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Old 07-08-2015, 14:49   #1
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Help with AC load calculations?

Hi all. Planning an AC installation on a newly purchased trimaran, and want to make sure my thinking is correct in computing the electrical draw of the system.

I'm looking at installing two separate units, one 5K BTU and one 16K BTU, both 120V AC. The 5K unit draws 4.4 amps running and the 16K unit 11 amps. Trying to plan on ample battery capacity so that when my solar/wind is not producing enough to power these units I'll be able to make it through the night and into the next charging cycle.

Inverter is a 3000W continuous / 6000W peak that runs at a claimed 90% efficiency.

In terms of the base calculations, I'm running under the assumption that the units will operate for 12 hours overnight on battery alone at a 50% duty cycle (more in the early evening as the heat of the day is dissipated, less towards dawn hours when the thermal load of the vessel and contents has been reduced.)

So, am I thinking correctly that the inverter draws a DC load from the batteries equivalent to the AC load plus the 10% lost in the conversion? In other words, does a 120 watt A/C light bulb (120V x 1A = 120W) draw 11.1 D/C amps from the battery bank through the inverter (12V x 11.1A = 120W/0.9)?

I understand it's not always linear as battery voltage drops off as the batteries discharge, but this would give me a starting point from which I could build in some additional capacity to make up for other loads and the expected dropoff in efficiency as the batteries get lower.

So in my test case above with the two AC units, I'm calculating a total draw over a 12 hour period from the battery bank of

15.4A combined draw * 120 volts = 1850 Watts

which would draw 171 Amps while running

12V * 171A = 1850W/0.9

or about 85 amps average over the course of an hour (50% duty cycle), and then extrapolated out to roughly 1000 amp hours of battery storage required to meet the draw over a 12 hour period, or roughly 2000 AH of storage to meet the load and maintain somewhere around 50% battery capacity in the morning.
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Old 07-08-2015, 15:27   #2
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Re: Help with AC load calculations?

Your power draw calculations are in the ball park. However, a couple of things to consider. Are you planning to charge your batteries by wind and solar exclusively? If that is the plan then I assume you do understand that it takes longer to put charge in a battery than it does to take it out, at least for any version of lead acid batteries. LiFePo I understand is different.

Also to generate that much charge with solar during the 4-6 hours of peak sunlight would take minimum 3000 watts of solar panels. Even cheap panels you're looking at $5-6,000 installed, covering most of your boat with panels and still it is very doubtful you would be able to charge your batteries back up to full.

More practical is to plan on usable capacity of 25-35% of the batteries. Even if you are able to get 50% of the battery capacity you're still talking about 20 six volt golf cart batteries, Trojan T105s or similar. Round numbers $2,000.00 or more and over 1000 lbs. If you go with LiFePO batteries then the initial cost would be much more, double or triple I think.

Have you thought about installing an AC generator? Would weigh less than the panels and batteries and in the long run probably be cheaper.
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Old 07-08-2015, 16:10   #3
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Re: Help with AC load calculations?

Thanks for the reply, skipmac. I have considered a gen set, and do plan on having a honda 3000 in the mix for emergency use, but the end goal here is to never have to plug into shore power and never have to buy fuel. For the mean time I will retain the Perkins diesel as a drive source but down the road would like to convert to electric propulsion so that I could be 100% self sufficient in the event that the zombie apocalypse happens

The trimaran is 25' wide, and will have a full width set of davits on the back that should allow me to support a total of 14 panels between the davits and the top of the 25' wide pilot house. Actually, the cockpit is deep enough that I could support two full rows of 7 panels each, giving me a total of 21 panels generation capacity without losing an inch of deck space.

I can get a pallet of 20 310W panels for $5,600 giving me a solar generation capacity of over 6000 watts. Combine that with twin 400 watt wind generators mounted to the rear davits and a home-made 500W towed hydrogenerator and I think I should have enough generation capacity.

I hadn't considered the charge vs discharge time. Any idea what the relative ratio there is?
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Old 07-08-2015, 16:39   #4
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Re: Help with AC load calculations?

On a related note, I came across this product on Amazon (I cant post a link) : ML35-12 - 12V 35AH Sealed AGM Battery

It's an AGM battery at less than $2 per AH of storage capacity. Assuming that I don't want to blow the entire bank acct on storage by going Lithium, this is about the cheapest AGM storage I've come across. Anyone seen an industrious person run a bank of, say 40 smaller batteries as opposed to 4 or 5 large 8D's?

How about a custom bank made out of lithium laptop batteries? Something like 100 batteries in a custom chassis? Looks like you could get about 500AH of lithium storage that way for about a thousand dollars.....
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Old 07-08-2015, 16:54   #5
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Re: Help with AC load calculations?

If you live without aircon you will make it no prob. I run a fridge compressor and a freezer compressor of 300w of panels and 500Ah batts I have an additional 900w of panels normally covered but available if I need it. I would buy some good quality fans if I was you. Not sure if my AC units still work!
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Old 07-08-2015, 16:57   #6
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Re: Help with AC load calculations?

Emmalina, if it were up to me I'd not put AC in at all. My wife, on the other hand, has had the first few introductions to hot flashes and it's unfortunately going to be a downright necessity
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Old 07-08-2015, 17:03   #7
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Re: Help with AC load calculations?

There are always complications in life
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Old 07-08-2015, 17:42   #8
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Re: Help with AC load calculations?

Quote:
Originally Posted by Serapium View Post
Thanks for the reply, skipmac. I have considered a gen set, and do plan on having a honda 3000 in the mix for emergency use, but the end goal here is to never have to plug into shore power and never have to buy fuel. For the mean time I will retain the Perkins diesel as a drive source but down the road would like to convert to electric propulsion so that I could be 100% self sufficient in the event that the zombie apocalypse happens
I know this isn't your question but how do you plan on replacing the batteries when the zombies are in command? With the amount of work you plan for your batteries you could easily be replacing them every 3-4-5 years. So how will you be self sufficient when it''s time for new batteries?

I guess you could open up the old batteries, melt down and refine the lead, cast new plates, make some sulfuric acid and reassemble the batteries.

OR, you could buy a multifuel generator, make a simple still and run it on ethanol or rig a biodigester and make methane gas.

Bottom line, if the global shirt hits the fan, running an air conditioner by any means is going to very unlikely and probably the least of your worries.

The other bottom line, running air con on batteries even in "normal" times is going to be very expensive and require some significant sacrifices in weight, space, performance, etc.

You're going to have to make similar sacrifices if you switch to electric drive.

Don't get me wrong. I am 100% in favor of energy independence, green technology and would love to go electric drive (I am after all an EE) but the compromises required are just too limiting for my cruising plans.
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Old 07-08-2015, 17:53   #9
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Re: Help with AC load calculations?

Quote:
Originally Posted by Serapium View Post

So, am I thinking correctly that the inverter draws a DC load from the batteries equivalent to the AC load plus the 10% lost in the conversion? In other words, does a 120 watt A/C light bulb (120V x 1A = 120W) draw 11.1 D/C amps from the battery bank through the inverter (12V x 11.1A = 120W/0.9)?
If you take the 0.9 as good then your numbers are roughly correct in amps and absolutely correct in watts.
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Old 07-08-2015, 17:59   #10
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Re: Help with AC load calculations?

Quote:
Originally Posted by Serapium View Post
The trimaran is 25' wide, and will have a full width set of davits on the back that should allow me to support a total of 14 panels between the davits and the top of the 25' wide pilot house. Actually, the cockpit is deep enough that I could support two full rows of 7 panels each, giving me a total of 21 panels generation capacity without losing an inch of deck space.
You need to think about weight. 20+ panels plus all the hardware for mounting is going to be approaching 1000 lbs. Add this to the 1000+ lbs of batteries you are going to be at or over the weight of an engine, generator and fuel.

Also need to consider the windage of all the panels. I would want to be able to remove them and stow them below in case of a bad storm.
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Old 07-08-2015, 18:07   #11
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Re: Help with AC load calculations?

Quote:
Originally Posted by Serapium View Post

which would draw 171 Amps while running

12V * 171A = 1850W/0.9

or about 85 amps average over the course of an hour (50% duty cycle), and then extrapolated out to roughly 1000 amp hours of battery storage required to meet the draw over a 12 hour period, or roughly 2000 AH of storage to meet the load and maintain somewhere around 50% battery capacity in the morning.
Your Ah capacity requirement is optimistic for three reasons.
  1. The rated Ah capacity you buy is based on C/20 discharge rate. Given that with the air cons running you will draw about C/12, the capacity you will get will be below rated capacity. You will need to increase your capacity to account for this (Peukert effect).
  2. You seem to be assuming that the batteries will hit 100% every afternoon and then draw down to 50% by sunrise. The 50% is OK but the 100% is optimistic in most cases. Note that battery acceptance is very low at the top end of the curve, hence for the last 10% or 20% or charging you need your controller will throttle back power just because the batteries will not take it quickly enough. This effect will be more marked with flooded batteries, less so with AGMs. I would be more comfortable with 80% instead of 100%. This mutiplies your capacity requirement by 5/3.
  3. As batteries age their capacity will drop below the rated capacity. Allow extra 20% at least for this.

All this is a long way of saying that there is a reason why folks who like their aircons while at sea tend to rely on generators.
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Old 07-08-2015, 18:53   #12
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Re: Help with AC load calculations?

In the end I strongly suspect that I'll go with just the one 5K BTU for our stateroom and run it off a Honda 2K gen set when the need arises (i.e. hot flashes ensue), but I like seeing what the options are and thinking down the road. If we decide to make a lifestyle out of this I'll be much more serious about putting $20K - $30K into making the vessel truly energy self sufficient.

One offset to the extra weight of the batteries and panels needed will be achieved by the removal of the big old diesel and associated fuel tanks and fuel. That alone should offset many hundreds of pounds I would think. Any idea what a Perkins 4.236 weighs? I tried looking it up online but came up empty.

Storage of the 7 panels on the davits shouldn't be an issue. One thing we have a TON of is storage space. The two rows of seven panels on the pilothouse would be permanently mounted to the hard roof and thus not exposed to wind.
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Old 07-08-2015, 22:27   #13
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Re: Help with AC load calculations?

Quote:
Originally Posted by Serapium View Post

1. ...but the end goal here is to never have to plug into shore power and never have to buy fuel. For the mean time I will retain the Perkins diesel as a drive source but down the road would like to convert to electric propulsion so that I could be 100% self sufficient in the event that the zombie apocalypse happens

2. I hadn't considered the charge vs discharge time. Any idea what the relative ratio there is?
1. Laudable goal, but until you finish your energy budget and input, you won't know. You're getting closer, but even a fridge is a big impact and that's only 60ah / day. AC? Wow! Zombies? Even better.

2. Look up (Google or any boating forum search engine) "battery acceptance." That'll answer your questions. It's NOT linear to return juice to a bank, only to take it out.
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Old 08-08-2015, 04:44   #14
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Re: Help with AC load calculations?

When you are looking for your AC unit, Try to find an "inverter" type, Low start up current and far more efficient, COP's around 5, basic reason they are better, they speed up and slow down compressor from start instead of big clunk on and off, ( & no start surge)
Also do not forget to factor in your other loads & losses, If you assume 30% loss from battery to AC unit, you will not be far wrong,

Battery charging, The general rule of thumb for FLA's & AGM's is it takes 20% of the time to bulk charge (reach it's absorb voltage) and 80% to absorb, Assuming reasonable charge currents eg around 10% of 20hr

Other battery points, In the off grid solar "world" 1 string is considered ideal, 2 strings "OK" 3 strings "hard work" In the Boating world I would go with the two strings, A little more work to maintain, But you get the "back up" value, I would also prefer to go at least 24v to cut cable size & loads down,

Tim
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Old 08-08-2015, 11:32   #15
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Re: Help with AC load calculations?

Even given the price at this size of installed power I really think you are in the lifepo battery range. They have a much faster acceptance rate, and a much deeper depth of discharge tolerance. Add in the weight savings, and on a very weight sensitive boat like a trimaran it really needs to be explored.

Tri's more than any other boat are highly suceptable to over loading. They have huge amounts of empty room, that honestly really needs to be left as empty room to function correctly.
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