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Old 08-19-2018, 02:17 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, 02:58 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, 03:34 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, 03:52 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, 04:21 PM   #5
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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, 04:25 PM   #6
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.

A quick chart

.
Attached Images
File Type: jpg AC-1.JPG (36.1 KB, 90 views)
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Old 08-19-2018, 04:38 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, 06:41 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, 09:22 PM   #9
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Default How long can you run AC from batteries?

Letís 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 donít 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 doesnít 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, 10:50 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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Old 08-19-2018, 11:57 PM   #11
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No electrical background here; just actual experience if that is helpful. Our RT is equipped with 800ah lithium with under hood gen. After driving about 10 hrs stopping for the night; get around 4to 4.5 hours of rooftop ac before the volt start kicks in. This is in Texas in the summer time.
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Old 08-21-2018, 08:52 AM   #12
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How long your appliance will operate from a particular battery is dependent on the appliance's power consumption in watts (or AC amps x 115V), and the battery capacity. Low inverter efficiency, and improper battery wiring during installation can also reduce run time.

Calculating Run Time

The following formula can be used to determine run time in most applications using a 12V battery or bank:
10 x (Battery Capacity in Amp Hours)
/ (Load power in Watts) = Run time in hours

Example:

How long will my 100 watt TV run with my PW150 from my car's engine start battery (60 Ah)?

The load is well within the PROwatt inverter's maximum power rating, therefore the inverter itself will not limit the run time.
10 x (60 Ah)
/(100W) = 6 hours maximum run time before the battery is completely discharged, or 3 hours run time before the battery is 50% discharged. You should still be able to start your car at this point.

Tips

Engine start batteries should not be discharged below 90% remaining charge-state, and marine deep cycle batteries should not be discharged below 50% remaining charge state. Doing this will shorten the life of the battery based on most battery manufacturers' recommendations.
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Old 08-21-2018, 02:03 PM   #13
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How long your appliance will operate from a particular battery is dependent on the appliance's power consumption in watts (or AC amps x 115V), and the battery capacity. Low inverter efficiency, and improper battery wiring during installation can also reduce run time.

Calculating Run Time

The following formula can be used to determine run time in most applications using a 12V battery or bank:
10 x (Battery Capacity in Amp Hours)
/ (Load power in Watts) = Run time in hours

Example:

How long will my 100 watt TV run with my PW150 from my car's engine start battery (60 Ah)?

The load is well within the PROwatt inverter's maximum power rating, therefore the inverter itself will not limit the run time.
10 x (60 Ah)
/(100W) = 6 hours maximum run time before the battery is completely discharged, or 3 hours run time before the battery is 50% discharged. You should still be able to start your car at this point.

Tips

Engine start batteries should not be discharged below 90% remaining charge-state, and marine deep cycle batteries should not be discharged below 50% remaining charge state. Doing this will shorten the life of the battery based on most battery manufacturers' recommendations.

Some additions to this would be that you shouldn't go by the rated amps or watts on an appliance in your calculations unless your are just roughly looking at use, as they are usually maximums under worst case conditions. It is best to measure the use. This will also include inverter inefficiency or 12 to other DC converter inefficiency if you are using one.


If you really want to know how long any given load will run constantly, like the example gives, you need to calculate the what the 20 hour AH rating (which is the normal rating given) amps are. In the example the 60ah battery would be at 3 amps for 20 hours to go to dead, so that is the amps that the rating applies to. Higher amps will give a lower AH rating and lower amps a higher AH rating. AH rating is really kind of a bogus rating anyway because it only tells you when the battery hits 10.5v at the amperage tested. It does not have anything to do with the actual energy stored in the battery.


IMO, the thing that shortens battery life the most is the fact that hardly any RV charging systems get the batteries totally full, without overcharging them, on a regular basis, and both short and over charging will shorten battery life.

In the example, the 100 watts is going to 8.8 amps, so the AH capacity of the battery to run that load continuously to 10.5v will be less than the 20 hour rating by some amount. Of course this also is based on the battery being 100% full when you start, which very, very, rarely is the case unless you have excellent charging control and a monitor to know you are full. It is also pretty likely that at a current higher than the 20 hour rated current, the battery voltage would hit the inverter low battery voltage cutout, but that is dependent on where that voltage is set, as some can be very low.



Most of the current starting batteries are the low water use variety that are more tolerant to not getting fully charged than the older versions, and that is good because most starting batteries never really get full and cycle in the 70-80% full range for most of their lives. The exceptions are in vehicles that travel very long distances on a regular basis, as it can take over 6 hours to top off a battery to full from 75%.


We have discussed the 50% discharge rule here quite a bit, and IMO it is very exaggerated. The charts show a 50% reduction in number of recharge cycles between 80% discharged and 50%, but the actual energy in/energy out is under 20% different. For that loss of life, you gain 60% of usable capacity, which is a lot, and save yourself from hauling lots of weight around. Since the discharge depths appear to average when applied to life, one discharge to 80% down plus one discharge to 20% down would put very similar wear on a battery as two 50% down cycles.
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Old 08-21-2018, 05:24 PM   #14
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............................

We have discussed the 50% discharge rule here quite a bit, and IMO it is very exaggerated. The charts show a 50% reduction in number of recharge cycles between 80% discharged and 50%, but the actual energy in/energy out is under 20% different. For that loss of life, you gain 60% of usable capacity, which is a lot, and save yourself from hauling lots of weight around. Since the discharge depths appear to average when applied to life, one discharge to 80% down plus one discharge to 20% down would put very similar wear on a battery as two 50% down cycles.

That's a great and very concise explanation that should often be referred to.
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Old 08-21-2018, 05:44 PM   #15
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That's a great and very concise explanation that should often be referred to.
Yep. "Rules of thumb" like the "50%" thing are a good way to get the masses into the ballpark, but nothing beats actual understanding.
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Old 08-22-2018, 10:27 PM   #16
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Thank you for taking the time to answer my question! Looks like my initial calculations were off by an order of magnitude (330 AH instead of 33 AH) and, like the compressor refrigerator, the AC will only use about one-third of the rated electrical needs.

I actually caught quite a bit of vitriol on a FB group for saying that 300W of solar would not keep up with an AC so I wanted to get the real scoop in case I was wrong. Glad to know that here, facts are facts and are based on physics and real-world experience rather than wishful thinking.
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Old 08-23-2018, 04:28 AM   #17
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. . .

I actually caught quite a bit of vitriol on a FB group for saying that 300W of solar would not keep up with an AC . . .

If you are referring to the FB Solar and boondocking group... forget it. That group is full of idiots. I quit before getting myself involved in their stupid ignorant arguments.

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Old 08-23-2018, 05:22 AM   #18
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FWIW, I was able to get 8+ hours of a/c time out of my 500 AH battery bank I set up in my Xplorer. Key to that was the a/c unit I picked - a high efficiency LG window type unit. Many of the Roadtreks have this style of air conditioner in them. They can do alot better than the rooftop style air conditioners. I measured on my BMK that the unit drew around 50 DC amps when it was running, and ran about 60% of the time. Solar could offset some of that during the day. As a rule of thumb for forecasting battery usage, I multiply the 110V amps by 10 to estimate the DC amps needed. It's not exact, but neither is energy draw of appliances.
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Old 08-23-2018, 06:10 AM   #19
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Here is what the depth of discharge (DOD) vs number of cycles looks like for my Amstron AP-GC2ís looks like. Limiting DOD to 50% increases the number of cycles substantially according to the manufacturer.

IMG_0334.jpg
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Old 08-23-2018, 09:53 AM   #20
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Quote:
Originally Posted by booster
Some additions to this would be that you shouldn't go by the rated amps or watts on an appliance in your calculations unless your are just roughly looking at use, as they are usually maximums under worst case conditions. It is best to measure the use. This will also include inverter inefficiency or 12 to other DC converter inefficiency if you are using one.
....................
The thing I said is a usual calculation in theory. Here you pointed out that we should take the real situation into consideration, I cannot agree with you any more.
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