Running 12V refer on battery only - Page 2 - Fiberglass RV


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Old 02-15-2009, 08:19 AM   #15
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- When ice is melted, then remove and discard bag. The principle here is to reduce the conduction area on the ice by draining the meltwater off (yes we are discarding some negative Btus but the ice will last longer)
One trick I've used on a boat (since I've been boating a lot longer than "egging"), is to drain that cold meltwater off into a bucket, and then use it to cool (or at least pre-cool) canned or bottled beverages. That way you are removing the conduction of the meltwater from the icebox, yet using the coldness of it instead of just draining it away.

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Old 02-15-2009, 12:19 PM   #16
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I like where this thread is headed - thanks for all feedback and PM so far.

I'm not an expert at this type of calculation, but to close the loop on the compressor fridge option, bear with me while I attempt to do some amp-hour math here based on some of your responses and further research:

Novakool site claims its small refrigerators consume about 30 watts.

Backwoods site claims the Novakool 4.3 cu.ft (bigger than what I need or want) runs about 30% of the time on a 70 degree day. Let's assume that running time doubles at 100 degrees. But the site also mentions that adding extra 2" foam insulation around the box (which I'm already doing with my 3-way unit) will cut run time in half.

So to keep things simple and add a comfortable fudge factor, let's say a smaller 2.5 cu.ft. 30 watt Novakool with extra insulation runs about 40% of the time over 24 hours in hot weather. That's about 300 watt-hours per day, or 25 amp-hours.

Solar panels are even more unknown to me, but let's say a 80-watt panel is able to produce 40 watts during 4 hours on a good day. That's 160 watt-hours or 13 amp-hours, about half of what the fridge will use.

If a 100 amp-hour battery actually provides about 50 amp-hours, a new and full battery should be able to run the fridge, in theory, for about 2 days. Each good solar panel day would extend that by half a day. I get a feeling that things would be far worse in the real world, though.

Make any sense?

The ice block option seems more popular and I can see why, but we've had poor results dealing with ice blocks ourselves. Maybe I had the wrong technique or equipment, but hunting for ice and the daily water removal routine was far more trouble than just switching propane tanks every couple weeks.
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Old 02-15-2009, 12:23 PM   #17
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Good suggestion Raya,

We sometimes have 2 coolers, one for beverages, one for food.

Drain the beverages, then use melt water from the food cooler to cool the beverages. Or one could keep the food cooler higher and run a hose between the two drain spouts and do it automatically as the ice melts.
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Old 02-15-2009, 12:31 PM   #18
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Maybe I had the wrong technique or equipment, but hunting for ice and the daily water removal routine was far more trouble than just switching propane tanks every couple weeks.
Not taking the ice out of the bag or breaking it up helps a lot on the first problem and the open drain helps on the second. I thought of the drain when I removed a defunct ammonia-cycle reefer in my old Jayco and replaced it with a big cooler that I already owned -- No way was I going to be taking the cooler and food and water back out to drain...
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Old 02-15-2009, 06:54 PM   #19
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Quote:
1) . . . Novakool site claims its small refrigerators consume about 30 watts . . .

2) . . . runs about 30% of the time on a 70 degree day . . .

3) . . . [let's assume that a] 30 watt Novakool with extra insulation runs about 40% of the time over 24 hours in hot weather. That's about 300 watt-hours per day, or 25 amp-hours . . .

4) . . . let's say a 80-watt panel is able to produce 40 watts during 4 hours on a good day. That's 160 watt-hours or 13 amp-hours, about half of what the fridge will use . . .

5) . . . If a 100 amp-hour battery actually provides about 50 amp-hours, a new and full battery should be able to run the fridge, in theory, for about 2 days . . .

6) . . . Each good solar panel day would extend that by half a day. I get a feeling that things would be far worse in the real world, though . . .

Make any sense?
Excuse my breaking your post down like that, but it makes it easier to refer back to as I answer your question.

1) 30 watts at 12 volts is 2.5 Amps (30 watts/12 volts) . Pretty good for a compressor motor!

2 & 3) 30% * 24 hours * 2.5 Amps = 18.75 Amp hours at 70 degrees; 40% * 24 hours * 2.5 Amps = 24 Amp Hours in hot weather . . . but I think your hot weather estimate is a wee bit optimistic. Even if the refrigerator box is better insulated the compressor still has to work much harder on a hot day to pump heat from the inside of your 'fridge into the hot air outside. Since you're trying to come up with a worst-case scenario I'd tend to stick with the more pessimistic 50% estimate for a 95 degree day, 60% for 100 degree day I've seen elsewhere. So 60% * 24 hours * 2.5 Amps = 36 Amp hours.

4) Different kinds of solar panels have different performance curves for different angles of the sun relative to the solar cells. Monocrystalline panels have the best performance, with a power production curve that produces very meaningful power levels even when the sunlight hits the panel at an angle*. Translated into people-speak this means that 50-watt monocrystaline panel really can make 20-25 amp-hours of 12v electricty on a sunny day.

It's worth mentioning, however, that these numbers assume several things, like 12 hours of sunlight, but during the hottest parts of the year the sun is up longer than 12 hours, so you actually gain a few hours of solar energy production during the part of the year when your 'fridge is likely to work hardest.

The downside of that summer advantage is that you loose out on solar energy production during the winter when the sun is up for less than 12 hours and is always at a steeper angle on the horizon. I'd count myself as lucky to get 7 amp hours of power from my 50 watt panel during the winter.

But if you don't boondock during the winter it's very realistic to think a single 80-amp monocrystaline panel could meet the needs of your 30-watt 'fridge even on a hot day.

5) You have the right idea. using my "worst case" numbers you'd be closer to 1.4 days from a single 100 amp battery.

6) You're right. Your real-world figures will probably be less than what we worked out above. Little things, like campsite tree shadows, can take a big bite out of solar power output, so I'd plan to have some excess capacity. Two 80-watt monocrystaline panels would do the trick during the regular camping season and give you extra power to run LED lighting and a few other goodies.

But let's do a reality check: Solar panels cost about $0.50/watt, so an 80 watt solar panel will run you about $400. Add to that the cost of a solar charge controller and an extra battery and you're spending $500 before you've even bought the 'fridge. Not only will $500 will buy a whole lot of propane, but after you factor in other maintainance costs for the solar system and compressor 'fridge (your batteries will wear out, your 'fridge compressor will give out long before you start having problems with the ammonium hydride evaporative cooling system in a propane-powered 'fridge), and buying a few dollars of propane every year starts looking a whole lot more attractive.

If you're having problems with your propane 'fridge keeping your groceries cold on a hot day, I'd suggest looking into ways to improve the efficiency of your existing 'fridge's cooling system. If your trailer doesn't have a convectiion flue for your fridge (many don't), consider installing one, or add a small muffin fan (0.28 amps) and a temperature switch that blows air over the coils when outdoor temps crest 90 degrees. These are much more cost effective -- and possibly even more environmentally friendly -- than the compressor 'fridge/solar alternative.

* For the science geeks out there: You'll appreciate the test results I came up with when I was playing with my 50 watt Siemens panel one sunny December day just before Christmas. Testing my panel's output as I rotated it in relationship to the sun I measured a maximum 2.7 amps output with the panel at 90 degrees to the sun. (Remember this is almost winter Solstice, when the Sun's energy is at its lowest.) When I rotated the panel I found it's output was fairly close to (Max Output)Cos(X), where (X) was the incident angle.
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Old 02-15-2009, 07:30 PM   #20
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See, I knew this was going to be a fun topic when you guys got involved!

Parker
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Old 02-15-2009, 08:23 PM   #21
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One more quick note on ice box (or cooler) drains: If you are going to have an "always open" free-running drain (someone suggested potentially running one down to a lower box), then you'll probably want to put a "trap" in it. This would be a loop that would hold a bit of water, and which will keep the cold air from "falling" right out of your ice box. Boats sometimes have an open drain on the icebox, and when they do they (should) have this cold trap.

Raya

PS: Of course if you have a typical cooler with the capped drain that you only open to drain the water off, then this isn't a concern.
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Old 02-15-2009, 10:38 PM   #22
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One thing that has not been noted is that the Norcold DC040 recirculates the heated air into the cabin through vents in the lower front of the unit. There is no venting to the outside. That could make quite a difference in cabin temperature in the summer.
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Old 02-21-2009, 06:37 PM   #23
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I have a DC0040, and here are my personal opinions after logging quite a few trips with it over the last while:

1. It beats the hell out of an icebox.
2. The venting system (as mentioned above,) is inefficient and poorly designed. To work well, the fridge's enclosure needs to be sealed. Even then it's pretty crappy. I'm going to do some modifying by adding 2 more CPU fans to see if I can move a bit more air over the compressor & bring the ambient temperature down to squeeze a bit more life out of the battery. I'll report back on that.
3: With a healthy 80AH deep cycle battery and the temperature setting on 3.5, it is good for about 36 hours. Which isn't very good, but if you run the tow vehicle, have a generator to bump up the charge, or can plug it in for a while somewhere, it's not bad.
4. It runs a lot cooler when the converter is plugged in to 110VAC and it's getting full juice.
5. It beats the hell out of an icebox.
6. With a couple 225AH 6V golf cart batteries or an 80W~ish solar panel, you could rule the world. With a single stock battery it has its limitations. Where to stow the golf cart batteries? Increased weight? Venting? It could be done rather easily, but there are some trade-offs to consider.
7. I didn't have to take a sawzall to the side of my camper to vent it.
8. The compressor is surprisingly quiet.
9. It beats the hell out of an icebox.
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