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 > DC Conversion and Amp Hours for CPAP

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ApexAZ

Gilbert, AZ

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Posted: 09/21/21 10:10am Link  |  Quote  |  Print  |  Notify Moderator

I had a 12v plug installed in our bedroom and now I need to try and figure out how much my CPAP machine uses.

I bought a Kill A Watt device and measured 8 hours of sleep last night. The device only does KWH and showed a measurement of .36, or 360 watt-hours.

The DC converter cable I bought for my machine is rated at 90 watts and has the following input and output values:

DC Input: 12/24v
DC Output: +24v / 3.75A

I take this to mean that when plugged into a 12v battery, the cable converts it to 24v.

My question is this: I'm not sure if I should be doing the conversion math based on the 12v battery, or the 24v conversion?
When I do the math to convert from Watt Hours to Amp Hours by dividing by 12v, I get a value of 3.75, which also corresponds to the 3.75A rating on the cord. So I'm assuming the correct value would be to use 12v, but just want to confirm.

Thanks,

Brian

2oldman

So Cal

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Posted: 09/21/21 10:33am Link  |  Quote  |  Print  |  Notify Moderator

It would be helpful to know the specs of the cpap.

ApexAZ

Gilbert, AZ

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Posted: 09/21/21 10:37am Link  |  Quote  |  Print  |  Notify Moderator

2oldman wrote:

It would be helpful to know the specs of the cpap.


It's a Resmed AirCurve 10 S bipap.

I think my pressure is 19 inhale and 14 exhale. Pretty high. The readings I got were with humidification.

DrewE

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Posted: 09/21/21 10:39am Link  |  Quote  |  Print  |  Notify Moderator

If we assume the DC-DC converter in the cable is equally as efficient as the AC power supply for the CPAP machine (which seems as reasonable a guess as any), then you can divide the 360 watt-hours by 12V to get 30 Ah for the night, or 3.75A average current draw from the 12V battery.

The 3.75A rating on the 24V output is irrelevant to this calculation; that's just the maximum rated output for the DC-DC converter in that cord. If the CPAP machine were to use the full 3.75A at 24V that the cord can supply, you'd have somewhere around 8A input on the 12V side as the converter can't create energy out of nothing (and indeed consumes/wastes a little itself in the voltage conversion process).





2oldman

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Posted: 09/21/21 11:19am Link  |  Quote  |  Print  |  Notify Moderator

ApexAZ wrote:

I think my pressure is 19 inhale.....
No, I mean the electrical specs

stevenal

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Posted: 09/21/21 12:11pm Link  |  Quote  |  Print  |  Notify Moderator

The meter you used measures kWh at 120V. If we assume the 120V and the 12V adapters have similar efficiency, I would use the same measurement at 12V. Multiply by 1000 to get Wh. Divide by 12V to get Ah. Divide by hours of use to see average amps at 12V.


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MEXICOWANDERER

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Posted: 09/21/21 12:24pm Link  |  Quote  |  Print  |  Notify Moderator

I have a Respironics Model T (obsoleted) and with the humidifier set on maximum, the unit consumes 5.13 ampere at 50F. The humidifier eats 85% of the wattage.

When I informed Respironics some folks left their unit plugged in while starting their motorhome they about had a conniption fit. No No No! They yelled. It can destroy the circuit board or corrupt the memory. This is assuming the device is powered by the engine starting battery.

So the conversation ended with "when powered by a battery, we recommend unplugging it after each use"

BTW I use a recording pulse oximeter, and it reveals 1000% more of the information than the machine AHI and Intermittent breathing numbers. And take blood pressure within 1 minute of waking up as a final proofing of not having life shortening events while asleep.

I have my eyeballs stuck to the page of a portable O2 concentrator. The Blabbery says it is good for as much as 1.5 liters. But it and the English advertisement are from China so maybe it is Thunderproof.

ktmrfs

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Posted: 09/21/21 12:55pm Link  |  Quote  |  Print  |  Notify Moderator

ApexAZ wrote:

I had a 12v plug installed in our bedroom and now I need to try and figure out how much my CPAP machine uses.

I bought a Kill A Watt device and measured 8 hours of sleep last night. The device only does KWH and showed a measurement of .36, or 360 watt-hours.

The DC converter cable I bought for my machine is rated at 90 watts and has the following input and output values:

DC Input: 12/24v
DC Output: +24v / 3.75A

I take this to mean that when plugged into a 12v battery, the cable converts it to 24v.

My question is this: I'm not sure if I should be doing the conversion math based on the 12v battery, or the 24v conversion?
When I do the math to convert from Watt Hours to Amp Hours by dividing by 12v, I get a value of 3.75, which also corresponds to the 3.75A rating on the cord. So I'm assuming the correct value would be to use 12v, but just want to confirm.

Thanks,

Brian

I suspect your CPAP is a resmed, they use 24V for the DC input. I have a resmed How much power it needs is HIGHLY DEPENDENT on the setup. Like most CPAP machines if it has and you want to use the humidifier, that alone will be by far the biggest power draw and how much depends on the ambient temperature.

And while the converter may say 90W, that does NOT mean it will draw that much. And finally what you observed on the kill a watt for power will be higher than the actual draw on 12V since the brick is a AC-DC converter to supply 24V to the resmed and is more efficient than the brick. (I've verified that on mine)

Now all that said, using the DC input only same configuration as in your house I'd expect for 8 hours about 300-325WH draw, or from the 12V battery around 25AH. I suspect based on your readings you have the humidifier running at night. now turn off the humidifier, tube heat, wifi and it should drop to about 10AH, that's about what mine draws in 1 night based on that configuration.

Now the resemed DC brick is VERY sensitive to input voltage on startup. if you have to much voltage drop on startup when the CPAP is first turned on, it will kick out. so make sure your wiring to the battery is adequate. If you tapped off another circuit in the trailer, it likely used 14 or 16 ga wire, and may be long enough to have to much initial voltage drop. Especially true if you keep the humidifier on, since it will try to kick on the humidifier heat right away.


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Bumpyroad

Virginia

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Posted: 09/21/21 02:14pm Link  |  Quote  |  Print  |  Notify Moderator

in addition to shutting off/down the humidifier, don't use a heated hose. since this is a bipap not a cpap I "assume" it uses more electricity ?????????
bumpy





gsander1

Birmingham, AL

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Posted: 09/21/21 02:26pm Link  |  Quote  |  Print  |  Notify Moderator

If the device used 360 WH you can expect it to use about 30 AH at 12VDC.


George in Birmingham, AL
03 Country Coach Magna

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