I just bought an ARB fridge/freezer. For those of you who don't know what this is, it basically looks like an ice chest but has in it a highly efficient compressor that allows it to maintain a temperature as low as zero degrees while running off a standard 12 volt outlet. I will use it as a freezer in our RV and then also use it in our car when we take trips without the RV. While it doesn't necessarily need an auxiliary battery when I use it in the car, I still want to have one. So I am trying to figure out what size of deep cell AGM battery I need.
This unit also runs off of AC and so I hooked it up to a Kill-A-Watt meter for three days. It was set on 15 degrees and the room temp averaged about 75. This was my feeble attempt to try to compensate somewhat for the lack of summer time conditions. Anyway, it ran for three days on 0.91 kWh's. I thought it would be easy to find info about how to translate AC kWh's into DC amp hours, but my searches have come up empty. I am pretty much lame when it comes to electricity. Any chance someone here could show me how to figure out what size battery I might need to run this fridge for three days, under similar conditions, off of a deep cycle AGM?
Bill, Carol & Striker The Cat
2005 Everest 323k, Anderson gooseneck adapter
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bearsnob wrote: I just bought an ARB fridge/freezer. For those of you who don't know what this is, it basically looks like an ice chest but has in it a highly efficient compressor that allows it to maintain a temperature as low as zero degrees while running off a standard 12 volt outlet. I will use it as a freezer in our RV and then also use it in our car when we take trips without the RV. While it doesn't necessarily need an auxiliary battery when I use it in the car, I still want to have one. So I am trying to figure out what size of deep cell AGM battery I need.
This unit also runs off of AC and so I hooked it up to a Kill-A-Watt meter for three days. It was set on 15 degrees and the room temp averaged about 75. This was my feeble attempt to try to compensate somewhat for the lack of summer time conditions. Anyway, it ran for three days on 0.91 kWh's. 910 watts I thought it would be easy to find info about how to translate AC kWh's into DC amp hours, but my searches have come up empty. I am pretty much lame when it comes to electricity. Any chance someone here could show me how to figure out what size battery I might need to run this fridge for three days, under similar conditions, off of a deep cycle AGM?
Because the compressor is 12 volts, then if it uses 910 watts in 3 days without opening the door, or adding thawed foods that need the heat removed, (this take a lot more energy) then you can do this with about 80 amp hours of power, if the voltage stays above 12 volts. So to do that, you will need about 220 amp hours of power, pair of group 27 batteries or a pair of T-105 golf cart batteries, and you are set.
If you want AGM, consider a pair of them, around 100 AH each, and you will get the job done without discharging the battery below 50% state of charge, and the battery will last much longer.
Watts are watts, whether 120V AC or 12V DC.
.91 KwH = 910 watts drawn for one hour or 91 watts drawn for 10 hours.
An amp at 12V DC = 12 watts or a rate of 1 amp-hour
To draw 910 watts from a 12V battery would require 910/12 = 76 amps, or 7.6 amps to draw .91 KwH over a 10 hour period.
As a previous poster said, to draw a battery to no less than 50% would require a capacity of 2x76 or 152 amp hours. 200+ would be better as the above calculations do not allow for any inefficiency.
Keith J.
1999 Sunnybrook 27RKFS Fiver.
2005 GMC Sierra 2500HD CC/SB/DA 2WD, LBZ air cleaner, 52 gal Titan tank, Bilsteins, Line-X, Westin steps, Prodigy, Retrax cover, 16K Superglide, 5th-Airborne pin-box, Multi-vex mirrors, TST TPMS.
Try a small battery running the cooler, and the kill-a-watt on a charger. Run the charger for 2 or 3 days without the cooler on the kill-a-watt to see what the charger wastes, then a second time with the cooler. subtract the smaller from the larger, divide by total hours, and you have a decent hourly wattage useage.
about 25.33 amp hrs per day from 12v if it runs at the same efficiency
there will be some difference as you will open it more often when using it in the car
next time you test it read the watts draw/used while the compressor is running
and
simulate car use age put some food in it and open it for drinks and to fix lunch etc..
this will give you a more accurate power use reading
IF your readings hold to the current level
one 80ampHr battery will work for road use If it gets a daily charge
charged while driving and overnight use to keep the freezer going
Options, always have options, and the journey goes much smoother ....
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Wow, this really seems to demonstrate how limited we are on battery power. In my mind I was thinking that I could use a much smaller battery simply because the AC power draw seemed so small. Here in Oregon, we pay less than 12 cents per kWh. With the fridge test coming in just under 1 kWh, I was thinking that was near nothing. Apparently not if if I'll need two Trojans to do the same thing. No wonder I'm so happy to get back to shore power after boondocking for a while.
So I think I need to rethink this because I really don't want to put an extra 150 lbs in my high mpg small car. There are other options. One thing I am always hearing about these fridges is people saying that they run them for days off their regular car battery (the fridge has a protection circuit in it that prevents you from drawing the battery down too far). I'm guessing these people are failing to mention that they are running the engine during those days. If my memory is right, a car battery charges quickly both because of the nature of the battery and the fact that the alternator is putting out more than 80 amps. But I think that means it discharges quickly as well.
I think I will defer to you guys at this point. What do you think might be my ideal setup for this fridge given these factors:
1. I will be driving the car every day for at least an hour 80 percent of the time.
2. Maybe twice a month (at most) the car would sit idle for up to a three day stretch of time. Obviously, I could run the car at these times, but I would prefer not to.
3. I will have with me a light weight 800 watt inverter generator that can power a 20 amp charger.
4. I have the option of running this as a freezer. What that means is I can freeze the contents while driving by setting it at zero. Then, when I turn the car off, I can turn the temp up to 32 to greatly decrease the time the compressor will run. Contents that can't handle the freezing temps would be placed in a small traditional ice chest. Cold drinks can usually stay in the freezer if placed in the small area on top of the compressor from what I'm told.
liborko wrote: 0.91kWh in three days equals 0.91kWh:3=0.30kWh/day
0.30kWh/day:12V=0.025kAh/day or 25Ah/day.
You will have to replace 25Ah every day you run the fridge, either by running the engine alternator, generator or solar power. Make your choice.
That actually sounds pretty good. A deep cycle AGM made for use in electric scooters is small (8x8x6), reasonably light at 23 lbs, and rated at 35 ah. But the rub, if I am understanding things right, is that I don't actually have 35 ah's available to me in that battery because I should only discharge it to 50 percent. Is that right? Does that mean the battery I am describing really only has about 17 available amp hours?
bearsnob wrote: Does that mean the battery I am describing really only has about 17 available amp hours?
Yes. You'd need a 50Ah battery minimum IF you only intend to use 25Ah per day. BTW, some of those smaller AGM's don't like high amps or high voltage for charging. Make sure you check the specs on the battery if you go that route.
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