air conditioner from batteries possible?

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air conditioner from batteries possible?

Postby Jonny » Wed Jul 18, 2012 5:37 pm

Hello all. Just put a deposit on a 1991 B190 and am spending evenings trying to digest all the information about this wonderful toy before I get my hands on it next week.

I know for the air conditioner to run you have to be on shore power or generator, by design. It looks like the AC is about 11,500 BTUs and people estimate somewhere between 10 and 15 amps when the compressor is running? I haven't found the specs yet for the inverter, but if it's too small for that, what would stop me from putting a couple of beefy AGM batteries under the couch and a fresh new inverter to handle the load.

Granted I'm no electrician, but if I have 200 amp/hours worth of batteries and a 50+ amp inverter, couldn't I use the air overnight before needing to charge the batteries via gen or shore power? My goal is off-grid for several months, but I gotta have that air :)
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Re: air conditioner from batteries possible?

Postby skater » Wed Jul 18, 2012 8:53 pm

No, not going to work, at least not without a ton of batteries. The basic problem is that you're forgetting that 12 volts DC has much less power than 120 volts AC.

The air conditioner draws (say) 15 amps at 120 volts. That's 1800 watts, or 150 amps at 12 volts. (Watts=amps*volts is the formula that rules.) Whenever the compressor is running. 200 amp/hours of batteries would keep it running for a couple minutes, tops. You'd need to fill the camper with batteries to make it work overnight.

This is why solar installations are never large enough to run air conditioners.
1991 Airstream B190 - bought, 2005; sold, 2011; bought 2017
1995 Airstream Excella 30' trailer

WBCCI #13270, Washington, DC Unit
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Re: air conditioner from batteries possible?

Postby Jonny » Thu Jul 19, 2012 7:56 am

Thanks for the info skater. I was talking to some friends last night and they were trying to give me the 'why' on less watts is way more amperage.

Some back and forth resulted in me not understanding details, but their answer was the same as yours. Too many batteries. One person did say it may be possible to use 4 (or 8) batteries connected to make 48 volts, then converting it to 110AC, but nobody could or would sign off on it actually working.

That would be somewhere around 40 amps, which still seems like quite a draw.
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Re: air conditioner from batteries possible?

Postby EricZ » Fri Jul 20, 2012 2:47 am

Skater is correct that running the A/C off of batteries for any significant length of time is not realistic.

It is, however, possible to run the A/C from batteries for short periods. I've installed three 100 Ah (amp-hour) AGM batteries under the couch in my B190 (note that ONLY sealed batteries, such as AGMs, should go inside the living space), for a total battery bank capacity of 300 Ah. I have a 2000-watt Xantrex PROsine 2.0 inverter with 4500 watt surge capability. (The A/C draws well over 2000 watts on start-up, so that surge capability is important. For example, I can't reliably run the A/C on a 2000 watt Honda EU2000i generator.)

Using this setup, I've run the A/C for about 30 minutes on batteries without problems. I saw battery current as high as 160 amps for short periods, but mostly it was between around 20amps (when the compressor was off) and 75 amps (when the compressor was on). I thus estimate that I could go as long as 2 hours without draining my battery bank past the halfway point (more than that dramatically reduces their life span). I'll be able to better estimate when I install an Ah meter (I have a Bogart Trimetric, just haven't wired it up yet).

But Skater is right, this would be completely impractical for all-night use.

As for power calculations, again, follow Skater's email: Amps*Volts=Watts. I recommend converting everything to Watts, as they're a consistent measure of power, regardless of voltage.
1 Amp at 120volts = 120 watts. 10 Amps at 12 volts = 120 watts
(But with inverter inefficiencies, it turns out that to get 120 watts I'm actually drawing more like 11-12 Amps.)

You also mention an inverter, Jonny. Airstream 190s (at least all the ones I've encountered) came from the factory with a CONVERTER (something that converts 120 volts to 12 volts to run the 12-volt accessories and charge the house battery), but not an INVERTER (something that converts 12 volts to 120 volts). Inverters can be installed, but they're not cheap. An upgraded converter might be about $200-$300. A nice inverter might be $1000-$2000. (Note that the stock converters are pretty bad. Also note that some inverters also include converter functionality.)
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Re: air conditioner from batteries possible?

Postby craigmar » Fri Jul 20, 2012 3:11 am

I'm not to much of a guy with all the electrical formulas flying around but can say the following. I used to work in a shop that made undercover vehicles for various law enforcement in Utah. We made simple air conditioners that ran all day and night for those vans etc by using a couple of muffin fans, pvc tubing, ice and a cooler. you can search the web for some that are pre-made but its just a simple process of putting one fan in the side top of the cooler drawing air from the interior of the vehicle and channeling it through the pvc pipeing that is layered in the cooler and covered with ice, and the second fan connected to the pvc on the bottom of the cooler or routing your pipe back up to the other side top to eliminate the need for caulking, sucking the air out expelling the cold air to the interior. sounds simple, is simple to make and cheap and can be used outside the vehicle such as in a tent, enclosed room etc. as long as the fans are hooked up to a battery or 12 volt source now maybe even a small solar panel. An added plus is the fact that they are quiet as well because the vehicles could not be in an area with the vans engine running when they are supposed to be incognito. In looking at some of the various offerings on the net, a lot of them just blow air over ice which can put moisture in the air and that problem is eliminated when you use an enclosed grid whether its pvc, copper tubing or even a used transmission cooler that is cooled by the ice or frozen water-bottles, ice packs etc. to cool the air inside the grid of your choice. Also, you can even eliminate the second fan and just use a vent cover if your fan is sufficient in blowing out the air. As in a regular vehicle the air will get cooler the longer it is recirculated and you may have to shut it off to keep from freezing. I'm not to hot on spending a lot of money when something simple can be made and the money saved can be used for a good dinner for me and the wife or something equally as entertaining, anyway.
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Re: air conditioner from batteries possible?

Postby craigmar » Wed Aug 08, 2012 11:48 pm

Just wondering what you have decided as far as air conditioning is concerned?
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Re: air conditioner from batteries possible?

Postby Jonny » Thu Aug 23, 2012 5:15 pm

Actually I just shelved the idea and embraced the generator, since it seems like I'd need quite the battery bank.
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