Air Conditioning on battery pwr alone? Well, on paper. . .

[H.A.]

Hrip.

[jss06]

How about eliminate the electric requirement all together.
Some idiot needs to suggest cryogenic cooling. A cryogenic cooled railcar can stay cold a couple of weeks. Scale that down to teardrop size... A couple weeks is a decent time for a camping trip.

The draw back is those rail cars are hyper insulated and hermetically sealed as well. The TD would be heavy and walls a lot thinker with insulation to come close to that and still be usable.

[daveesl77]

Vigilant1 - it is the initial surge requirement of the AC that is doing the overload on the inverter. AC does show LRA, but the plate does show running wattage at 515. It isn't unusual for startup draw on cheap ACs to hit 5-6 times running requirements. As to the battery, yes it was low, like just barely 12v unloaded, and the inverter output was down to 106. I have it fully charged now and inverter output is 116. I plan to test it again in a while and will report back. The dorm fridge actually would do the same thing if the battery was low, so it might be that. No matter, the cost of the capacitor is worth it, I'm just surprised I never thought of it before. Old age.

Edit: Did the secondary test and yep the initial current draw is just a bit too much for the inverter. Now to see how the hard start works, or not. Supposed to get here next week. The dorm fridge fired right up and no I didn't try them at the same time.

dave

[Talia62]

Here's a 2500 Btu unit: http://www.climaterightair.com/products/climateright-2500-btu-mini-ac-heater.html for $399.

That's significantly cheaper than the ones I found.

[vigilant1]

Edit: Did the secondary test and yep the initial current draw is just a bit too much for the inverter. Now to see how the hard start works, or not. Supposed to get here next week. The dorm fridge fired right up and no I didn't try them at the same time.

Dave,
Maybe it's time to install another hard-start kit in the 'fridge.
If, after you install the capacitor and relay in the AC unit, the circuit breaker still pops, there's something else you could try: disconnect the fan motor and try it again. I'm assuming the fan motor and the compressor motor both come on at the same time, so the starting surges are additive. If there's enough capacity in your inverter to get the compressor motor started, you could add a short timer circuit so the fan starts a few seconds after the compressor does. Another work-around would be to leave the fan running in "low" all the time (so, no starting surge at the same time as the compressor). Running the fan is often helpful in these window units anyway to reduce stratification and also so that the thermostat "sees" a more accurate air temp for the living area (but it would use up some precious battery power).

Happy experimenting!

Mark

[daveesl77]

Update test - AC with SPP6 hard start on 5k btu window shaker.

I got in the SPP6 hard start capacitor/relay today and installed it in the 5k "goldstar" window shaker I got at a yard sale a couple weeks ago for $20.

Test 1- to make sure that my inverter/battery system was still working as it did in the past, I cranked up my Haier 3.3 ft3 dorm fridge, using just the old truck battery and the inverter. Unit started and ran perfectly, just like always. Shut it down, turned on charger to bring battery up to full power. Plugged in the modified window AC into the inverter. Started fan first, no problems, but when tried to fire up the AC inverter dropped on overload protection. This could be either the 6 year old truck battery, but it may also be the cheap inverter. Tested, unload voltage was at 110v, load with fridge was 106 then rose up to 108. That is right on the edge of the 10% variable range.

So for now, looks like I'll just use the inverter system to run the fridge when traveling or if in a place without plug-in power. I know from my past test that once I got the fridge cooled down by running the night before being plugged into shore power, I could run it for at least 11 hours on just battery alone at a setting that kept the internal fridge temp at 28F. With my 50watt solar setup, it ran continuously for 2 days and nights, with the solar charging the battery. At the end of 2 days/nights, battery power had finally dropped to just below 12v. Another 50watt solar panel would take care of this and/or new deep cycle battery.

Test 2 - got out my little HF 800 watt 2 cycle generator to see how it would do. Fired up the genny, warmed it up, plugged in the modified window shaker and it came right on. Generator did lag for maybe 3-5 seconds, but then came to full song.

Test 3- since I now knew the generator could run the AC (no, I'd never run this noisy thing at night or near others), I wanted to see if the SPP6 was actually doing anything to help. So, I unplugged the SPP6, let the AC sit for 10 minutes, kept the generator running, then tried to start the AC. Without the SPP6, the genny just would not fire up the compressor at all. So, the Hard Start SPP6 does work and works properly.

While I'm not quite ready to trade in the really old (but free) truck battery for a new deep cycle unit, what I may try is to do a parallel test using my van battery with the camper battery to see if that will keep the inverter on. In reality, I care more about my fridge being able to run when boondocking than the AC. Adding in the additional 50watt solar panel should probably allow it to run continously, unless I have cloudy weather, then I can just charge the trailer off the van.

Conclusion is that a good deep cycle battery and/or better inverter would probably have no problem running the 5k window unit, as it was close to starting on test 1. Like I said, in my first post, I'm using a 6+ year old mopar truck battery and a Centek 2k watt mod-sine inverter that I got on sale for like $40 several years ago.

dave

[vigilant1]

Dave,
Thanks very much for doing the test and for writing up your findings. It sounds like the hard-start kit will likely be the ticket for running these small window AC units from battery power if we are on a limited capacity inverter.

If my back-of-the-envelope calculations in the OP are close, it might take 600 Watt/hours to keep a small sleeping area cool for 8 hours. That much could be stored up by running a small generator during the day for less than an hour at some convenient time, hooking up to the tug and drawing juice for about the same amount of time, or even gotten with a day of good sun on a 100-200W solar rig.

As you've probably guessed, I don't like sleeping in the heat.

Mark

[Shadow Catcher]

Kimberly Karavans has an article/Ebook where they discuss solar/battery/AC Basically convincing you need a LOT of battery and a LOT of solar and a 1000W generator and a AC unit with a hard start capacitor.

[vigilant1]

Kimberly Karavans has an article/Ebook where they discuss solar/battery/AC Basically convincing you need a LOT of battery and a LOT of solar and a 1000W generator and a AC unit with a hard start capacitor.

I saw on their site that they sell some apparently very nice split-system air conditioning units. (http://www.kimberleykaravans.com/water-heating-air-conditioning ). They are definitely appealing to the "high end" of th market. Their trailer walls/roofs have less than the equivalent of 3/4" of XPS foam, so the insulation is limited. And, if they want to cool an entire full-size trailer 24 hours per day, away from shore power, then I agree it will take a lot of batteries + solar or generators.

[RonS]

Would using dual batteries, in series for 24V be a solution? I know they make 24V DC to 120V AC inverters. One that I saw said about running refrigerators and microwaves....

Also, you don't want to oversize the A/C unit where it only runs a couple minutes at a time. That doesn't allow it to properly dehumidify the air and you end up cool, but clammy/damp.

[vigilant1]

Would using dual batteries, in series for 24V be a solution? I know they make 24V DC to 120V AC inverters. One that I saw said about running refrigerators and microwaves....

I don't think it would change the situation appreciably. The total capacity of the two batteries would be the same whether wired in series or parallel, so we'd expect them to power the AC unit for the same amount of time. I don't know enough about battery chemistry to say whether there'd be a difference in peak startup wattage available depending on parallel/serial wiring. But, then, I don't even understand why Dave's 2KW inverter is clicking off on a load that is less than 1 KW (regardless of the health of the battery involved.

Also, you don't want to oversize the A/C unit where it only runs a couple minutes at a time. That doesn't allow it to properly dehumidify the air and you end up cool, but clammy/damp.

Yes, we're having to adapt to the realities of the marketplace (very small capacity AC units--less than 5K BTU-- are not mass-produced "commodity" items, so they are expensive), and also probably physics/engineering (for whatever reason, as we go down in size, these units appear to produce less cooling per watt consumed). Still, if we can stick with the 5K BTU unit and introduce some thermal mass, maybe we can dehumidify that air more effectively. The problem you cite happens in home AC units when they are "oversized", and therefore they don't have to run very long to get the home cooled down. As a result, a relatively small amount of the home's air gets routed over the evap coil with the resultant removal of moisture. However, if we had some thermal mass immediately downstream of the evap coil (think a bunch of full AL cans, each wrapped with simple heat sink of aluminum fins, or something similar), they'd be cooled down by the AC unit and the air that comes out of the system and into the living compartment would be warmer than it otherwise would be (because it got warmed a little by the cans), so the AC would run longer each time. At the end of the "run" we can continue to run a fan over the cans to take advantage of their coolness, and if they remain below the dewpoint of the cabin air they'll continue to remove moisture (gonna need a pan under them and a drain).
In our hypothetical sleeping area (queen size bed size, 4' high), we've got 130 cu ft of air = 10 lbs of air. If we had 4 gallons of fluid in the cans (about 35 lbs), they would effectively more than quadruple the thermal mass of our system, allowing the AC to run a lot longer each time (and have fewer startups over the course of the night). By allowing the compressor to run longer, I think we'd do a better job of removing humidity, and probably also improve the comfort by keeping temps more even in the sleeping area.

All the above assumes very efficient thermal transfer to the cans, etc. There are lots of possibilities for different ways to increase the thermal mass. Ideally, we'd use something we already have aboard and need (water, etc).

[daveesl77]

The reason the 2k inverter is kicking off during startup is two-fold...
1) Required starting amperage on a compressor can be 3-8 times the under-load amperage. So, a 500 watt running load can hit possibly 4,000 watts for a few seconds at start up, but probably more in the range of 2,000-2,500 watts.
2) The inverter I have is a cheap model and the AC is also a cheap model (goldstar), so neither is efficient at doing their job. They both do what I got them for and I only paid $40 for the inverter and $20 for the AC, so even if they won't work well together, they do work well separately.



dave

[Shadow Catcher]

daveesl77 You might be able to get yours to start with a hard start capacitor http://www.homedepot.com/p/SUPCO-120-288-VAC-1-Phase-1-2-10hp-Hardstart-Relay-and-Start-Capacitor-SPP6/203566183

[vigilant1]

The reason the 2k inverter is kicking off during startup is two-fold...
1) Required starting amperage on a compressor can be 3-8 times the under-load amperage. So, a 500 watt running load can hit possibly 4,000 watts for a few seconds at start up, but probably more in the range of 2,000-2,500 watts.
2) The inverter I have is a cheap model and the AC is also a cheap model (goldstar), so neither is efficient at doing their job. They both do what I got them for and I only paid $40 for the inverter and $20 for the AC, so even if they won't work well together, they do work well separately.



dave

Dave,
You wrote ("Test 2") that the AC (with the hard-start capacitor) will start up when powered from your 800W HF generator. That generator can't be putting out more than, what, about 1100W even for a short period? So a 2K watt inverter should not be maxing out and tripping a circuit breaker. That's why I don't think its as simple as the AC drawing too much power (because your generator can start it). Unless the 2K inverter really isn't a 2K inverter. If it can power, say, a 1500W hair dryer without tripping, it would seem capable of starting the air conditioner. And if the battery wasn't up to snuff, that wouldn't cause an overcurrent condition--just the opposite. Of course there could be all kinds of other esoteric stuff happening (true sine wave vs pseudo-sine causing issues, etc).

Mark

[citylights]

Evaporative cooling is much more likely from battery power. You would need to run a fan and small water pump. All the cooling energy comes from the evaporating water. Super efficient evaporative coolers achieve a 30 degree temperature differential, but very common is 20 degrees. IE. 90 degree outside temperature could be conditioned to 70 degrees.

Simple and most common setup is 100% outside air and 100% exhaust air. With very dry outside air, it is also possible to set up an air split, say 50% outside air, 50% recycle, and exhaust for better efficiency and greater temperature drop.

Evaporative cooling is practical in the desert southwest US where the relative humidity is often below 10%. In the wet south east US, where the relative humidity is 80% plus, forget about it and stick to AC and generator, or keep working on that battery powered AC.