CPAP Machine Power Analysis

For those that don’t know what a CPAP machine is, it is a simple air pump that increases the air pressure going into your lungs while you’re asleep. As you get older, things like chronic bronchitis, previous pneumonia, damage from infection like Covid to the lungs and sleep apnea can cause major health problems. I was a “denier”, of course “I do NOT have sleep apnea!” Then I took a sleep test because I was getting up five times at night to go to the bathroom. Yes, as we get older, the prostate gets bigger and you have other problems. Well, after the sleep study, the results were that I had stopped breathing for a significant amount of time 60 times a night. So I reluctantly agreed to try out a CPAP machine. The first night with this thing clamped onto my face was an uncomfortable one. It was also uncomfortable because you felt like you couldn’t breathe, but yet you could. I calm down and decided that I was gonna make this work. I would take a breath in through my nose and breathe out through my mouth until I calm down and then I closed my mouth and started to breathe through the nose and the machine. It was absolutely amazing. Best sleep I have had in 15 years. Only got up once that night to go to the bathroom not five times. My blood pressure dropped. I started to lose weight. A lot of things started to reverse. In the month and a half that I’ve had it, I have had more sleep than I have had in the last 10 years. An amazing difference. The only problem is you’re tied to 110 V outlet to power the machine. They do make a little mini version of the CPAP machine so you can go traveling with it, but once again it uses 110 V. I like to go sailing or camping so what do you do now when there isn’t 110 V readily available all the time? I decided to check out different ways of powering this little mini CPAP machine and here are some of the test results. I managed to purchase a 12 V to 24 V DC – DC 90W converter that the CPAP manufacture ResMed makes. It was a bit expensive and it’s a heavy device so I figured at 90 W it would consume a tremendous amount of power. On a sailboat you also don’t have unlimited power. I also purchased a small 12 V DC – 110 V AC converter so that maybe I could plug-in the 110v small 20W power supply that came with a mini CPAP machine. I figured there’s a chance that that smaller 20 W unit would be a lot more efficient than the bigger 90 W unit. Boy was I wrong. (I’m getting used to being wrong!)

Here’s how I tested it on a 20 amp hour lithium ion battery that I put into an old emergency power supply for starting cars. It’s output is 13.8 V and I made an accessory socket and connected it to an in-line amp meter that would tell me how many watts and amps each unit was using. Below are the pictures of the set up and of the results.

The Results:

I first tried using the ResMed mini on 12 V (20 AH LiFePo battery) and both of them ran very well and you couldn’t tell any difference in performance. The power consumption, however, was quite different. The 12 V ResMed 90 W power supply would use 1.5 W 0.12 A while the AST used 3.5 W at 0.26A in just standby. There was a difference in the amperage draw from 4.2 W or 0.3 A exhaling a breath all the way up to 8–9 Watts, on deep inspiration. The deeper the inspiration, the higher wattage would be required to maintain airway pressure. So the average between the low and the high was the difference between 4.2 and 8. Using an average 6.2 W/hr Ave breathing cycle, 14 breaths a minute, is about 1860w for a 5 hr night power consumption.

ResMed 12v power supply:

Expiration: 4.2 W 0.32 A
Inspiration: 8.5 W 0.65 A
Standby: 1.5 W 0.12 A

——

Using the 12 V – 110 V AC car inverter and the 110v power supply, there was a 40 or 50% increase in the power consumption so that exhaling power consumption was 6.8 Watts, and inspiratory went up to over 11. There’s a very significant difference in the average power consumption rate. It worked very well but was significantly less efficient.

AST inverter to 110v ResMed 20W power supply:

Expiration: 6.8 W 0.52 A
Inspiration: 11.4 W 0.87 A
Standby: 3.5 W 0.26 A

The difference makes sense to me because the ResMed 12v power supply is a DC to DC inverter taking 12 V DC and converting it to 24 V DC for the unit versus the AST converts 12 V  DC to 110 AC and then the power supply for the mini takes it from 110 AC to 24 V DC. The good news is I have two ways of powering my CPAP when I’m out on the boat or camping. A 20 amp our lithium iron battery would last all night either way but would last longer with the 90W unit.

Sweet Dreams!
N4PVH

---