Interested in making a cheap AC inverter out of parts you can often get for free?

If you happen to have a broken UPS backup computer power system lying around (or can find one cheap), you could possibly turn it into an inexpensive power inverter for your camper.

Uninterruptible Power Supplies are often thrown away or donated to thrift stores when their battery goes bad. Once they do, you could simply replace the dead battery with your RV's deep cycle battery and enjoy a decent quality inverter that often -- at least on newer ones -- includes 5V USB power for your phone -- in addition to several standard AC outlets.

cheap camper inverter from ups

Typical UPS units you find in thrift stores

I actually did this, after watching the video below that I found on YouTube. It's an interesting project. The main idea is simply to unhook the leads going to the internal battery and then solder in some leads that will connect to an external battery. The most elegant solution -- to make it function like a store-bought inverter -- would be to run small leads to screw terminals on the outside of the case.

The second simple bit of electronics is to turn off the emergency alert -- the high-pitched squeal that normally is designed to warn you when the electricity goes out and you're running on battery power -- and you can disable it simply by finding the small buzzer element and cutting the leads and removing it.

There are other hassles you can run into. I found the outside case can sometimes be unreasonably difficult to crack open. You might also find that the circuitry is already fried, or in such bad shape that it sizzles or breaks as soon as you connect the battery. But after a few, you might just find one that opens easily and works really well.

The one big disadvantage for a small camper is the weight -- the transformer inside is quite heavy, especially on large UPS units, and especially when compared to the lightweight transformer-less inverters you can buy on sale for $40-80.

It's also no less prone to shorting out -- A badly-timed surge can fry any inverter pretty easily, so it's nice to have the option to exchange it for a new one. Then again, it's also nice to know you can build one for next to free, so it's well-worth learning how to do it!
(Note: Modified Sine-Wave AC output by cheaper UPS inverters may not be suitable for all devices. For quickly charging your laptop and mobile electronics, it should do fine, but for continuous power, beware that it could cause problems, so be sure to google and download the UPS manual. If you're concerned about this, check out his next series on how to make a sine-wave inverter. )

Watch The Video And Comment Below!

instant air cooling

"Cool Your Camper With A Fan And A Wet Towel!"

A misting fan

A misting fan (Photo credit: Wikipedia)

What if I told you that you don't need to use a traditional air conditioner in the desert. (Crazy right? But under certain circumstances, totally true!)

For an RVer one of the most expensive drains on your reserve battery power is running the AC. And if you live in a hot climate, it might seem ridiculous to consider living without it.

If you live in a hot and HUMID climate you may be stuck with using expensive traditional air conditioning. (At least for a little while longer, until technology catches up.)

But if you happen to live in a hot and DRY climate, you can often get away with something that may be far cheaper.

The one essential ingredient to make this type of non-AC cooling system work is something that most of us have in such ready supply that we don't even think about it: WATER.

Ironically, that's the one thing that runs in short supply in HOT and DRY climates like the popular Desert Southwest snowbird destination.

But if you happen to have a full water tank or -- much better -- are close to a ready water source, then you can have the makings of a quick and dirty evaporative cooling system.

If you're not from the American Southwest, you may not be familiar with the concept of "swamp coolers". These are what residents of hot dry climates use to cool their houses cheaply, without the use of air conditioning. Essentially a swamp cooler -- technically an "evaporative cooler" is nothing more than fan that blows hot dry air through a water soaked fabric mesh. As the hot air passes through the mesh, some of the water evaporates into water vapor, carrying some of the heat with it, up and out of the building. In a dry climate, a swamp cooler has the benefit of not only cooling the air, but adding some desirable moisture to the parched air as well.

(If you're from the humid Southeast, you might be wondering why the heck you'd want to add moisture to the air! Trust me. If you ever visit Arizona you will figure it out within the first 24 hours, when you wonder why you're so dehydrated.)

A typical home air conditioning unit.

A typical home air conditioning unit. (Photo credit: Wikipedia)

A traditional air conditioner uses so much energy because it's doing something that's very difficult. It works on a heat pump principle, that uses some laws of how liquids and gasses behave at different pressures to force little bits of heat to go from a cold place to a hot place. A heat pump is always working "up hill," and the steeper the temperature difference between the cold and hot place (think of your freezer) the more it has to work.

But an evaporative cooler only requires a simple and efficient DC fan, and sometimes not even that.

For illustration:

  • A great way to cool down your motorhome on the hottest and driest of days is to soak some towels and lay them onto the roof. If it's really hot and dry, it may take a few buckets of water, but it'll give you some quick relief from the "oven" effect.
  • To cool the inside, simply use a fan and a wet cloth. For a quick blast of cool air, lay a soaked towel over a fan.
  • For a semi-permanent "mini-installation", hang a wet washcloth over your incoming air vent (with the fan on obviously) and spray it with water when it gets dry.
  • If you're handy, you can turn this into a full swamp cooler installation by using a more efficient wicking mesh, a small water basin, and a small water pump.

A key thing is to make sure the air on the intake side is the dry outside air, so that it can absorb the water vapor. Once the inside air is saturated, you've hit your cooling limit.

Keeping the inside air dry enough to keep cooling it is where advanced air conditioning technology is required. There have been a lot of small developments in swamp coolers technology over the years, but getting past this hurdle has been a tough challenge for cooling engineers. (Check out the Coolerado system.)

But these ideas should be enough to get you started experimenting with evaporative cooling tricks. In future articles I'll be discussing different plans for more sophisticated DIY camper cooling systems.

Can I make this more permanent?

Absolutely! If you want to have a more permanently installed "swamp cooler" in your camper, you'll essentially need 1) a permanently installed fan, 2) a porous absorbent mat in front of the fan (optionally removable when you don't need the cooling), and 3) a way to keep the mat wet.

Assuming you can figure out the fan part, either by using a cheap AC fan (from the thrift if you want to save money) or a DC fan you can connect straight to the battery via a switch (which can come from a junked computer tower)...

You'll need some type of absorbent mat to place in front of the fan. It needs to be porous enough for the fan air to blow through and absorbent enough to hold water. A Shamwow works really well, but you could do better with an actual swamp cooler pad. A popular type available in home improvement stores in the Southwest is the Aspen Cooler Pad.

Then you'll need a way to keep the pad moist. While a more sophisticated operation would involve rigging up a traditional swamp cooler mechanism that continuously sprays the pads (pretty much like a drip irrigation system does), if your pad wicks well, you may be able to get away with a pan of water under the pad that you can refill after it's wicked all the water out... OR if you don't need much other than a quick blast to cool the cabin, you could just spray the pad with a water bottle a few times until it's cooled down enough.

In my own homemade camper...

I haven't bothered with a permanent installation (though I may do it in the future). Instead I use a DC clip-on fan (made for truckers that runs from the cigarette lighter outlet) that I just clip over the door, and I put a wet Shamwow over it, just like it the video. Since my camper is really well insulated, I don't need to run it all the time -- once I get the cabin back to a sane temperature, it will stay that way for a while. (It really only gets "too hot" when I've been away from the camper for awhile and left the skylight windows uncovered.)

I should add, though, that my main strategy for keeping cool in the summer goes along with my overall philosophy for having wheels on my tiny house in the first place -- When it approaches summer, I just drive away to somewhere it doesn't get too hot! (I try to save Sedona for springtime.)

DIY air conditioner hack

Have you ever wondered if it's possible with a bit of DIY hacking to Build Your Own Air Conditioner?

Maybe even one that saves energy?

Check out this instructional video by Amit Ranjan showing how he made his own "poor man's air conditioner" out of a few spare parts.

Here's another variation

Watch how desertsun02 creates a Simple Homemade Air Conditioner using essentially the same setup as above, but quite a bit neater, and in a way that separates the parts in a way that makes it easier to see what's happening.


As explained in the video, the whole setup uses only 45 Watts, suggesting that unlike a typical energy-hogging air-conditioner, this one could conceivably be run off the grid on battery power or solar panels. 

So, how does it work?

Copper is a great conductor of heat. When ice-water is pumped through the copper tubing, the heat within the warm forced air from the fan tries to equalize with the cold water in the tubing. In a sense the ice water in the tubing steals the heat from the air and delivers it back to the ice box. In doing so, it melts the ice, but if you have enough, you can keep this effect going long enough to get to room feeling comfortably cool again.

Believe it or not, this is essentially what's happening in an ordinary air conditioner (For HALF of the cycle). This is what's different:

  • Instead of using water as a coolant, a typical air conditioner uses a gas refrigerant (like R22) to absorb heat from the air.
  • But instead of sending the heated refrigerant to an "icebox," an air conditioner has to pump the heat outside using a complex compressor and coil system, fueled by electricity.

Refrigerators and air conditioners run on a "heat pump" principle to pull heat out a cold place and pump it into a warm place, using knowledge of what happens to liquids and gasses under different pressures. This takes quite a bit of energy, which is why air conditioners use so much electricity.

In the case of the homemade AC unit, you applied this energy before you started, by making the ice water!

So in some senses, you've actually cheated. You haven't really saved any energy -- you've just borrowed it from somewhere else. This might not be a big deal in your apartment, where the cost for the ice is measured in mere cents, but if you're attempting to boondock in a camper... well, you can't! Not for any length of time, anyway, without running out of ice. So this system is good for a day, maybe two, so you're talking... about $1-2/day for the ice + gas for the commute!

But it raises an interesting question...

Can you actually make a self-running air conditioner that uses less power... that could potentially be run on energy from, say... the Sun?

It's an interesting question. It turns out that because the concept of a heat pump is so straightforward, it's possible to play with different combinations of liquids, gases and materials that boil and condense at temperatures and pressures that can be hit without requiring so much energy.

In fact your typical propane RV refrigerator uses exactly this idea. In any type of refrigerator, the freezing is accomplished by part of a chemical cycle in which the refrigerant absorbs enough heat to cause it to evaporate. But in a Dometic style camper "absorption refrigerator", rather than use an electrically-driven mechanical compressor (like an air compressor for the gas refrigerant) to return the evaporated refrigerant to liquid form, it uses the science of chemistry to do the job. Instead of electricity, an absorption refrigerator uses propane to boil an ammonia-water-hydrogen mixture, pushing it through a series of tubes that causes strategic pressure and temperature changes that continue the refrigeration cycle.

It's a bit ironic to realize that refrigeration can be accomplished by adding heat, but that's how an absorption refrigerator works! Unfortunately this old technology is less efficient and convenient to maintain than using an electric compressor, so it's only seen its revival in RV refrigerators. But to someone interested in renewable forms of energy, it's natural to wonder if instead of propane, you could use heat from the sun to do the same thing. And in fact, some researchers interested in helping communities in developing nations have been rediscovering and trying to adapt this idea with new combinations. A promising one uses the class of volcanic rocks called Zeolites, and a few companies have actually started selling expensive solar camping coolers that run on a simple system using a zeolite/water mix. You can imagine that the air conditioning applications could be just around the corner.

Of course, if you're strictly interested in low-power air conditioning, then it will pay to become familiar with evaporative coolers (a.k.a. "swamp coolers"). Those who live in a dry climate like the Desert Southwest grew up with evaporative cooling systems, which cool the air by blowing hot dry air across a water-soaked mesh. As the water evaporates into the air, it takes the heat with it.

Traditionally the limitation of evaporative coolers is that they don't work in humid climates. But a number of researchers have come up with clever ways to pre-dry the air using "desiccants", so that evaporative coolers are evolving so that they one day might be feasible in any climate.

And once you've solved that issue, then (provided you have a source of cheap water) you've got yourself a source of low-power cooling, requiring no more than a fan. You can read more in my article about evaporative air cooling by clicking below: