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Old 08-25-2014, 01:48 PM   #11
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Ironically, a conventional compressor frig with 12 volt power uses probably only 25% as much power as a RV model.
I wish that were true. Even the smallest marine compressor fridges drew 5 amps running. The trick was to insulate the fridge enough to drive the "on" time to less than 50%. The rule of thumb was you had to add 2" insulation, and preferably 4" around the stock ice box to get a reasonable (non-battery killing) duty cycle. Just like in our campers, space for insulation comes out of space for the fridge or other valuable storage. A close friend installed one of the units in his camper, but had horrible results due to lack of insulation.

If you wanted a decent-sized fridge (or a freezer) on a sailboat, you either had to go to an engine-driven compressor with hold-over freeze plates (again loses space in the fridge) or 110 volt generator power.

Even though compressor units are more somewhat more efficient, the requirement for electricity to drive the compressor is a killer in small campers. The electricity issue is why much of the less-developed world still uses ammonia absorbtion refridgeration. It's simple, it works, and it's much easier to carry enough LP than it is to maintain a battery bank. In the marine world, LP is a frowned-upon fuel, and open flames are a no-no.

just my experiences
Fred W
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Old 08-25-2014, 02:35 PM   #12
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I wish that were true. Even the smallest marine compressor fridges drew 5 amps running. The trick was to insulate the fridge enough to drive the "on" time to less than 50%. The rule of thumb was you had to add 2" insulation, and preferably 4" around the stock ice box to get a reasonable (non-battery killing) duty cycle. Just like in our campers, space for insulation comes out of space for the fridge or other valuable storage. A close friend installed one of the units in his camper, but had horrible results due to lack of insulation.

If you wanted a decent-sized fridge (or a freezer) on a sailboat, you either had to go to an engine-driven compressor with hold-over freeze plates (again loses space in the fridge) or 110 volt generator power.

Even though compressor units are more somewhat more efficient, the requirement for electricity to drive the compressor is a killer in small campers. The electricity issue is why much of the less-developed world still uses ammonia absorbtion refridgeration. It's simple, it works, and it's much easier to carry enough LP than it is to maintain a battery bank. In the marine world, LP is a frowned-upon fuel, and open flames are a no-no.

just my experiences
Fred W
My boat unit with the plates where R-12 when installed, I was able to get that all over the world, but when I returned to the USA I had to convert to r-134, mine also was 110v and ran off a inverter with a 3 stage charger(pure sine wave) supplied by 4 battery banks of D-8's charged by solar plus a water maker that produced 7 gals per hr. from sea water. Plus ran my navigation system, short wave and weather fax. All done by solar, if I didn't produce enough juice because of clouds, I had to kick on my 50 hp engine with a 150 amp alternator. for 2 hr's per day. Those were the good old days.
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Old 08-26-2014, 10:34 PM   #13
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My point is that a DC heater element on 24 hours with an 8 amp draw uses at least 4 times more energy than a 4 amp compressor that cycles on and off. Please compare dc to dc and not DC to Propane.

A RV refrig with Ammonia absorption, should not be used on 12volt unless no propane is available. That was the OP original Q.

USE the PROPANE for an RV frig, unless plugged into shore power.


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Old 08-26-2014, 10:57 PM   #14
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Originally Posted by pgandw View Post
I wish that were true. Even the smallest marine compressor fridges drew 5 amps running. The trick was to insulate the fridge enough to drive the "on" time to less than 50%. The rule of thumb was you had to add 2" insulation, and preferably 4" around the stock ice box to get a reasonable (non-battery killing) duty cycle. Just like in our campers, space for insulation comes out of space for the fridge or other valuable storage. A close friend installed one of the units in his camper, but had horrible results due to lack of insulation.

If you wanted a decent-sized fridge (or a freezer) on a sailboat, you either had to go to an engine-driven compressor with hold-over freeze plates (again loses space in the fridge) or 110 volt generator power.

Even though compressor units are more somewhat more efficient, the requirement for electricity to drive the compressor is a killer in small campers. The electricity issue is why much of the less-developed world still uses ammonia absorbtion refridgeration. It's simple, it works, and it's much easier to carry enough LP than it is to maintain a battery bank. In the marine world, LP is a frowned-upon fuel, and open flames are a no-no.

just my experiences
Fred W

it is true when on 12 volt.


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Old 08-27-2014, 04:38 PM   #15
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it is true when on 12 volt.


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I disagree. Make sure you are comparing like cooling capabilities - watts/BTU, or at least same size fridges. A 5 amp/12 volt freon compressor - I've never seen one actually draw less than 5 amps - won't cool more than 3 cubic feet, and less if you want a 50% or less duty cycle. I can cool the same 3 cubic feet with a 75 watt (6.25 amp) element running continuously on ammonia absorption. So at best, the freon compressor rig is slightly more than twice as efficient.

As things scale up in size, the freon compressor does better and the ammonia absorption does worse, particularly on 12 volts. As size of cooled space goes up, the ammonia absorption needs added fans and heavy duty wiring to maintain performance. Freon compressors don't scale down well below a minimum size - the dorm refridgerators compared to a full size fridge are an example of how much less efficient small freon systems are.

I assume that because this thread is located in the truck camper section that we are talking a small fridge, certainly less than 5 cu ft. And in that size range, a freon compressor rig is just not 4 times as efficient as ammonia absorption.

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Old 08-29-2014, 04:19 PM   #16
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Gosh Fred, I sure don't want to disagree. But you are being ENTIRELY theoretical. and it could be as easily 4 times as 2 times.

THE POINT IS TO NOT RUN YOUR AMMONIA FRIG ON 12 VOLT, which is the OP's Concern.

Lets let it go.

I've been sailing for 40 years and have had a variety of compressor frigs and NON of them have drawn more than 4 amps, either holding plate units such as the technautics or the adler barbour conventional. And mostly they have a 25% duty cycle, not 50%.

Your experiance may vary, obviously, but let me have mine.



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Old 09-02-2014, 05:25 PM   #17
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For the OP, that's a very nice unit. However, all the add -ons may have parasitic electric draws that add up (in addition to the heater and refridgerator). I'm talking items like the propane and CO detector, battery monitor, fire detectors, any tank level indicators, stereo, etc. Those combined can add up to an amp or two continuously. Which is why the battery should be disconnected when the unit is not being used.

The amp draw of the fridge can be found from the specs in the fridge manual - take the DC element wattage and divide by 12. Typical draws for the smaller fridges without freezers in fold-down campers are 5-6 amps when running on DC. Your heater fan also draws 4-5 amps when running.

The problem is that your battery is likely a single size 24 (unless you deliberately upgraded). The capacity of said battery when fully charged to fully discharged is about 80 amp hours. Usuable capacity (that doesn't ruin the battery) is only 40 amp hours. As you can see, you can run out of battery real fast with both the heater and fridge.

I have an A-frame (A122) where the only 12 volt loads when dry camping are smoke alarm, propane and CO alarm, LED interior lights, water pump, and the biggie - the heater. Fridge is running on propane in that situation. We realized a single size 24 battery was not going to cut it for a long weekend in Colorado fall/spring weather. So we added a second size 24 battery to double our reserve for running the heater.

Because it is a towed trailer, battery charging while driving is a natural with the tow vehicle fully wired with the 7 pin connector. So I arrive at the site with a fully charged batteries, and with the fridge on DC. I switch off the fridge before I disconnect and then re-start it on propane or AC after disconnecting and leveling.

Other have said it - use AC for the fridge when available. Use propane when vehicle is not in motion, and no AC available. Use DC when driving. This assumes your Palomino battery is being charged by your truck alternator when the engine is running.

I'm not familiar with truck bed campers, so I'm not sure that the electrical connection between camper and truck is actually happening. The brochure talks about a battery charging station and battery quick disconnect - are those correctly set up for your camper battery to charge while driving? If the truck is not supplying power to the camper, putting the fridge in DC mode while driving will eat the battery in a matter of hours.

OTOH, I would not want to drive a truck with camper attached in the bed, and a fridge with open flame running (propane operation). I would certainly want to turn the fridge off of propane while refueling the truck. So making sure that the truck is charging the camper while driving is important (to me).

for garbonz, from the Adler Barbour Cold Machine Manual:
AC/DC O
PERATION

For the ColdMachine, figure approximately 5 amps when running. The average draw, as the ColdMachine cycles on and off, is 1.8 to 2.4 amps for most 4 to 8 cubic foot/0.11 to 0.23 cubic meter iceboxes with average (3 inch/7.6 cm or more) rigid polyurethane foam insulation.

(My comment) The duty cycle calcs based on these figures come out a minimum of 36% for the 4 cu ft fridge with 3" (or more) rigid foam insulation.
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Old 09-02-2014, 05:45 PM   #18
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Just make sure you have a diesel truck and don't worry about it. Unless someone has a major whoops of spillage, it is infinitesimally rare for the lower explosive limit of gasoline in air to be reached during refueling, even relatively close to the point of flow.

Among the things there are to worry about, that would not be one. If you are smart and have a diesel truck, your problems are solved. I don't even like driving in a gasoline vehicle AT ALL, they are just a bomb waiting to happen.

MOST truck campers are wired exactly like a trailer is, and use their own running, brake and turn signals since the back of the camper blocks the truck lights. Many have a standard 7 pin plug on them and a short pigtail to run from the truck trailer plug to the camper. Lance have their own proprietary plug with high gauge wire to carry the current and thus are well equipped to handle the refrig load.

Most recent Refrigs (my 2005 included) don't even have a 12 system.

I don't know what the point of the comments on the Adler Barbour are, but I gave up on going down that dark alley.

Regards
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