Anyone do this calculation without removing shoes aand socks? - Fiberglass RV


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Old 06-07-2012, 07:27 PM   #1
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Question Anyone do this calculation without removing shoes aand socks?

I've been looking at the conversion formulae for 12VDC battery/inverter produced 120VAC. I have a trailer-mounted AC dorm fridge which draws 1.5 amps starting and .87 amps continuous run for the compressor motor. I have a 1500watt pure sine wave inverter installed supplied by a single group 27 deep cycle batt. I am considering adding an additional group 27. I also have an 80 watt solar panel as a battery charger. By empirical trial, I've powered the fridge for a total of 16 hrs from a fully-charged battery This time lincluded perhaps 6 hrs. of strong sun on the panel. When shut down the battery was showing 12.3 volts. If I add the second battery for additional amp hrs. and assuming optimum clear summer conditions for the pv panel, how many hours/days runtime to 20% DOD and 50% DOD? I figure if I could get this rig to deliver refrigeration for 2-3 days of weekend/holiday camping, that's about all I could ask. The problem is I can plug in the numbers and do the math for amps and watts DC but still don't have a clue about how the "runs a third of the time" rule of thumb as applied to fridges effects the calculation. Please help!!

jack
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Old 06-07-2012, 07:32 PM   #2
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...........The problem is I can plug in the numbers and do the math for amps and watts DC but still don't have a clue about how the "runs a third of the time" rule of thumb as applied to fridges effects the calculation. Please help!!

jack
If it truly runs 1/3 of the time, calculate how long it would run if running continuously, then triple that time. Don't forget to calculate in inverter efficiency- usually about 90%
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Old 06-07-2012, 07:58 PM   #3
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start with your battery amp hours, use 200 for example, which means 80 is available. Take your refer draw 1 amp per hour over 24 hours @ 33% run time would mean 8 amp per day. Disregarding the 90% efficiency right now, thus you can go 8 days before you will need to charge. Now with solar adding back 6 am/hour over 6 hours per day means 36 a/h going back to battery. Thus each 24 hour period you have a negative 8 for refer and a positive 36 for the solar results in indefinite run time with these numbers.
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Old 06-07-2012, 08:44 PM   #4
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Now I'm further confused, Jim. The roughly 1 amp fridge draw is AC. Doesn't that mean 10 amps DC? Not that I wouldn't love your scenario of sustainability as I am not about to use a generator. At present I also don't charge from the tug. Have a seven pin connector but the feeds to the battery are not there. If I could get 2 days and then a day on the road with the alternator charging and avoid the genset, I would be delighted.

I bought a cheap molded tool box tonite (Husky 26"). With a bit of modification, it will be the battery box. Batteries will have to sit end to end as I don't know where else to put the 30lb propane tank except up in the narrow end of the tongue where it already is. I thought about T105s and AGMs inside but I have a practically new DEKA battery decided I'd get another and hope they're fairly well-matched in terms of condition.

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Old 06-07-2012, 09:00 PM   #5
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sorry, I thought the refer was a 12v model. ok 1 amp x 120v=120watts/12v=10 amp in 12 v mode. So using my numbers refer draw over 24 hours @ 1/3 run time =80 amps, with solar adding back 36 your net use is 44 amp hours so with a 200 bank you can go 2 days.
You would have to double your solar to get 4 days or get a 12v refer.
they draw about 2-3 amps in 12v mode which is 1/3 of your current model. This would mean you could go 6 with one solar or 12 days with the double solar. The conversion to 12v using an inverter is inefficient IMHO.
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Old 06-07-2012, 09:14 PM   #6
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Yes, I can see the point of the Novakool NOW but the 70 buck Walmart Wonder will have to do. Thanx for running the numbers, Jim.

jack
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Old 06-07-2012, 10:57 PM   #7
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Freeze some water bottles and put them in the pre-cooled fridge along with your pre-cooled food - frozen is better and you should get 2-3 days of a cool fridge. More if it is cold out less if you are in hot weather. This is without using AC, DC or propane.
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Old 06-08-2012, 08:58 AM   #8
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Now I'm further confused, Jim. The roughly 1 amp fridge draw is AC. Doesn't that mean 10 amps DC? Not that I wouldn't love your scenario of sustainability as I am not about to use a generator. At present I also don't charge from the tug. Have a seven pin connector but the feeds to the battery are not there. If I could get 2 days and then a day on the road with the alternator charging and avoid the genset, I would be delighted.

I bought a cheap molded tool box tonite (Husky 26"). With a bit of modification, it will be the battery box. Batteries will have to sit end to end as I don't know where else to put the 30lb propane tank except up in the narrow end of the tongue where it already is. I thought about T105s and AGMs inside but I have a practically new DEKA battery decided I'd get another and hope they're fairly well-matched in terms of condition.

jack
Some random suggestions:
  • Supplement with ice, as Brooke has suggested, or dry ice
  • Add insulation to the outside of the fridge if space allows
  • Make sure fridge heat exchanger has adequate ventilation
  • Charge trailer batteries directly in camp from TV with a heavy jumper cable set
  • Wire TV with 4 to 6 gauge wire (I buy at recycler for $2 a pound)
  • Add an additional moveable solar panel to supplement power in
  • Fill empty space in fridge with empty containers to minimize air that "falls" out when you open the door
  • Buy a used propane fridge like I did for $50..........
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Old 06-08-2012, 10:17 AM   #9
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Some random suggestions:
  • Supplement with ice, as Brooke has suggested, or dry ice
  • Add insulation to the outside of the fridge if space allows
  • Make sure fridge heat exchanger has adequate ventilation
  • Charge trailer batteries directly in camp from TV with a heavy jumper cable set
  • Wire TV with 4 to 6 gauge wire (I buy at recycler for $2 a pound)
  • Add an additional moveable solar panel to supplement power in
  • Fill empty space in fridge with empty containers to minimize air that "falls" out when you open the door
  • Buy a used propane fridge like I did for $50..........
I would add that we filled the freezer with blue ice packs with good results. They provide enough of a "bank" once frozen to keep fridge cool for many hours (like all day) even with fridge off.

There have been some threads about adding insulation to surrounding space making the ice box hold ice for 4 days. Seems that may offer a big performance improvement even for a fridge. So I second that as a good suggestion.

Recycler for heavy wire hmmm. Tom you are a constant source of ideas that appeal to my git-r-done cheap side.

We had a 12 volt RV compressor fridge (lower power draw than your example) no solar but three deep cycle batteries. We could go a week but only ran the fridge during the cooler part of the day (morning & evening/night) letting the blue ice hold the temp during the hotter part of the day when heat exchanger would be less efficient.

I'm not sure of how the extra run time to freeze blue ice vs time one can remain off would graph to find the efficient sweet spot, I think most of our "gain" came from ambient air being cooler when we ran the fridge.

We also kept drinks in ice chest, and had a an ice water jug for a cold drink or source of ice cubes for cocktail hour. Both of which avoided a lot of open and close activity on the fridge. Those two would only last 3 maybe 4 days depending on if we had block or cube ice in the cooler. By that time leaving the woods and going to a store sounded like "fun".
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Old 06-08-2012, 01:02 PM   #10
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................Recycler for heavy wire hmmm. Tom you are a constant source of ideas that appeal to my git-r-done cheap side...................
As I've mentioned before, electricians scrap out the little short 25' pieces off 600 foot rolls, so most of the wire that I've purchased was actually new. It sells for a tiny fraction of Home Depot prices.
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Old 06-08-2012, 01:08 PM   #11
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man thats too much thinkin....pass me one of them beers outa there while there still cold wouldya?
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