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Old 21-06-2010, 18:06   #1
rja
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12vdc Air Conditioning Systems

Hello,

Does anyone have experience with the 12v A/C systems on the market? I'd like to convert the A/C in the master cabin to DC so that I can run it off the house banks at night. Google found a few with the MES 6000 looking the best..

Cruise N Comfort USA MES 6000 12 volt Marine A/C:
About the MES 6000

Looks to be a true 12v system that uses quite a bit less power than the others.. has anyone tried one?

Thanks,
R. Jason Adams
s/v Kapalua
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Old 21-06-2010, 18:33   #2
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Jason:

I haven't used one, but I have done the math. They use essentially the same watts of energy per BTU of cooling as an AC powered system. And they cost twice as much.

At the dock or with a genset running you are better off with AC. At anchor without a genset you can run a 5000 BTU AC system from a 2000 watt inverter almost as easily as running a DC air conditioning system from batteries.

So, I don't see the point.

David
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Old 21-06-2010, 18:34   #3
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Haven't tried one, but be very careful about your calculations of needed 12V power. Their "recommended batteries" of two Group 31's is a joke.

2 x 110AH = 220AH total

Of this total, only 50% maximum is really available (110AH) because you don't want to discharge your batteries below about 50%.

On Medium power their 6,000BTU unit draws 35A. Thus in only 3 hours at 35A it would deplete the available power. Running it overnite....even if you really could use low power or 18A....would draw about 150AH, plus whatever it took originally to cool down the cabin.

On a hot day/night with the unit running at high power (about 50A) you'd draw down a lot of power.

And, remember, you've got to somehow replenish that energy in the house batteries.

I'm not saying it's not do-able, and in some cases it might even be a good idea, but beware of the real power required for cooling.

Bill
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Old 21-06-2010, 18:44   #4
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The link you provided is pretty strait forward about performance and energy demands but if you do the math you will discover there is a big road block. My boat needs 16K BTU's to cope with Chesapeake summers, where temperatures on the water at night reach the nineties. Six thousand BTU would marginally cool one stateroom running at full blast all night long. (overnight lows are in the high eighties!) Forty eight Amps would drain a large battery to 50% in one hour. You don't want to discharge a battery that much. So eight hours in a single cabin will need a minimum of 8 EXTRA large deep cycle batteries, for a total of somewhere around 400 extra pounds.

Where are you going to put them? How are you going to charge them? Who is going to LIFT them?
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Old 21-06-2010, 18:44   #5
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Originally Posted by btrayfors View Post
Haven't tried one, but be very careful about your calculations of needed 12V power. Their "recommended batteries" of two Group 31's is a joke.
Yea, I noticed that. Plan is to expand our house bank to around 1200AH with 8x 130w solar panels (maybe wind genny also). I'm thinking the solar would be more than enough to replenish anything pulled overnight by the A/C. It's a lot of work/cost for A/C.. but the wifey wants it. I'm sure we've all heard that before.

-r. jason adams
s/v Kapalua
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Old 21-06-2010, 18:51   #6
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Originally Posted by sandy daugherty View Post
The link you provided is pretty strait forward about performance and energy demands but if you do the math you will discover there is a big road block. My boat needs 16K BTU's to cope with Chesapeake summers, where temperatures on the water at night reach the nineties. Six thousand BTU would marginally cool one stateroom running at full blast all night long. (overnight lows are in the high eighties!) Forty eight Amps would drain a large battery to 50% in one hour. You don't want to discharge a battery that much. So eight hours in a single cabin will need a minimum of 8 EXTRA large deep cycle batteries, for a total of somewhere around 400 extra pounds.

Where are you going to put them? How are you going to charge them? Who is going to LIFT them?
Damn.. don't rain on my parade! I really don't want to run the genny every night (all night) for A/C.

DC Genny (existing genny is going to need replaced soon anyway)
1200AH Li-Ion house bank
Victron 5kw inverter
Existing 16k btu in the cabin (we'll turn off the 16k btu in salon and the 12k btu in the guest side hull).

Might be doable. Should be able to refill batteries in under 2hr with DC genny (vs running 8hr every night). Damn A/C.

-rja
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Old 25-06-2010, 20:33   #7
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But wait! There's more!

You are going to add 600 to 800 pounds of batteries, plus a couple hundred pounds of solar panels and wind generator, to run an air conditioner all night? And they won't be Li-ion batteries unless you have unusually deep pockets.
Most deep cycle batteries have a finite number of recharge cycles in them before they need to be replaced. How many summers of use?
If you have guests, will they be sleeping in the guest alma with its own generator-run AC or in the master alma with the DC air conditioning?

Michael
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Old 25-06-2010, 20:50   #8
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But wait! There's more!

You are going to add 600 to 800 pounds of batteries, plus a couple hundred pounds of solar panels and wind generator, to run an air conditioner all night? And they won't be Li-ion batteries unless you have unusually deep pockets.
Most deep cycle batteries have a finite number of recharge cycles in them before they need to be replaced. How many summers of use?
If you have guests, will they be sleeping in the guest alma with its own generator-run AC or in the master alma with the DC air conditioning?

Michael
Actually, I'm most likely going to go with a 800-1000AH LiFePO4 bank, Victron inverter/charger, and keep the 110v air cond. Cost (for LifePO4) isn't to unreasonable for what you get. We'll drop around 200lbs vs our current house bank and have 2x capacity. After running the AC in the cabin for a few nights (with a TED watt meter) the draw isn't as bad as I thought. It cycles off/on often enough to bring the overall kwh usage down. If we have guests, we'll use the genny more. Either way, having a large bank and a large charge capacity will cut genny hours in half.

If it takes AC for hot summer nights to make the wife happy that's what I'm going to do. If she's willing to go off and cruise for a few years I'm willing to make her comfortable.

-rja
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Old 25-06-2010, 22:04   #9
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If it takes AC for hot summer nights to make the wife happy that's what I'm going to do
I understand the imperative.

Thanks for the idea of LiFePO4 batteries. However, researching them on the web, the only source I could find for large amp-hour batteries was ThunderSky,

ThunderSky LifePO4 Batteries | Electric Autosports Inc.

There a 3V 200 Ah battery costs $300 Canadian. Need four of them to get to 12V at 200 Ah, so the cost becomes $1200. Need four of those sets to get 12V at 800 Ah, so the cost becomes $4800. Ouch! But they do weigh only half as much as lead acid batteries. May also require a "battery management system" for recharging different from normal regulators.

Michael

P.S. Conversion from $C to $US is about 1:0.95, so $300 CDN becomes about $285 US
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Old 25-06-2010, 23:34   #10
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maybe instead of using solar panels to charge a battery to run an air conditioner (which is all very expensive and inefficient) you could instead pump water through white pipes in the sun which have additional (seawater) pumped onto them which evaporates. This cools the water which can be pumped inside large tanks which moderate the boat temperature even keeping it cool long after sunset.

There is probably a more clever way to do it with vacuum sealed tubes, but remembering it is for cold water (bonus points if it can make hot water too!)
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