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Old 09-20-2021, 12:40 AM   #1
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Default Inverter - Battery Use

I am trying to figure out how long a battery will last using an inverter to power a device.

I suspect this has been answered here before but apparently my search skills are not up to snuff.

Lets say I have a 110 AC device that uses 120 watts of power. To calculate how long that device can be operated via the inverter do I:

1. Divide 120 watts by 12 volts to get 10 amps;
2. Assuming my battery is rated at 200 amp/hours and considering I only have 100 amp/hours available for use, divide 100 amp/hours by 10 amps to get 10 hours of use?

I do realize this is an oversimplification, realizing inefficiencies of the inverter and ignoring any other battery draw.

I was asked how long a CPAP could be used on the inverter. I thought I knew the process but now I am second guessing myself.

Thanks for setting me straight.

Mike
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Old 09-20-2021, 06:51 PM   #2
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You are on the right track.
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Old 09-21-2021, 05:34 PM   #3
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Thanks for the response. Mike
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Old 09-21-2021, 08:23 PM   #4
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You need to be careful about the battery amp hour rating. Lithium can be run down to 10 or 20 percent of charge, but lead acid only to about 50 percent of charge without dramatic loss of life, according to my dealer and what I read on the internet.
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Old 09-23-2021, 03:00 PM   #5
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You also need to be aware that battery Ah ratings, assuming FLA deep-cycle chemistry, are normally based on a 20 hour discharge time. As you shorten the time to discharge by increasing the current draw, the total amount of energy actually decreases. So putting a 100A load on a 100Ah battery, assuming that rating is based on a 20 hour period (C/20) will not get you 100Ah - it will be dead long before that.

Going 8 hours under load rather than 20 hours might net you 70% of what it could do over 20 hours, so consider that when you look at how much of a battery bank you need to support the overnight load while not dropping below whatever threshold you intend to use (50% or even lower).

Here's a bit of light reading. I don't know anything about this company, but what they're saying seems consistent with what I know - which admittedly is quite limited.

https://www.power-sonic.com/blog/wha...tery-c-rating/
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Old 09-23-2021, 03:40 PM   #6
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I joined a DYI solar forum or two just to try to get a handle on battery technology today before I dump much money into electronics for my older ERA , and it seems that getting satisfactory performance from batteries and solar can be quite a money pit. You also have to consider inverter losses which are substantial on big inverters.

I basically use propane for everything I can, and am looking into 12 volt A/C just to stay away from hauling a ton of batteries...YMMV.
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Old 09-23-2021, 04:26 PM   #7
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Originally Posted by ArtDeco View Post
You also have to consider inverter losses which are substantial on big inverters.
My Xantrex Freedom SW3012 inverter/charger (3000W) shows 90% as a "peak" efficiency, without stating what the corresponding output wattage is, and the corresponding input voltage. But that's a pretty big hit when running the A/C unit off the battery, which I do a lot, and the efficiency could be worse than the published 'peak' of 90%.

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I basically use propane for everything I can, and am looking into 12 volt A/C just to stay away from hauling a ton of batteries...YMMV.
Later versions of my van come with a DC air conditioner. Cuts out the middle man, and it's more powerful (20,000 BTUh rather than my 11,000 BTUh). Not worth it for me to try to retrofit, and if I was going to do something approaching that much work, I'd replace the A/C unit with a quieter one. That sucker is noisy, and it's right over the bed.
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Old 09-23-2021, 05:23 PM   #8
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Originally Posted by GBLandau View Post
I was asked how long a CPAP could be used on the inverter. I thought I knew the process

Mike

transforming ac to dc or dc to ac has inefficiencies/waste


if this is the cpap, get one which is DC native


Mike
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Old 09-23-2021, 05:37 PM   #9
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Quote:
Originally Posted by KanzKran View Post
My Xantrex Freedom SW3012 inverter/charger (3000W) shows 90% as a "peak" efficiency, without stating what the corresponding output wattage is, and the corresponding input voltage. But that's a pretty big hit when running the A/C unit off the battery, which I do a lot, and the efficiency could be worse than the published 'peak' of 90%.

90% is actually pretty good. I read a lot of 80 to 85 percent figures. And a phantom load when turned on but not powering anything. What kind of battery bank do you have?

Later versions of my van come with a DC air conditioner. Cuts out the middle man, and it's more powerful (20,000 BTUh rather than my 11,000 BTUh). Not worth it for me to try to retrofit, and if I was going to do something approaching that much work, I'd replace the A/C unit with a quieter one. That sucker is noisy, and it's right over the bed.
That's exactly why I'm looking to replace my A/C . Blower right over the bed, and the generator needed to run it without shore power right under the bed. We call it a "noise sandwich ". Not even with earplugs could I sleep through that.

Do you know the the make and model of that A/C . I am very interested in one.
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Old 09-23-2021, 06:37 PM   #10
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Do you know the the make and model of that A/C . I am very interested in one.
This is what Coachmen is putting on the Galleria.

https://www.premierproducts.net/orde...v-and-48v.html

There are quieter options in AC-powered roof units, too. Like this one.

https://www.airxcel.com/rv/coleman-m...0-dq-ndq-ac-hp

I heard about the Coleman Mach 10 NDQ from this video.

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Old 09-23-2021, 07:56 PM   #11
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Yes I watched a bunch of James and Stephanie videos when I was shopping for my van. And I hate videos as a rule.
He did a newer video about normalized runtime on the A/C where the comments thread ran into the weeds and got me looking at 12 volt heat pumps, since a quiet A/C over a noisy generator still means no sleep ...
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Old 09-26-2021, 08:44 PM   #12
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get a 12vdc cpap machine....methinks that most machines allow for either input
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Old 09-27-2021, 03:06 AM   #13
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1) I have found it easier to convert everything to watts A*V=W. So a 200ah battery at 12 volts has 2400 watt/hours of rated capacity.

2) As noted by someone else, you can't use all your stated capacity. Depending on the battery, 50%-80% is the max that is useable.

I believe some/most lithium batteries won't allow you to draw their full capacity and some manufactures incorporate that into their capacity ratings. So in those cases you really can use all the battery's stated capacity.

At 50% of stated capacity a 2400 Wh (200ah) battery will have 1200 watts.

2) Inverters use a lot more power than I would expect. According to Roadtrek, mine uses 100 watts. So over an 8 hour night it will use up 800 watt hours of that 1200 watts. My CPAP at 2.5 amp/120 volt uses 300 watts over 8 hours for 2400 watt/hours., so you would end up with 3200 watt hours for a good night's sleep. That means three batteries.

3) Direct 12 volt is much more efficient. My cpap at 5 amps and 12 volts uses only 60 watts. I need only 480 watts for a good night's sleep. Both my cpaps use 12 volts natively and, with the proper cord, can just be plugged directly into a 12 volt outlet.

4) None of that takes into account the impact of drawing current at a higher rate than 1/20 of your capacity in an hour. But the numbers we are using anticipate a drawing down less than half the battery capacity over 8 hours. That is a draw rate of 1/16 per hour of the total capacity which is not far off from 1/20.

BTW - I also use 120 instead of 110 largely because it creates easier comparisons to 12 volts. Our measures of battery capacity and use are not all that precise anyway so we are working with rough estimates.
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Old 09-27-2021, 12:25 PM   #14
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1)2) Inverters use a lot more power than I would expect. According to Roadtrek, mine uses 100 watts. So over an 8 hour night it will use up 800 watt hours of that 1200 watts.
Really??
What kind of inverter do they supply?
My 2800 watt Outback inverter is spec'd by the manufacturer at 2.6 watts in search mode and ~20 watts at full AC output, with a conversion efficiency of 90%. If the numbers you cite are correct, the unit must be pretty poor.

outback efficiency.jpg
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Old 09-27-2021, 12:52 PM   #15
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Really??
What kind of inverter do they supply?
My 2800 watt Outback inverter is spec'd by the manufacturer at 2.6 watts in search mode and ~20 watts at full AC output, with a conversion efficiency of 90%. If the numbers you cite are correct, the unit must be pretty poor.

Those are similar to our Magnum but a bit better. The MS2000 specs less than 8 watts on search and 25 watts inverting with no load.



We don't use the search feature as we rarely need the big inverter since we added a couple small, very low parasitic ones for the AV stuff and device charging. We actually see about 18 watts when inverter on but no AC output.


Be aware that in the post Avanti replied to there is some mixing of watts and watt hours so take that into account when evaluating what is actually battery energy used over time vs instantaneous use by the devices.
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Old 09-27-2021, 01:12 PM   #16
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Quote:
Originally Posted by avanti View Post
Really??
What kind of inverter do they supply?
My 2800 watt Outback inverter is spec'd by the manufacturer at 2.6 watts in search mode and ~20 watts at full AC output, with a conversion efficiency of 90%. If the numbers you cite are correct, the unit must be pretty poor.

Attachment 12028
My 3000W Xantrex is similar, but shows current as being no higher than the published values. Disabling the inverter function reflects about a 3A change on the BMS display, though I don't remember exactly what the change is.

Quote:
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BTW - I also use 120 instead of 110 largely because it creates easier comparisons to 12 volts. Our measures of battery capacity and use are not all that precise anyway so we are working with rough estimates.
Just as well, as the voltage standard and official name (hence the term "nominal voltage") in North America actually is 120V (and 240V) at the service drop, +/-5% (per ANSI C84.1, which is a utility standard), with an additional 5% drop due to premises wiring (NEC, which is everything after the service drop). It hasn't been 110/220V since the 30's or thereabouts I believe (115/230V), and first standardized at 120/240V in the 50's, but it still lives on in common useage. "220, 221; whatever it takes."
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File Type: jpg Inverter Specs.JPG (156.5 KB, 4 views)
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Old 09-27-2021, 01:41 PM   #17
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My 3000W Xantrex is similar, but shows current as being no higher than the published values. Disabling the inverter function reflects about a 3A change on the BMS display, though there is no decimal place so I don't know just how much less than 3A it draws at idle.
In our rig, turning on the inverter shows an increase of 2.3A, which is 27.6 watts (compared to "~20 watts" in the spec. However, this also includes the clock on the microwave, which comes on when I turn on the inverter. So, at least in this case, the spec is pretty close, and in the same ballpark as what you are seeing.
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