presumably getting in the frig often.
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Old 06-05-2014, 09:16 PM   #12
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If you have an invertor run it on 120V when driving the engine alternator will keep the battery charged, switch to propane if parked for a while (lunch-dinner-hike) and when connected to shore power then you use 120v mode. If you are using propane make sure the fridge and contents are cold as propane will maintain but not cool very well
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Old 06-06-2014, 11:25 PM   #13
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I have run on LP driving down the road for over 30 years and switch to AC when plugged in. My Class B has a 3-way but I still just use LP when traveling. It really doesn't use very much propane on LP. When you use 12V it has a device that after a short time turns it on and when you shut your key off it shuts off or at least that how mine works.
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Old 06-14-2014, 08:12 AM   #14
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In my opinion, I would first get my fridge cold by hooking up to electric the day before we leave.. Now your starting out with a cold fridge so you can have the option of gas or battery and each would and should hold the fridge cold until you reach your destination. At that time I would use electric if available (why use your energy supply when you can use the strongest energy supply that's included with your site) to keep your fridge the coldest. For us, when we take our first trip in the new Trav, we are going to try the battery option on the fridge, maybe just because we've never had that choice and want to see how well it's going to perform. And when we stop for fuel we won't have to worry about remembering to shut the gas down when filling up. So to answer the original question, I would turn my fridge switch to AC when connecting my land line.(I assume that now power to the fridge will no longer go through my house battery, just come directly from the land line.
So, Happy Trails!
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