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Old 08-07-2013, 19:28   #1
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DC Frig amps

I have a AC/DC refrigeration unit. I usually just daysail for a few hours so I just unplug from shore power and the it stays cold long enough to keep the beer cool. This weekend we are leaving, with some other boats, on a cruise to the Lake Erie Islands. The first day should take about 6 hours to get to the first port. I would like to turn on the frig for some of that time.
The unit uses 40 watts when on 120 VAC.
After reading some things here in this forum and elsewhere, I get the usage as 3.33 amp/hr at 12 volts and with 90% efficiency give me 3.66 amps/hr usage. (The mfg. doesn't publish DC watts or amps usage). My batteries are 100 min RC and 25amps drawing down to 7.25 volts or so the manufacturer says. So I think I can safely run the Frig for an hour or so without drawing down the house battery too far. I have 2 batteries.
Does this sound right or am I way off somewhere?

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Old 09-07-2013, 04:59   #2
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Re: DC Frig amps

Yes, 3-4 amps is typical which won't run down your battery for just a few hours use,.


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Old 09-07-2013, 05:34   #3
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Re: DC Frig amps

DC compressors are amazingly efficient so you will probably be fine, but I would stick a good multimeter on it and figure out what it actually draws. Deepends on the make/model of compressor and the specifics of your installation, but the 3.33 amps you are using is typically for a small DC compressor like a Danfoss BD35.

Note that typically you are better off just to leave the unit powered up because once the box is cooled down it takes a lot less energy to keep it there (compress only cycles on as necessary to maintain temp). Turning on and off will ultimately use more juice because it has to chill down all over again.

One of the specs you state on your batteries are "RC" (Reserve Cranking AMPS I assume). This is normally given for "start" type batteries not "deep cycle" batteries. Are your batteries deep cycle?
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Old 09-07-2013, 15:50   #4
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Re: DC Frig amps

Yes both batts are AGM Exide group 34. I got the RC from their Website.
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Old 09-07-2013, 17:55   #5
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Re: DC Frig amps

Put a block of ice in there to help it.
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Old 09-07-2013, 20:12   #6

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Re: DC Frig amps

The "reserve capacity" usually refers to an SLI (car starting) battery not a deep cycle. A quick look at that shows a 60AH capacity for the battery, perhaps yours is higher if it is a deep cycle. But with any AGM battery, if you pull it below 50% capacity you shorten the life. Below 70-80% capacity, you shorten it even more dramatically.
And if you have an SLI (car starting) battery, deep cycling it radically shortens the life, no matter what kind it is.

Since "watts is watts" and ignoring the ac/dc conversion factors, let's say your ice box pulls 4 amps, times a six hour cruise that's 24 amphours. No problem for a deep cycle battery with a 60AH capacity, no problem for an SLI battery as long as you don't make a habit of it. SLI batteries are mainly designed to never be used for more than 10% of their capacity, as few as 5-6 deep discharges (80-90%) can ruin them. Those kind of numbers will vary with the maker, the battery type, and how long the salesman's nose grows as he tells you about it.

If you really want to make sure you have no power problems, see if a supermarket or bait shop near you sells dry ice. Stick a bag or two in the cooler before you go, and it may not even turn on during the whole trip.

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