Air conditioner running off batteries

[colinhulse]

Consider if you will a cruising vessel with a center cockpit and a bedroom aft. I carefully insulate that room and install a small 6000 BTU air conditioner. The draw is 4.5 amps @ 120 volts. So I need approx 50 amps per hour from a 12 volt battery bank (Losses to inverter etc). I have...say 900. I can use about half of that, so I should be able to run it continuously for 9 hours. I run this while I sleep and recharge the next day with wind, solar or an on-board generator. (I don't want to sleep with a Genny running) This would be for hot sticky nights on the hook while traveling. The boat would have a bigger AC unit for shore power but this would allow for a good nights sleep when it is nasty out there. Any thoughts?

 

[colinhulse]

inverter!!

My best guess why this won't work is the lack of an inverter that will convert 50 amps at 12 volts to 4.5 amps at 120 volts. Maybe a 24 volt battery bank is better suited?

 

[CharlzO]

My guess is that you would need a lot of solar, genny running, etc. to top off a 450 amp deficit in a day.

 

[Steve Dion]

Here is a thought. Get a portable generator and put it up on the bow and let it run your AC.

 

[JohnShannon]

Can't you get a 12V unit? that would avoid double conversion. 6000 BTU is about 1700W of cooling which sounds like a lot, you only put out 100W, the other heat is hot air/heat coming from the outside through the insulation. How many square feet of wall/roof/hull in this cabin and what is the R value?

Starting current on the airco is usually several x the running current which may crunch your inverter.

 

[JohnShannon]

Here is a link to http://rencoolusa.com/dc-products/air-conditioner-compressor-drive-unit/
This claims 11000 BTU for 50A at 12V so roughly 2x the efficiency of the inverter to AC motor approach..

 

[kloudie1]

Dometic makes a small 12V unit called the Cuddy. I think it is about 3500BTU and draws around 25 amperes. http://www.dometic.com/4fc9ef64-bd40-404d-b1c0-1a68775ba27a.fodoc
This would be ok once the sun load is off the boat, and only for one cabin. Probably fine for sleeping, but would not do much with the sun heating. We used to use a little 5000 btu unit for nightime only and it would cool the entire boat fine for sleeping.

 

[bobtigar]

4.5 amps at `120v=approx. 40 amps at 12. Then factor in the inverter. That's each hour. Bob

 

[Maine Sail]

4.5A @ 120V is 45A at 12V + add about 20% inefficiency for the inverter or 54A.

This does not even take the Peukert effect into account. On a 900Ah bank of flooded batteries this 54A load equates to a Peukert corrected load of about 57A.

Of course you have been off cruising so your bank will rarely if ever be back much above 80% SOC. So we really only have about 270 Ah's to work with or 80% of 900Ah's. 720Ah's - 450Ah's = 270Ah's to 50% SOC.

Lets not forget the other house loads which may average another 10A on top of the AC.....

So 54A from the AC + 10A for other house loads like refrigeration, anchor light etc. This means 64A and when Peukert corrected yields approx a 71A load the bank is seeing..

This means you have about 3.8 hours before you hit 50% SOC but now you have to put back in 270Ah's + 15% for charge efficiency or 270 X 1.15 =311 Ah's to get back to 80% SOC....... With a 100A alternator (running hot at 75A) this would take approx 4 hours of continuous charging to go from 50% SOC to 80% SOC on a 900Ah bank.

When you run out the math you can see why AC's are generally not run off 12V lead acid batteries....

If the bank is smaller than 900A this only gets worse as Peukert really takes a deeper bite...

The 64A load when Peukert corrected on a 500Ah bank would be about 83A......

 

[woodster]

you can run a lot of camfo fans on that kind of power.....just saying

 

[cephius (Dave)]

Quite a bit of technology management going on here.

We just sleep on deck when the cabin is hot!

 

[alexco38]

How about one of this cool mist over the boat.
http://www.koolfog.com/misting-systems/

 

[Winkfish]

I know I'm from a hot humid climate already, but I've found that when I go to places further south without AC, I get used to it after a day, maybe two, so that a fan feels like a blast of cold air. Wind scoops and fans are your friends.

 

[Skipper]

I know I'm from a hot humid climate already, but I've found that when I go to places further south without AC, I get used to it after a day, maybe two, so that a fan feels like a blast of cold air. Wind scoops and fans are your friends.

Also canvas the boat. Large boom tent gunwale to gunwale canopied out on poles, and a bow tent, adding to your cockpit coverage (bimini\dodger, etc). Shaded decks makes for a giant difference inside the boat. Add low draw fans and good ventilation and it can be bearable, even comfy.

At night the boat will already be cooler from the shade during the day. Still warm, though. But that's what you want it's the beauty you've dreamed of during the winter of snow and ice.

Alesco's mister looks like fun too.

 

[seadaddler]

Cooling

I put canvas off the deck for shading but also still allow a breeze and than
run the AC with Gen for a hour or 2 for topping off the batteries and the AC
to cool the cabin and make coffee with my electric drip for the next morning
and than let fans run all night for cooling.
Nick

 

[Dave602]

We also have a large shade from the bow back to the dodger, the wife also made reflective inserts for the windows. It's amazing how much difference that made.

 

[Benny17441]

I tried that once at home with a 100 amp battery just for the heck of it. The unit ran well for around 15 minutes before the low voltage switch on the inverter aborted the job. What happens is that if a battery is discharged quickly under a high load the capacity declines markedly. No substitute for a generator.

 

[CarlN]

I think Mainesail's numbers are closer than yours but you could do better with a few assumptions:

Have a genset for lots of reliable electricity. Use it to fully charge the batteries during the day. You could even look at installing lithium batteries that give you a wider range and faster charge acceptance.

I realize none of this is cheap but you posed the question without a cost limit.

Pre-cool (and dehumidify) the aft cabin with the genset. More than half of A/C energy goes to dehumidification (latent heat).

At night, you won't have any sun heating outside surfaces but you still need to control warm, humid air getting into the cabin. Seal up air leaks.

With luck, you can reduce the run time of the compressor to well under 50% during the night.

Another route is to install a genset so that you can't hear it at night (or at least it is quieter than the running A/C). Really not that hard if you mount it well away from your berth, use good sound insulation, and a water separating exhaust.

But all this is sort of silly expensive compared to fans, ventilation, and a late bed time.

 

[CarlN]

Or just spend the summer months cruising in Maine.

But you'll need extra blankets.

 

[colinhulse]

Thanks for the replies

Great responses. I will save this. If the air was operated off the Genny until bed time that would mean the room would be cool and the unit would not run constantly. True however that the start loads would be very hard on an inverter. I bring this up because I have never heard of it being done and wonder if it is possible. Thanx again.