DC Conversion and Amp Hours for CPAP

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.