Running 12V refer on battery only

[Daniel V.]

I've picked up scattered clues about this question in some past threads, but didn't find one dedicated to it (or sorry if I missed it).

Can one run a 12V compressor refrigerator (high-efficiency danfoss type) strictly on battery power and for how long? We like our propane fridge because it can run for days and even weeks on a single propane tank. But we would consider the 12V as our next unit if there was a way to run it for at least 4-5 days in a row in fairly hot weather without setting up a gigantic solar array, a generator, or dragging a separate trailer full of batteries, i.e. basically anything that is more trouble than just a propane tank.

What is the real-life Amp-hour draw of, say, a 2 cu.ft. 12V compressor fridge?

Thanks

[Parker Buckley]

I've picked up scattered clues about this question in some past threads, but didn't find one dedicated to it (or sorry if I missed it).

Can one run a 12V compressor refrigerator (high-efficiency danfoss type) strictly on battery power and for how long? We like our propane fridge because it can run for days and even weeks on a single propane tank. But we would consider the 12V as our next unit if there was a way to run it for at least 4-5 days in a row in fairly hot weather without setting up a gigantic solar array, a generator, or dragging a separate trailer full of batteries, i.e. basically anything that is more trouble than just a propane tank.

What is the real-life Amp-hour draw of, say, a 2 cu.ft. 12V compressor fridge?

Thanks

I'm looking forward to one of the science guys answering this with numbers, but I bet you'll have to go a long way to compete with all the energy stored in 20# of propane. Something showing BTUs removed to cool the fridge, the efficiencies of that process, the efficiency of the propane fridge, and having 366,000 BTUs available in the propane tank. Thanks for posting. This will be fun.

Parker

[Don F.]

I'm looking forward to one of the science guys answering this with numbers, but I bet you'll have to go a long way to compete with all the energy stored in 20# of propane. Something showing BTUs removed to cool the fridge, the efficiencies of that process, the efficiency of the propane fridge, and having 366,000 BTUs available in the propane tank. Thanks for posting. This will be fun.

Parker

It might last overnite - with a single battery...

[Chester Taje]

In My Trillium i had a 12 volt fridge. I had dual batteries, but was on the move each day or had hookups. The fridge worked fine but I never did try a long stay. I did overnight with it and had no problems what so ever.

[Pete Dumbleton]

Whoa, we are drifting off -- OP wants to know about COMPRESSOR (Danfoss type) reefer, not typical ammonia-cycle RV reefer -- Big difference -- Danfoss-style was designed for 12VDC operation.

A lot will depend, of course, on the ambient temps and the subsequent need to run it, plus the actual box the reefer uses, in terms of insulation -- A front door versus top door will also make a difference.

This site:

http://www.backwoodssolar.com/catalog/refrigerators.htm

sez "WARNING: COMBINATION GAS-ELECTRIC refrigerators are usable only as gas powered, not as electric. An electric heater running nearly full time replaces the gas flame. They use much more power than any electric compressor, so they are not practical electric units.

SUNFROST products have 4 to 6 inch insulation, and the compressor is on top where it can’t put heat back into the box. The RF-16 model uses a total of 750 watt hours a day, about 300-400 watts of solar modules, depending on the climate.

NOVAKOOL products are smaller DC powered refrigerators made for RV & marine markets. With the same Danfoss compressor as the Sunfrost, these are smaller refrigerators but use about the same power. Adding 2 extra inches foam insulation to all surfaces except the door cuts that power use in half."

Looks like a 100 AH battery is going to have 1,200 Watts, so the Sunfrost 750W unit is not going to last two days, the more-insulated Novakool less than four days.

However, the site wasn't specific about the reefer size and I suspect they were looking at the larger ones, so more research is indicated.

[peterh]

I looked into the compressor-style 'fridges a while back and found that the smaller (e.g. FG trailer compatible) units consumed under 20 amp hours a day when the temps were moderate (in the low 70s) but that figure doubled as the temps climbed toward 100 degrees. With figures like that you'd deplete a full battery in 36 hours in moderate temps and drain that same battery dry by late afternoon on a hot day.

As for how much solar panel you'd need to meet the needs of a 'fridge like that, I figure a quality "monocrystaline" solar panel setup placed where it'll get a lot of sunlight will put out around 25 amp hours on a sunny summer day for every 50 watts of panel capacity. Given that hot days are also generally very sunny days that would translate to needing an 80-watt solar panel and a pair of batteries to power one small compressor refrigerator with no left over capacity to run your lights or other appliances.

If your heart is set on a Danforth-style 'fridge and you don't do any winter (when there's much less sunlight) boondocking, don't have any other energy-hungry appliances to feed*, and convert any lights you use for more than a few minutes a day to LEDs you could probably get by with 110-120 watts of solar panels and a pair of batteries. Just don't go cheap on the panels, batteries, or charge controller.

* Thinking of power-hungry appliances, the forced-air furnace in many FG trailers would qualify, but because you don't generally run the furnace when its 100 degrees outside and the 'fridge is at the top of its energy demand curve you might not need to add more solar capacity to cover your furnace's needs.

On edit: Clarified when I meant "amp hours" when I said "amps."

An amp is the amount of current (electricity at a specific voltage) that a device uses when its running. An amp-hour is the amount of current the appliance uses when it runs for one hour. So a 3.1 amp motor uses 3.1 amp hours of electricity if it runs continuously for an hour and 31 amp hours if it runs continuously for ten hours.

Deep cycle batteries (like we use in our trailers) are rated in terms of the number of amp-hours they store. In theory a 50 amp-hour battery should be able to pump out 50 amp-hours of electricity before it starts to loose its punch (e.g. before the battery's voltage starts to drop significantly), but in practice battery ratings are always outrageously optimistic. I tend to figure a battery is good for 50-60% of its rated capacity, so a 50 amp-hour battery stores somewhere around 25-30 amp hours of usable power, enough to run our favorite 3.1 amp motor for eight or nine hours.

[Roger C H]

Assuming you have the smallest Danfoss compressor which = 1/20 hp.

http://www.danfoss.com/North_America/Busin...Compressors.htm

1hp = 746 watts. Therefore 1/20 hp = 37.3 watts. 37.3 watts ÷ 12 volts = 3.1 amps.

If you have a 31 amp hour battery, it will run for 10 hours. But you only want to discharge your battery by 1/2 so you should have a 62+ amp hour battery for 10 hours (constant) running of the compressor.

Naturally all these are approximate. There are I²R losses in the wiring, controls and what have you. But it does give you some ballpark figures to start with.

[peterh]

. . . If you have a 31 amp hour battery, it will run for 10 hours. But you only want to discharge your battery by 1/2 so you should have a 62+ amp hour battery for 10 hours (constant) running of the compressor . . .

Well, yes and no. Like a furnace that switches on when the temperature drops below a certain point and run until the temperature has warmed up by a degree or two above the set point, refrigerator compressors switch on when the temperature inside the 'fridge rises above a certain point, then cools the refrigerated space down a degree or so below that point and switches off.

While the compressor is running it probably does consume 3.1 amps or so, but the 'fridge uses little or no energy when its refrigerated space is below the set point. On a hot day the inside of the 'fridge will warm and rise above the set point more quickly and the compressor will have to run longer to cool it back down than on a cold day, which is why power consumption is so much higher on a hot day than a cold one.

There are things you can do to reduce the refrigerator's power needs, too. First, you can add insulation, which reduces the transfer of heat from the outside to the inside of the refrigerator. Just be sure not to insulate the compressor motor and casing or around the pipe-like coils. The coils are where the heat that the compressor motor has just pumped from the inside of your refrigerator is radiated away into the air outside your 'fridge; insulating more than slows this process down, it'll actually stop your refrigerator from working right and make the insides warm up when the compressor runs!

Which brings us to the other thing you can do to improve efficiency: keep the coils clean and make sure they have ample ventilation so air can move freely around them, making it easier for them to throw heat off into the air around them.

[Pete Dumbleton]

It occurs to me that the reality is that there isn't all that much improvement with the compressor reefers or they would be standard on upscale RVs and optional on mid-range ones.

Also, the ammonia-cycle RV reefers have a choice of three different power sources, so they are a lot more flexible -- Carrying an extra 20lb LP tank along is a lot simpler than adding more electricity (batteries, solar panels, bigger generator).

Also, give serious consideration to why one needs a reefer in the first place except knee-jerk reaction because one has one at home.

One of the super-coolers and some block ice might be just as effective at bottom line for absolute necessity like keeping prescription drugs cool -- When I lived in Europe, I learned to enjoy drinks, even beer, without them needing to be ice cold, and a few canned or boxed meals on a camping trip are not going to hurt us -- Food will stay fresh for surprisingly longer than the Use By dates, especially prepared food, except some of the more critical raw stuf -- Skim milk can be made as needed with Milkman dried milk and cool water.

There are tricks that can be used to make living with a cooler more fun than it used to be!

[Roy in TO]

We prefer block ice for the coolers but it can be difficult to find in many locations. Cubes seems to be the norm now.

[JEAN-L]

You can freeze water in a larger bottle before going on the trip ,that keeps it cool for a while, And the water stays in the bottle, keep everything else dry !

[peterh]

Pete, I'm not sure why someone would prefer a 12v compressor model over a propane powered model in an RV. As long as they're vented correctly they're very efficient, silent, and because they have almost no moving parts (just the thermostat, the ammonia hydrate refrigerant, and the propane) they run trouble-free for years and years.

[Pete Dumbleton]

Here are the cooler tips that I found to work for me:

1. Redo the drain fitting to accomodate plastic tubing so you can drain the melt-water overboard (Or out the back door everytime you open it if you have it in back seat of car -- Be SURE to get tube outside every time!) -- What we don't want here is a deep puddle with franks and old milk floating in it.

2. At Wally or Home Despot or where ever they sell plants, get some of the open trays that the plant pots come in -- Turn upside down, cut and piece to fit in bottom of cooler to raise food out of puddle.

3. When you buy ice, cubes in bag or whatever, do NOT break up into smaller chunks -- The bigger the mass is, the less surface area it will have and the slower it will melt (We are going for long-lasting effect, not trying to flash-freeze warm beer) -- Do NOT remove the ice from bag (protects from convection melting).

4. Punch a few holes in one side of the bag and place in cooler, on top, with holes down -- As ice melts, water will drip down on food and out drain system -- 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) and to reduce the convection area on the ice by keeping its plastic jacket on (Again, contents won't be as cold, but they will be cool enough longer).

If you have a lot of food and cold drinks, consider two coolers so the food cooler is opened less -- Buy stuf that is already cold, esp drinks.

Keep the cooler(s) in cool shade with maybe some insulation on *bottom* to prevent heat (rising) into cooler -- Don't put cooler in trunk of car over exhaust system, for starters!

[Raya]

We prefer block ice for the coolers but it can be difficult to find in many locations. Cubes seems to be the norm now.

And don't even get me started on those "fake blocks" that are just a bunch of small cubes "pressed" into a block shape Useless!

This won't help most people when travel-trailer camping, but when boating we had only an ice box, and we liked nothing better than to set anchor in a new port in someplace like Central America. When you're looking for ice, the "less developed" countries are you friends! First exchange was close to this: "Hello, we are happy to be in _______""Where is the ice plant?"

The ice plant would have good, clear blocks of ice the size of small houses, and the propieters would saw you off the size hunk you could use and then "tong" it over to you. Now that stuff lasted! Of course we sometimes went a month or more between blocks, but it was happy time when we did get it.

One note if your forays are going to be ten days or less in length: With a decent, insulated ice box, and homemade "ice blocks" that you freeze in a dishpan before you leave (or one or two real ten-pound block), it will last quite well -- ten days or so.

Two things that help are having a top opening ice box (like on a boat), with "stepped" lids, and then keeping a piece of something like Reflectix lying over the top of the food. Then when you open the lid you just reach your hand under the "blanket," but don't expose all the food and ice to warm air. It helps to visualize where you're going to go in the box before you open it, to eliminate "shopping."

When we had ice available, we used to sometimes fill a portable cooler with just drinks and ice. That gave us a bit extra, and kept us from opening the ice box except to get things out for meals.

I do have a friend who lives aboard now and powers a 12v refrigerator/freezer (Danfoss type compressor) with "just" solar panels and a wind generator (which of course he uses the power for other needs as well), but it's a larger array than you'd want on most trailers, I'd think.

Raya

[Raya]

- 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.

Raya

[Daniel V.]

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.

[Roy in TO]

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.

[Pete Dumbleton]

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...

[peterh]

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.

[Parker Buckley]

See, I knew this was going to be a fun topic when you guys got involved!

Parker