

Well, this is the second pass at this web page. The first pass crashed due to running Music Match plus. I'm kinda grumpy about it. The ProblemI have obstructive sleep apnea. Basically, when I sleep, my throat closes up and I choke. My body then wakes me up to get me to clear my air passageway, and I fall immediately back to sleep. I of course realize none of this. All I know is that I would wake in the morning extremely tired. So I now have a CPAP machine (I forget exactly what the acronym stands for) which allows me to no longer have this problem. I can't get a restfull night of sleep almost anywhere I go. Almost. See, I'm in a group called the SCA that holds a lot of events at primitive and semiprimitive sites. Sites where there is no electricity or where its extremely difficult to get to (think "digging 100 foot long trench"). So I needed a way to get power. My CPAP.I have a Devilbis 9001 CPAP. It states that it uses 30 Watts of power at maximum. 30 Watts of power at 120 volts means that the machine drains 0.25 Amps of current constantly. On a 12V battery, that means it draws around 2.5 Amps of current (although since the inverters do not convert the battery's DC power to AC power perfectly) it is typical to apply a 1.1 efficiency factor to the calculation, so that instead of drawing 2.5 amps it draws 2.75 amps. So for each hour of operation, my CPAP uses 2.5Ah of capacity, if its running at maximum draw. A typical 8 hours of sleep would use up 22 Ah of power.
My studySo I decided to see how much power my CPAP machine really needed. I assembled the following items: My procedure
My ResultsI'm including a chart of my results here:
The first four columns after the date were all data read from the tool or the DMM. The estimated average power was calculated from KWh reading divided by the time. The Estimated inverter Amps was figured by divided the average power by the AC voltage measured. And the estimated amp hours on the battery were calculated by inverter output amps multiple by 10 and by a 1.1 efficiency standard and by the hours used. Theoretical ResultsHere's what is should look like for an evening using the maximum power as specified by the instructions.
InterpretationI was able to run the CPAP machine for almost 85 hours before the inverter started warning. The inverter is designed to cut out when the voltage level on the input drops to 10.5 volts. If we were using the theoretical maxium power usage of the it should have been using almost 2.75 Amp while running. Instead on average (ignoring the two points where I think I turned the CPAP off) we used around 0.84 Amps (on average). This could be a reflection of the fact that my CPAP machine is set to run @ 8 cm of water, and the maximum pressure of 20 cm of water. 8/20 is only 25% of rated pressure, and .84/2.75 is 30% of rated current. Reading this site where it states that "takes between 12 and 14 amps of 12volt DC current to make one amp of 120 VAC power" that an efficiency approximation of 1.1 is not quite correct, and that an efficiency rating of 1.2 might be a bit more accurate. Regardless, using 1.1 and 1.2 calculates that we used between 67 and 74 Ah of a theoretical 115 Ah capacity. We still had around 0.6 volts to go, but that still would not have made up the difference of almost 40 hours of capacity. It might be due to weather conditions, rate of use, or in inefficiencies in the inverters.
How to do it for yourself
I'll be updating and improving this essay as time goes on, so check back in and send me feedback.

