Run a/c off batteries

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tadel001
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Tue Jan 31, 2017 11:10 am

We have a 6000 BTU ac unit. Looking to see if we can run that off a battery for 10 hours or so. It would recharge on shore power. Has anyone done this?

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tego
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Tue Jan 31, 2017 12:52 pm

You don't say what your battery bank capacity is, nor what type of inverter you have. If both are of sufficient capacity, then yes, you can run that long. It does require a large battery bank of 5-600 AH for intermittent use of the A/C. If it's a hot day and the A/C runs continuously, you're going to need a generator. Ben

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Tue Jan 31, 2017 12:59 pm


tadel001
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Tue Jan 31, 2017 4:43 pm

My current battery/inverter are not so important as I will run this off a new battery bank/inverter. I am just trying to see if this is practical.

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DesertAlbin736
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Thu Feb 02, 2017 2:19 pm

I wish I had that problem to worry about. But here's some spec numbers on the Pompanette Thru Hatch AC ($1,200 at West Marine) for example. May not be the same unit as yours, but it's a 6,000 BTU unit for comparison, which draws 5 amps at 115V AC, with a 14 amp starting current. That's a 1,610 watt momentary starting load, and a 575 watt running load. Assume that's max, and the spec doesn't mention if that's real load, ie taking into account power factor, or includes reactive load. So that translates to 48 amps @ 12V DC continuous and 134 amps @ 12V DC for starting current. What would that starting current draw do to your inverter? Let's say rounding up to 50 amps DC continuous accounting for inverter losses. So in 10 hours running time that's 500 battery amp-hours, plus your other house loads. Of course if you're at a dock you could run off AC shore power. I think if it were me and there was space available I'd be thinking in terms of at least a 2000 watt minimum genset, preferably 3000 watt & diesel powered to run off your main fuel tank. I would think just running your main engine at anchor to keep the battery up would not be a good idea as diesels don't like to be run for long periods at light load.
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WillieC
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Thu Feb 02, 2017 2:27 pm

I think DesertAlbin meant that it would NOT be a good idea to run the main for the light loading of battery charging.

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DougSea
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Thu Feb 02, 2017 10:50 pm

Possible - Yes. Practical - Not so sure. When we had our 28 I briefly looked into it. Traded up before I got too far. Depending on the efficiency of your A/C unit and your inverter you're probably going to need an awfully big bank of batteries. Have you considered a small Honda generator? EU2000. Several members of the forum have them. I realize this means bringing gasoline aboard, but if done right it can work safely and well. Make sure you have a CO detector and run it up on the hardtop.

At the end of the day it's just math telling you how many batteries you need. I guessing that most of the inverter companies could help validate the calculations. You might also look at setting up a 24volt system as that should improve inverter efficiency. (I looked into a 48v system for our Island based Yacht Club. Higher input voltage definitely helped)

I found this calculation overview pretty complete - https://electricalnotes.wordpress.com/2 ... tery-bank/
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mboatworks
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Sun Feb 19, 2017 11:21 am

I think DesertAlbin736 is pretty much summed up the amp draw. I would estimate it could be more around 60A/Hr or 600A over 10-hours, by the time you factor in the efficiency losses through the installation and inverter.

Your typical Group 27 Deep Cycle battery has about 90AHr. Anytime you drain a wet cell battery more that 50%, you greatly reduce its life or cycles. This is one of the advantages of Lithium and Carbon AGM batteries, which gives almost 90% of usable capacity.

So if you use that 50% of that Group 27 battery... Then you would need about 13 batteries to run the A/C unit for 10-hours. You would need roughly a battery for every hour you wanted to run that A/C unit.

I feel it is the duration that make this a unrealistic situation. Drawing that much power (600W) through an inverter is fine, even for your smaller 1000W inverter/charger units. I would have no problem using an inverter for periodic heat or cooking, but I would not recommend it for powering an A/C unit.
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