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Old 12-07-2017, 08:23 PM   #1
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Using less battery overnight.

DW has a CPAP. I see that on its wall wart @ 120 volts it uses 2.1 AMP,

I read somewhere that would translate to 21 amps from the battery at 12 volt in round figures.

I notice its wall wart says the output is 12 volt @ 5 AMP.

8 hours x 21 Amps = 168 amps drawn from the battery via the inverter.
8 hours x 5 Amps = 40 amps drawn directly from the battery, no inverter needed.

Am I completely misunderstanding something?
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Old 12-07-2017, 10:01 PM   #2
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The numbers may be the max draw and max output.

It sure sound out of whack.

You would need to plug it into a Kill A Watt meter to see the actual amp draw.
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Old 12-07-2017, 10:36 PM   #3
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Quote:
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The numbers may be the max draw and max output. It sure sound out of whack. You would need to plug it into a Kill A Watt meter to see the actual amp draw.
I tried that a couple weeks ago and the dang thing won't run if its plugged into a killawatt. Never had anything else have that issue. I currently have that same killawatt feeding the coach 20amp cord.

I cannot explain why the resmed machine is a no go when plugged into a killawatt. The only thing i can see is the machine doesn't have a ground wire, not sure why a killawatt would need that.
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Old 12-08-2017, 02:05 AM   #4
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The problem arises in AC calculations because power is not Volts X Amps, it is Volts X Amps X Power factor and for many appliances with transformers that power factor can be a number considerably less than 1.

For your purposes, the current draw from 12V will be a maximum of 5 amps and probably somewhat less so the draw from your batteries overnight will be say 8 hours at 4 amps so 32Ah out of your battery. If you supply the unit from an inverter, the draw from the battery will be more than 5 amps, but nothing like 168Ah. Perhaps 60 Ah but would need measurements to confirm that.

If your machine had a humidifier mode then the draw would be much higher.
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Old 12-08-2017, 06:11 AM   #5
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If your wall-wart is putting out 12 volts, then you should be able to run the CPAP directly on 12 volts. The important thing will be determining correct polarity for the connector. See the link, its for a TV, but will be the same for the CPAP.

Will a 120 volt TV work on 12 volts DC – MAYBE – 2001 Coachmen Mirada 300QB

Conserve energy.

When boodocking, Your main consumers will be lights, the furnace and the CPAP. You can significantly reduce the lights consumption buy changing your lights out to LEDs.
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Old 12-08-2017, 06:38 AM   #6
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CPAPs are sometimes quite sensitive to voltage level. It all depends on the make and model. Remember we talk about 12 volts as the battery voltage, but it actually can vary from 11.5 to 14.5 depending on what is running (engine, converter, etc.) to many CPAP manufacturers THAT IS NOT 12 VOLTS! I agree that the consumption off an inverter will be much lower that the pate value that you wrote. Turn off the humidified and tube heater if it has one as these are major load sources. If you use an inverter, safest would be pure sine wave as some units do not like MSW waveforms.
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Old 12-08-2017, 07:44 AM   #7
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Did you use the Kill A Watt on shore power or MSW inverter power ?

MSW messes with meters.
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Old 12-08-2017, 07:55 AM   #8
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Quote:
Originally Posted by BigBillSD View Post
DW has a CPAP. I see that on its wall wart @ 120 volts it uses 2.1 AMP,

I read somewhere that would translate to 21 amps from the battery at 12 volt in round figures.

I notice its wall wart says the output is 12 volt @ 5 AMP.

8 hours x 21 Amps = 168 amp-hours drawn from the battery via the inverter.
8 hours x 5 Amps = 40 amp-hours drawn directly from the battery, no inverter needed.

Am I completely misunderstanding something?
FTFY
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Old 12-08-2017, 09:51 AM   #9
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Did you use the Kill A Watt on shore power or MSW inverter power ? MSW messes with meters.
I was using shore power at the sticks & bricks. Also, I replaced our MSW with a higher wattage PSW shortly after we purchased the coach.
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Old 12-08-2017, 10:20 AM   #10
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Is the output from the wall wart AC or DC ? If it's DC the machine should be able to run directly off the battery. If it's AC , it still might run off the battery , but possibly not. You should be able to get some info from the manufacturer about it's power requirements or maybe even an adapter to go from a 12v battery source to the CPAP.
On most equipment , when the input is 12 volt , it will operate from 10.5 to 16 volts .
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Old 12-08-2017, 10:26 AM   #11
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I wouldn't risk it. CPAPs are extremely sensitive, complex...and expensive. There are DC to DC converters available for them (some run on 24 volts.) PSW inverter to power supply is the safe way and I doubt that it is much less efficient than the DC route since the DC converters are active as well.
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Old 12-08-2017, 01:40 PM   #12
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If the manufacturer of the CPAP offers a 12VDC plug set, you can use it plugged into batteries. When I had a CPAP, I had a separate AGM battery in a box that I ran the machine off of so that my house batteries were saved mostly for the furnace at night. I turned the humidity level to zero and then recharged the battery off a solar panel during the day. It worked really well.
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Old 12-08-2017, 01:58 PM   #13
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Double check your machine or the brick. My wife's ResMed machine converts it to 24 volt-not 12. I bought a converter from them ($$) and spiced into the closet 12v for hers. I think you can get a 12-24 converter cheaper than what I did. She doesn't use the heater and hers draws very little amp hours overnight.
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Old 12-08-2017, 04:50 PM   #14
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I called the manufacturer, they gave me the part number for the car adapter, $35 on amazon.
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