CPAP On Batteries

Fighting Pages

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 Problem

I 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 semi-primitive 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 study

So I decided to see how much power my CPAP machine really needed.  I assembled the following items:

My procedure

1. Every night before bed I would check the voltage across the terminals of the marine battery.
2. I would then turn on the inverter, which would apply power to its outlet, and it would reset the Kill a Watt tool.
3. Go to sleep.
4. Wake up in the morning, and write down the elapsed time, the power used (measured in KWh or kilowatt-hours), and the voltage across the battery terminals

My Results

I'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 Results

Here's what is should look like for an evening using the maximum power as specified by the instructions.

Interpretation

I 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 12-volt 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

1. Find out what your maximum power of your CPAP is.  For example, mine is 30 Watts.
2. Divide this by 120.  This is your AC amperage.  For example, mine is 0.25 amps.
3. Multiply this by 10 and then by 1.2.  This is your DC amperage.  Mine is 3.0 amps.
4. Muttiply this by 8 hours.  This is the amount of power used in a normal sleep night.  Mine is 24 Amp-hours (Ah)
5. Multiply by the numbers of days you need.  I needed a week. That's 168 Ah.
6. Adjust the value.  For example if your maximum power is 30 Watts at 20 cm of Water, but your setting is only 8 cm of water, it is reasonable that you will only need 2/5's as much power, assuming linearity.  For me 2/5's of 168 Ah is 67.2 Ah.  If you only sleep 6 hours a night, adjust that too.
7. Buy a battery with at LEAST as many Amp hours as you need.  Best safety practice would be to buy one that has TWICE as much capacity as you need.
8. Run it at home before you depend upon it out somewhere.  Record how much you use it before it has to be recharged.

I'll be updating and improving this essay as time goes on, so check back in and send me feedback.