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Old 16-09-2016, 08:18   #1
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My Real Life 12 Volt AC Experience

Air conditioning threads are often like those on firearms and anchors, where opinions are shot fast and facts sometimes break loose. Add 12 VDC as the power source and the discussion could get crazy. I’ll start one anyway and see how it goes.

For several years I had dreamed of a 12 volt AC system for use on a 28 foot boat in South Florida’s waters–particularly when cruising in August on the bay side of the Keys or in the Bahamas when the water temperature tops 90 degrees and the mosquitos are fixing for a feasting as soon as the sun goes down. Thread contributions by those on this subject who haven’t experienced these conditions are, well, not exactly on point, and those who have suffered under these conditions but can fit 4KW generators are, well, not in the target audience.

First, I assumed that limiting the space to be cooled was the way to go. Going further, the problem to be solved was refined: 1) evening sleep for 8 hours (with a small refer drawing 4 amps and an LED anchor light also in the overnight mix), with 2) no more than 50% discharge from a "full" 500 amp-hour AGM battery bank, and 3) next day recharging with use of the engine/alternator for only a "reasonable" period of time in the morning before the sun hits the panels. To get to the point, it can be done.


The basic AC hardware in my system is a 6000 BTU Pompanette unit and a 12 volt March pump. The unit and its electronics sit in a hot lazarette, but there is some forced air circulation. 2/0 cables, 15 ft round trip, carries the compressor/fan load. Installation was very careful so that only filtered air from the quarterberth is sucked in and only air from the evaporator is sent back. The pump is on a 3/4 inch through hull with a small sea strainer, and there is 3/8 inch ID tubing until the larger AC intake.


Combined AC-pump draw is usually less than 40 amps. The key, of course, is run time, and the first way to reduce that is to reduce the cubic feet of the cooled space. So far, using curtains to shrink cabin size, the directly cooled space is somewhere between 100 to 150 cubic feet. I don’t have an easy way to measure run time over the course of an evening, but my estimate is that over an average South Florida summer night it is about is about 4 hours. This is with the AC fan, compressor, and water pump cycling together. Those who are truly interested in this thread have just done the math for amp-hour drain.

There are improvements that have yet to be made. Insulation for the air duct (about 6 ft long, no sharp bends) and for the hull side of the cabin are on the drawing board. I am also thinking of a smooth walled air duct, and plan to do testing of other hardware cycling configurations.

Sailors who are confident in their diesels may want to know that I made some modifications to my 2GM20F for handling a large alternator. With beefed up bracketing, a serpentine belt, improved alternator/engine/battery cooling, and controls which allow current to reach maximum (120+ amps) when engine output permits, my Lifeline batteries are typically "full" in about two hours. Lithium batteries are not in the immediate future for the system, but three 100+ watt solar panels are a near term project.

Hopefully some will find my experience useful–and find small boat tropical cruising more enjoyable. Any ideas which could improve the concept of 12 volt AC on a small boat?
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Old 16-09-2016, 08:32   #2
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Re: My Real Life 12 Volt AC Experience

I have tried it too, I have a 5,000 BTU Webasto under our bunk so duct length is about 2' maybe 3'.
It draws with the March pump about 6 amps AC, so about 60 amps assuming 100% conversion efficiency, which isn't possible, but you have to use something right?
I don't know draw as it comes from my inverter that is wired directly to the bank without a shunt. I know AC draw though and infer DC draw from AC draw.
We could I guess go with a 50% duty cycle and incorrectly assume no current draw in the off time, mine the fan runs continuously. So for just argument 5 hours a night at 60 amps draw is 300 AH. I have a 660 AH bank, so it's theoretically possible, but to recharge I have to burn Diesel as you say.
So instead I just run the generator at night and cool the whole boat, and not cycle my batteries which is worth something money wise.
To run the generator for that 10 hours is 2.5 gls of Diesel, and I'm not running the big motor in the morning.
So far I do not frequent crowded anchorages, and the once or twice when I do, I tuck up near the big motor Yachts that are running their generators too or I'm the guy away from everybody, down wind.


I think to make it viable it's going to take at least a KW of Solar and LifePo batteries.
It's doable, if you can get enough Solar and a big enough bank. It's just power of course so answer is you have to have a little more going in than you have going out is all.
Then you need to determine if its worth it or not, just cause it can be done, doesn't mean always that it needs doing?
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Old 16-09-2016, 13:15   #3
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Re: My Real Life 12 Volt AC Experience

What about eliminating the March pump entirely?Keel Coolers
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Old 16-09-2016, 14:32   #4
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Re: My Real Life 12 Volt AC Experience

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when engine output permits, my Lifeline batteries are typically "full" in about two hours.
How are you doing that?

Even with a .4C charge or 40% or Ah capacity in charge current (200A for a 500Ah bank) it still takes about 5.5 hours to go from 50% SOC to 100%. This is with very healthy AGM batteries and a lab controlled charge source that does not drop to float prematurely. Once your Lifeline batteries hit 14.4V, no matter what you have for current, you can only charge them as fast as they will accept it at 14.4V and XX SOC.

You will want to be very careful you are not walking the batteries down from continued PSOC use. Because your AC loads are a bit beyond the 20 hour discharge rate, at 40A, you would be best to set your bottom voltage (where you begin recharging) at about 11.9V - 11.95V or so. With a 25A load your 500Ah Lifeline batteries would cross 50% SOC at approx 12.1V so allowing for the additional 15A on top of the 20 hour rate 11.9V - 11.95V is a pretty safe bet & likely where you'll cross the 50% SOC threshold under a 40A load.
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Old 16-09-2016, 14:45   #5
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Re: My Real Life 12 Volt AC Experience

twelve, an adjunct to insulating the hull sides is to insulate the cabin top, under the headliner. Makes a big difference in cutting down solar heat gain during the day.

In fact I would recommend you do the headliner insulation first.
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Old 16-09-2016, 15:08   #6
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Re: My Real Life 12 Volt AC Experience

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How are you doing that?

Even with a .4C charge or 40% or Ah capacity in charge current (200A for a 500Ah bank) it still takes about 5.5 hours to go from 50% SOC to 100%. This is with very healthy AGM batteries and a lab controlled charge source that does not drop to float prematurely. Once your Lifeline batteries hit 14.4V, no matter what you have for current, you can only charge them as fast as they will accept it at 14.4V and XX SOC.

You will want to be very careful you are not walking the batteries down from continued PSOC use. Because your AC loads are a bit beyond the 20 hour discharge rate, at 40A, you would be best to set your bottom voltage (where you begin recharging) at about 11.9V - 11.95V or so. With a 25A load your 500Ah Lifeline batteries would cross 50% SOC at approx 12.1V so allowing for the additional 15A on top of the 20 hour rate 11.9V - 11.95V is a pretty safe bet & likely where you'll cross the 50% SOC threshold under a 40A load.
Well even at 2 hours run he's still burning 1.5 gallons of diesel. At 5 hours recharge he's closer to 3.5 gallons. Why not run a Honda eu2000 to run your ac , And only use 1 gallon of fuel for every 5 hours of runtime while saving your expensive batteries from a short life and keep hours of your Eng hour meter ?
If you know your going to be running the next day great no biggie but otherwise your wasting a lot of $ I would think. In the long run the fuel savings could probably buy you a new Honda each year, and you could be cooling your entire boat not just a little cabin. For the record I have done ac installs on cats that wanted to do what your doing in one cabin or Two. In those cases they had a 850-1000ah bank, used a 5k btu ac unit, and had 750-1000 watts of solar to help recharge the batteries.

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Old 16-09-2016, 15:32   #7
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Re: My Real Life 12 Volt AC Experience

Or run a little Diesel gen and burn one gl every four hours, and keep the batteries fully charged, light the outside of the boat so drunks see you, watch TV and all kinds of things?
Answer is of course noise and the gen has only X number of hours of life, so I only run it if I'm laying there trying to sleep soaked in my own sweat, or like last weekend there were an enormous amount of mosquitos, There were so many I was trapped in the boat until 10 AM, never seen it that bad before.

Once free of being a wage slave, I do hope to chase fair weather, and be wherever it's comfortable and not use the AC, but I'm planning on a twice weekly generator run to get the batteries to 100% twice a week.


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Old 17-09-2016, 03:54   #8
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Re: My Real Life 12 Volt AC Experience

Greetings and welcome aboard the CF, twelve.
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Old 17-09-2016, 10:41   #9
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Re: My Real Life 12 Volt AC Experience

Gosh, two rational AC threads two day in a row. Not just the OPs, but the replies too. CF is becoming mature at least as to AC. Twelve, yours is a good question that makes sense, with plenty of facts, and if you continue to participate actively at that level, we will all benefit. Welcome aboard!
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Old 18-09-2016, 14:43   #10
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Re: My Real Life 12 Volt AC Experience

The comments and suggestions are on point, and hopefully I can contribute further to the discussion. I’ve only time to reply to one right now: keel cooler devices. Interesting idea, but it has to be considered that even a small AC unit has a much larger condenser than a moderately sized refer. And the work to modify the coolant lines is certainly more than I can do. Anyone know the effectiveness of these devices in really warm waters? Also, it is worth considering that the pump (March model 893) that would be replaced draws only 1.5 amps.

I should be able to post another reply in the next day or so.
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Old 18-09-2016, 15:56   #11
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Re: My Real Life 12 Volt AC Experience

I think you have hit on the cusp of the problem, is it even worth it? Only you can decide that.
1.5 amps at 120 VAC is of course 15 amps at battery voltages or about three times what an average fridge draws.
Water temp is irrelevant as either way, it's water cooled by the same temp water whether keel cooled or pumped so if one will work, the other will too.
Either requires a hole in the boat, lot more refrigerant line though and now your not talking about a drop in box like a normal package marine AC is, now your separating and installing components, vacuuming down systems, determining correct refrigerant charge etc.
For me it's not worth it, but it may be to the person that was determined to make the most efficient air-conditioner possible, cause really I don't think it matters if it's AC or DC, efficiency is what matters, for instance even though we all think of the Danfoss compressor being a DC machine cause we wire it in to DC power, It's actually an AC compressor, and may be three phase, I'm not sure about that.
So I think to make this viable you need to consider first and foremost what is the most efficient way, not most compact, easiest or least expensive which is I think the primary considerations for off the shelf marine AC units.
Lot of work, and may not be cheap either?



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Old 19-09-2016, 12:09   #12
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Re: My Real Life 12 Volt AC Experience

I’ll try to get to the discussion about battery re-charging soon, but first:

Pilot, thanks for your ideas, and I guess it would be helpful to be clear what the amp-hour loss from the pump really is. The pump draw is 1.5 amps at 12vdc. The estimate amp-hour drain on the battery bank over night is thus about 6. Not much at all compared to the rest of the system.

Adding under deck insulation. I’m not sure what effect this would have during a typical summer day, but am concerned about the effect on heat dissipation at sunset from the cabin and quarter berth. Right now I’ve got fairly effective sun "shields" on my ports and hatches, which brings daytime cabin temperatures while underway (with everything closed but the companionway) all the way down to about 100 degrees. At anchor during daytime, opening hatches and using passive means to increase breeze gets the temperature in the cabin–so long as the thermometer is in the airflow–in to the mid to high 80’s. But even then temperatures on or about the inside hull, on the shaded side of the boat, still top 90. Would under deck insulation lower cabin daytime temperatures? I don’t know. Incidently, I do have a bimini and dodger, and even though adding large awnings could do more, I’m pretty sure that topside under shade is always going to be cooler than below, whether there is under deck insulation or not. (Virtually no time is spent in the cabin during the day, while at anchor or while underway).

So, the sunset cool down period–with or without AC–is a problem because 1) inside hull temperature stays at 90+ no matter what, 2) passive air flow in the quarter berth is poor and fans must be used, and 3) the engine box is mostly in the quarter berth so, depending on the cruising program, engine heat could still be undergoing dissipation. Since heat rises, could under deck insulation actually slow sunset cool down of the quarter berth and/or cabin?

On the other hand, at 90+ degrees the hull serves as a radiator for continuous sea heating. Hull insulation has got to help. To what extent, I don’t know.
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Old 20-09-2016, 10:44   #13
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Re: My Real Life 12 Volt AC Experience

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It's actually an AC compressor, and may be three phase, I'm not sure about that.
The BD compressors are fitted with a brush less direct current motor, which is electronically commutated by an electronic unit. Danfoss.
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Old 20-09-2016, 11:04   #14
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Re: My Real Life 12 Volt AC Experience

I have to imagine insulation has to help. Even if you trap heat during the day if you open the top hatch the first 10 minutes of operation you should clear it. You may get some more loading for the first hour but run times after that should drop. Insulating the hull sides would help too. Really if you get the insulation part down the power required should drop a lot. Also when at the hook during the day you could rig a fore deck awning that should keep some of the heat away.
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Old 20-09-2016, 11:58   #15
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Re: My Real Life 12 Volt AC Experience

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The BD compressors are fitted with a brush less direct current motor, which is electronically commutated by an electronic unit. Danfoss.

I think, but cannot verify that the module through an inverter converts to three phase AC and that is what runs the compressor

Point is though, that a DC airconditioner will not necessarily draw less power than an AC one. It may be that even with conversion losses an AC one is more efficient, certainly more available.
Best way to make air conditioning off of your battery bank viable is by finding one that is very efficient, not necessarily DC.

Just as Solar propulsion is possible, Solar air conditioning is too, both likely requiring large banks of batteries and panels and air conditioning would benefit from efficient insulation and shading. If I had to guess I'd guess that very effective insulation and blocking heat gain would be key to making it work, cause if you insulate well enough and block heat gain, maybe a 5K would cool the whole boat? Maybe 2K would the sleeping area? If you could get it down to 2K, than that looks a lot more easily done off of a battery bank.


Starting to sound a lot like making your fridge work.




On edit, I suppose it is possible that the Danforth is pure DC, but pulsed DC. I'm not smart enough on brushless speed controllers even though I have flown many a model airplane and dove with a Scotter that was Brushless DC with a speed controller.
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