DC Conversion and Amp Hours for CPAP

[ApexAZ]

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

[wa8yxm]

360 watt hours at 12 volt is 30 amp hours.

The CPAP likely does not use all that.. but (Due to conversion loss in the power converter/brick/wall wart) but that's figure 30 amp horus.

NOTE there are other loads on the battery 24x7

[ktmrfs]

Bumpyroad wrote:
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

also, most/all of the newer CPAP/BiPAP machines have internet connection as well. I turn mine off when using it in the trailer. Every little bit of power reduction helps.

And for the heated hose, Resmed makes a insulated sleeve for the hose. I have one on mine. if you want/need to use the heated hose it will reduce the power draw.

another option for the water tray is to fill it with warm water before sleeping. Even w/o the heater on it will offer some humidification and warm water helps for at least part of the night.

I can get 3 nights out of my portable battery pack if heater is off, hose heat is off, wif is off. If the water heater is on I can usually get one night, on cold days, not even one night.

Big clue on what draws power!

[gsander1]

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

[Bumpyroad]

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

[ktmrfs]

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.

[MEXICOWANDERER]

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.

[stevenal]

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.

[2oldman]

ApexAZ wrote:
I think my pressure is 19 inhale.....

No, I mean the electrical specs

[DrewE]

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).

[ApexAZ]

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.

[2oldman]

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