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Old 22-04-2014, 15:50   #1
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Are my Maths Correct?

I have a 240 volt AC unit rated at 800 watts.

Am I correct that in theory to run this through an inverter it would draw 67 amps per hour from a 12 volt battery bank? 800/12 = 66.6.

Actually, I gather it would draw maybe 20% more because of losses in the inverter. So starting with a 600ah battery bank, that should never be allowed to drop below 50% (300 ah) could I power the AC for a maximum of 3.75 hours?

The bottom line is that I only want to run it for about an hour at a time. I find that sufficient just to get the boat back into a comfort zone.
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Old 22-04-2014, 16:04   #2
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Re: Are my maths correct?

You can expect a 'best in class' inverter to have an efficiency of around 85%. If you were to desire an output of 800W then it'll cost you 800/.85=940W from the battery banks, bringing in your draw at the inverter input to around 78A. Note that the AC's power rating will be steady state, and that the initial start-up current draw of the blower motor and compressor may well be higher than 800W, due to the higher torque at startup.

Your best bet is to hook a current loop (fluke) onto the 240V input of the AC unit as it starts up to see what the peak instantaneous current draw is at the moment you start it. Most good loops have a "max value" memory so you can see if your inverter is capable of delivering the necessary current at startup according to the manual.

Once that's verified then you can size your battery bank according to the steady state draw of 800W.

I would personally aim to size the system such that you do not drop below 70-75% charge state for such a high-draw demand.

This may seem to cost more upfront but you'll be rewarded with a much greater lifetime on your batteries!


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Old 22-04-2014, 16:05   #3
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Re: Are my maths correct?

That's just 67 amps

(not per hour - it will draw 67 amps for as long as it is operating. It will draw 67 "amp hours" per hour ).

That nit picking apart, you are correct in your ballpark figures.
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Old 22-04-2014, 16:38   #4
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Re: Are my maths correct?

Quote:
Originally Posted by Alenka View Post
Actually, I gather it would draw maybe 20% more because of losses in the inverter. So starting with a 600ah battery bank, that should never be allowed to drop below 50% (300 ah) could I power the AC for a maximum of 3.75 hours?
Unfortunatly your calculations are little optimistic.
The battery capacity depends on the discharge rate. Your actual battery capacity at this high rate of discharge will be less than 600AHrs. In addition it is unlikely your batteries will be a genuine 100% charged with their brand new capacity.

The load will reduce your battery to 50% in a much shorter time than 3.75 hours. You will also need to add the other electrical demands.

In practice to stay above 50% SOC with AC 1hr running will be marginal.
It is also worth rembering that this power needs to be replaced.
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Old 22-04-2014, 16:46   #5
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Re: Are my maths correct?

Noelex,

Once the saloon is cool enough, replenishment of charge through a dynamo connected to an elevated bicycle frame will become a viable option! ;-)


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Old 22-04-2014, 16:59   #6
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Re: Are my maths correct?

Air conditioners are meant for shore power or whilst a generator is on.
I dont think anyone on this forum has one that operates off batteries.
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Old 22-04-2014, 17:01   #7
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Re: Are my maths correct?

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Noelex,

Once the saloon is cool enough, replenishment of charge through a dynamo connected to an elevated bicycle frame will become a viable option! ;-)
....
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Old 22-04-2014, 17:08   #8
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Re: Are my maths correct?

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Noelex,

Once the saloon is cool enough, replenishment of charge through a dynamo connected to an elevated bicycle frame will become a viable option! ;-)


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A high end bike generator with an in shape rider generates about 100 watts/hr. Assuming charging voltage of 13.5V you are going to make about 7.4 amps/hr. So to replace the 300amp-hrs you used you have to ride for 40 hours. I don't think this is a practical solution.
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Old 22-04-2014, 17:19   #9
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Re: Are my maths correct?

I dunno, I think this is worth some consideration.

Just imagine the glee that some desperate befuddled cruiser will feel in the future when they're faced with the dilemma of whether to sacrifice 40hours of hard graft just for an hour of smooth, blissful and sweat-free joy, or just to sit with a mostly charged warm battery bank in a urea-solution drenched berth next to a disused bicycle. What if this was a boat with 40 people on board with 40 bicycles, would it be worth it then? Could one hour's worth of a 800W AC unit keep the troops in check? These are the sort of questions that the self-obsessed society we live in today couldn't ever envision asking.

I really think we're pushing the envelope of CF's knowledge base here and I've never felt so alive!


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Old 22-04-2014, 18:12   #10
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Re: Are my maths correct?

I'd be willing to bet that several bike riders pushing hard would generate more heat than the AC can carry away.
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Old 23-04-2014, 02:01   #11
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Re: Are my maths correct?

Quote:
Originally Posted by noelex 77 View Post
Unfortunatly your calculations are little optimistic.
The battery capacity depends on the discharge rate. Your actual battery capacity at this high rate of discharge will be less than 600AHrs. In addition it is unlikely your batteries will be a genuine 100% charged with their brand new capacity.

The load will reduce your battery to 50% in a much shorter time than 3.75 hours. You will also need to add the other electrical demands.

In practice to stay above 50% SOC with AC 1hr running will be marginal.
It is also worth rembering that this power needs to be replaced.
Yes, I guess my initial assumptions were based on everything in peak condition. I was thinking of an Efoy fuel cell to replenish the power. I don't mind this running for a few hours or more through the night because the noise is so low.

The alternatives are for a fixed diesel Genset installation or maybe a suitcase petrol genny such as a 2kw Honda. The cost of fitting the former was eyebrow raising to say the least and the latter is not the perfect solution either. Both make noise!

(sorry guys Efoy is not a make of bicycle!!!!!)
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Old 23-04-2014, 05:26   #12
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Re: Are my maths correct?

I've run our primary 1400W AC with a Honda 2000i plugged into the system before. If you're in a lonely anchorage (as to not annoy your neighbors) you can float the Honda in your tender - that way the sound of the generator is actually indistinguishable from that of the AC blower.


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Old 23-04-2014, 12:36   #13
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Re: Are my maths correct?

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Originally Posted by Cavalier View Post
I've run our primary 1400W AC with a Honda 2000i plugged into the system before. If you're in a lonely anchorage (as to not annoy your neighbors) you can float the Honda in your tender - that way the sound of the generator is actually indistinguishable from that of the AC blower.
I must admit that I am leaning this way, but the noise is a big issue.

Good to hear the Honda can take the start up loads of a system nearly twice the size of mine.

Thanks
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Old 23-04-2014, 13:08   #14
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Re: Are my maths correct?

General note:

Please don't believe any post which includes the nonsense unit "amps/hr" or "amps per hour"

Not to be confused with "amp hours" or "amp.hrs" which are valid units.

Similar problem with watts/hr or watts per hour, which appears here and also came up recently on another thread.

Next time you hear someone talking about watts per hour, ask them how many horsepower their car develops per hour, then say "Gee, that works out to be a LOT of horsepower per day!"
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Old 23-04-2014, 14:22   #15
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Re: Are my maths correct?

Andrew.

I know I always get it wrong. And I generally post my inability to deal with electricity. Can you provide a primer for me to try and get this stuff right in my head?
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