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Old 08-19-2018, 01:17 PM   #1
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Default How long can you run AC from batteries?

I need some help with my math and, also, with real world experience.

If I have a 13,500 BTU AC, does that translate to 3,956 Watts?

Does 3,950 Watts at 120V translate to 33 Amps?

Therefor, does a 13,500 BTU AC require 33 AH?

If so, then in theory, I could run an AC for a couple of hours on a 100 A battery.

But, practically speaking, that load is only while the compressor is running and the compressor doesn't run all the time, correct? So, how long could you run an AC from a 100 A battery?
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Old 08-19-2018, 01:58 PM   #2
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Those that have been running theirs regularly will be able to fill in the blanks, but I think you are way low. Most of the AC units pull nearly 100 amps when the compressor is on, and much, much, more starting. That would include the losses in the inverter.


I doubt that any AGM battery at 100 amp hours would be able to come close to starting the AC even when full. Wet cells would do even worse.


IIRC, Avanti has said that about two hours of AC is reasonable off of 440ah of AGM batteries, starting from full and stopping at around 50% down. Below 50% there will be some point that the batteries will cause a low voltage cutoff of the inverter when the compressor cycles, but would be variable with system parts.


It takes a lot of batteries and also all the larger power sources to recharge them, to run AC even for relatively short periods of time.
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Old 08-19-2018, 02:34 PM   #3
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An anecdote is that we recently rented the ARV unit named Giddy Up for a trip from Ohio to Boston for a few days and back. Daily highs were in the upper eighties to lower nineties, lows were in the mid to lower seventies, and East Coast humid, which after fourteen years in CO was uncomfortable without AC.

Giddy Up has 800 ah with a very good AC (but not the quiet Australian AC which is reportedly more efficient). On the first night we ran the AC from ten until six when the 20% capacity alarm started beeping. After that we found it sufficient to run the AC for a couple of hours before going to bed and then opening up windows with the Maxxfan running for the rest of the night, maybe we were getting used to the humidity.

While running, the monitor claimed 10-11 amps at 120 vac, which is closer to 120 amps at a 12v battery.
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Old 08-19-2018, 02:52 PM   #4
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Quote:
Originally Posted by Phoebe3 View Post
I need some help with my math and, also, with real world experience.

If I have a 13,500 BTU AC, does that translate to 3,956 Watts?

Does 3,950 Watts at 120V translate to 33 Amps?

Therefor, does a 13,500 BTU AC require 33 AH?

If so, then in theory, I could run an AC for a couple of hours on a 100 A battery.

But, practically speaking, that load is only while the compressor is running and the compressor doesn't run all the time, correct? So, how long could you run an AC from a 100 A battery?
Without a huge lithium battery bank and a large efficient converter, don't even think about running your a/c on batteries. AGM's and wet batteries simply do not have the capacity and can't deliver that kind of power (and subsequent discharge rate) without damage.
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Old 08-19-2018, 03:21 PM   #5
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Quote:
Originally Posted by Phoebe3 View Post
I need some help . . .


If so, then in theory, I could run an AC for a couple of hours on a 100 A battery.

. . .

You are mixing up DC Ampere with AC Ampere.

You need to match apples with apples, and oranges with oranges

I know, it sounds complicated. . . especially when you throw in the watts and voltages too.
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Old 08-19-2018, 03:25 PM   #6
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.

A quick chart

.
Attached Images
File Type: jpg AC-1.JPG (36.1 KB, 73 views)
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Old 08-19-2018, 03:38 PM   #7
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Without a huge lithium battery bank and a large efficient converter, don't even think about running your a/c on batteries. AGM's and wet batteries simply do not have the capacity and can't deliver that kind of power (and subsequent discharge rate) without damage.
I don't really agree. You just need to set your expectations. As Booster said, my 440Ah AGM battery will happily run my very ordinary rooftop AC for about 2 hours without issue. This is far from "all night", but it is very welcome on a hot afternoon after a long hike, or to cool down the rig before bed.

I do agree that trying to run an AC on a 100Ah battery is hopeless. Probably won't even get it started.
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Old 08-19-2018, 05:41 PM   #8
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I don't really agree. You just need to set your expectations. As Booster said, my 440Ah AGM battery will happily run my very ordinary rooftop AC for about 2 hours without issue. This is far from "all night", but it is very welcome on a hot afternoon after a long hike, or to cool down the rig before bed.

I do agree that trying to run an AC on a 100Ah battery is hopeless. Probably won't even get it started.
I stand corrected. Still, two hours is not sufficient for me to consider upgrading batteries and inverter when I've got my Onan 2800QG generator. If I keep my gas tank full, the generator will run down to 1/3 tank which is essentially longer than I'll even need it. "Quiet hours" at campsites would be my biggest obstacle.
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Old 08-19-2018, 08:22 PM   #9
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Default How long can you run AC from batteries?

Let’s ignore the startup issue and assume that the chart BBQ posted is right that a 13500 BTU AC uses an average of 1250 W. If it was run flat out with no cycling for an hour this would be 1250 * 1 = 1250 Wh.

A 100 Ah battery @ 12 v can put out 100 * 12 = 1200 Wh. Except that you don’t want to run batteries like AGMs below 50% charge so this is now 1200 * 0.50 = 600 Wh. But you have to have a 90% efficiency inverter so this is now 600 * 0.90 = 540 Wh.

So the runtime would be 540 / 1250 = 0.43 hours ignoring the startup issue. This is similar performance to Avanti who has more than 4 times the battery capacity and can run for 2 hours.

So this doesn’t work to well unless there is a major battery upgrade and a big enough inverter to handle the startup.
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Old 08-19-2018, 09:50 PM   #10
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one AC amp is about 10.5 DC amps at 12 volt

your air conditioner tells you AC amps needed.
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