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Old 08-07-2022, 05:37 PM   #1
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Dometic DM2852 power usage

I just ran a test to see how my absorption refrigerator would run on battery. In this case a 120VAC/LP so the battery test involves an inverter. The reason I did this is that normally I'm on 120VAC at camps or home, and on LP while traveling or boondocking. Our ferry system doesn't like LP flames on the boat so I was curious what the inverter usage was. Yesterdays test running nearly 8 hours in a 75+ degree ambient temperature, resulted in an average of 22.46A per hour (174.68A total). I can handle a ferry ride with no problems. Of course when this thing dies, I'll be installing a residential refrigerator.
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Old 08-07-2022, 05:55 PM   #2
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How many amp hours is your battery bank?
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Old 08-07-2022, 07:16 PM   #3
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I just ran a test to see how my absorption refrigerator would run on battery. In this case a 120VAC/LP so the battery test involves an inverter. The reason I did this is that normally I'm on 120VAC at camps or home, and on LP while traveling or boondocking. Our ferry system doesn't like LP flames on the boat so I was curious what the inverter usage was. Yesterdays test running nearly 8 hours in a 75+ degree ambient temperature, resulted in an average of 22.46A per hour (174.68A total). I can handle a ferry ride with no problems. Of course when this thing dies, I'll be installing a residential refrigerator.
The term “amps per hour” doesn’t really have a meaning. An Amp is a rate, like gallons per hour, miles per hour, or Watts, so saying amps per hour has no meaning. My guess is that if you have a mid sized refrigerator with around a 300W heater, the inverter uses 22.46A to supply the heater when it is on. It uses 22.46A the instant it’s turned on and continues to use 22.46A whenever it’s on. If it stays on for 7.8 hours without cycling, which is unlikely, it would have used 174.68AH. But the power the inverter uses isn’t the whole story. The control circuits and fans draw another 2 or 3 A directly from the battery
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Old 08-07-2022, 07:36 PM   #4
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He said it averaged 22.46 amps per hour OR 22.46 AHs. Same thing.

It will draw more then that while running, something like 33 amps, but to get a true AH reading, you have to include down time.

22.46 AHs for 8 hours is 174.68 AHs.
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Old 08-07-2022, 07:44 PM   #5
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He said it averaged 22.46 amps per hour OR 22.46 AHs. Same thing.

It will draw more then that while running, something like 33 amps, but to get a true AH reading, you have to include down time.

22.46 AHs for 8 hours is 174.68 AHs.

No. Amps per hour would be Amps divided by hours. An Amp-Hour is Amps multiplied by hours. The difference is fundamental and critical to making the math work. An amp is a Coulomb per second. A Watt is a Joule per second.
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Old 08-07-2022, 07:57 PM   #6
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Dometic DM2852
120VAC 325W element at 2.7A
12VDC Climate Control Heater at 0.5A (always ON if Fridge ON)
12VDC Fan (optional) at 0.4A
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Old 08-07-2022, 08:31 PM   #7
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2.7 amps at 120 volts = 27 amps at 12 volts. Add 15% for inverter conversion losses gets you to about 30 amps.
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Old 08-07-2022, 11:36 PM   #8
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I have 3 batteries, one 280Ah and two 277Ah in parallel. I totaled the amps available in each battery at the start and again when completed. 174.68A went away. Bottom line, it will work for a ferry crossing (typically about an hour).
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Old 08-08-2022, 12:21 AM   #9
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Bottom line, it will work for a ferry crossing (typically about an hour).

"typically about an hour"..... so about 20AH used. Or, you could probably get by with just turning off your refrigerator and not opening the door for an hour...



Your reply ("about an hour") would have been even funnier had you waited until there were 30-40 more back and forth posts by others first....lol


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Old 08-08-2022, 12:50 PM   #10
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"typically about an hour"..... so about 20AH used. Or, you could probably get by with just turning off your refrigerator and not opening the door for an hour...


This is what we do!

Unfortunately, they want the valve shut off at the tank. When you disembark the ferry, there's not always a convenient place to pull over, get out, and open the tank valve. So, sometimes that hour turns into 2 or more.
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