Portable Power Supply

[Yooperman_57]

I live in the Upper Peninsula of Michigan and many of my friends have camps with no electricity. Being a deer hunter and also a CPAP user I needed to find a solution so I could continue to hunt. I’ve read lots of posts here that talk about various options for running a CPAP machine on battery power, but few offer many specifics. I hope my experience will help.

I use a Phillips Respironics REMstar Auto A-flex 550P, set at 11cm of pressure. After a lot of research I was unable to find any definitive data regarding the power usage of my machine. So I decided to do a study myself and borrowed a “wattmeter” from our local library. Knowing that the heating element in the humidifier would draw way too much power for a battery application, I ran my power test with the humidifier removed from the unit, using the unit’s standard 120V AC to 12DC power supply. I ran the test for several nights and found the unit’s average power use was approximately 250 watts for an 8 hour period. At 12V this equates to 20.83ah (amp hours).

I looked at a number of portable power options. While the commercially available portable CPAP power supplies were packaged nicely, most did didn’t have enough capacity to meet my needs and they are very expensive. I also considered several “jump start” units, but they are designed for starting loads, not “power” and again most don’t have enough capacity. Ultimately, I decided to use a deep-cycle marine battery and purchased one from Wal-Mart for $78 with a 105ah rating (105ah/21ah=5 nights).

I purchased the genuine Respironics 12V power cord (with male cigarette lighter end) and battery adapter (female cigarette lighter receptacle on one end and large alligator clips on the other) from a CPAP supply on the internet for $23.75 and $24.90 respectively. The quality of these items is excellent! It had been my intention to try this whole system out at home for a few nights before I went hunting. Unfortunately, I procrastinated to long and got the power cords on the day I was leaving. So the was no “dry run”.

The results couldn’t have been better! I used the machine for 25.5 hours over a 3 day period without recharging the battery and it worked flawlessly. When I got home and put the battery on the charger it showed that it still had about 50% of its charge left. So my guess is that I could have gotten at least 2 more nights. My only concern there is that I am not sure how the CPAP’s electronics will react to low voltage as the battery charge declines.

[Burkebang]

It's good to hear that you have a well functioning power solution for your camping.

Your meassurements shows dramatically higher power use than other people have come up with. Is there a possibility that you have gotten something wrong here?

Other people have meassured the same machine you have and have come up with a current draw of about 0.7 amps. 6.6 AH Lithium Ion batterys are readily sold as CPAP batterys:
https://www.cpap.com/productpage/ion-li ... -pack.html

The CPAP.COM calculator reports that this battery will run your machine for 9.66 hrs at a pressure of 11.
6.6AH / 9.66hrs = 0.68 amps current drawn from the battery when in use. 0.68 X 8 = 5.4 AH needed to run your CPAP for 8 hours. You numbers show allmost 4 times more?

CPAP.COM calculator is found on the bottom of this page:
https://www.cpap.com/cpap-machine/CPAP. ... Cable.html

But you have also tested it. How detailed is the scale showing the charge level on your charger? Because by CPAP.COM's calculations, your 105AH battery should last for 19.4 nights. Is it possible that the battery could have retained a charge level much higher than 50%?

[Yooperman_57]

Very Interesting! Yes, it is entirely possible there was more power left in the battery, the 50% number just came from the intial reading off the charger's meter.

The "Kill-A-Watt" meter showed .25kW used in 8 hours (or 32.25 watts/hr) with the 120V power supply. 32.5 watts/12V = 2.7 amps X 8 hours = 21.6 amp hours. If the rational behind my calculations are correct, the only place the error can be is in the power consumption measurement?

Thanks for the info.

[Goofproof]

If you use a 12 volt DC to 110 Volt A.C. inverter, you loose twice, you have power lose converting and again with the normal AC power brick. By using a 12 volt dc to xpap cord, you don't have those losses. Jim

[Burkebang]

Very Interesting! Yes, it is entirely possible there was more power left in the battery, the 50% number just came from the intial reading off the charger's meter.

The "Kill-A-Watt" meter showed .25kW used in 8 hours (or 32.25 watts/hr) with the 120V power supply. 32.5 watts/12V = 2.7 amps X 8 hours = 21.6 amp hours. If the rational behind my calculations are correct, the only place the error can be is in the power consumption measurement?

Thanks for the info.

Your calculations makes very good sense. I think the error is in the meassurement. My watt meter can show amps used in the moment and watts used in the moment, this is shown in watts and amp. I'm thinking that the KW/h meassurement scale is too coarse to make accurate meassurements on such small loads? I'll try this with my own watt meter tonight.

My charger only shows full charge, half charge or empty, so unless the battery is actually either full or empty, it shows 50%. Is it the same with your charger?

[archangle]

I ran the test for several nights and found the unit’s average power use was approximately 250 watts for an 8 hour period.

That would be 250 watt hours, not 250 watts. Average power would be about 31 watts.

My PRS1 only draws 30 watts average even with the humidifier running, so that number doesn't seem right. If you have a lot of leaks, that might cause more power usage, though.

[Burkebang]

I have now done the test with my own watt meter. I set my machine to 11 cm h2o pressure in CPAP mode with C-Flex and humidifier turned off. The meter varied on inhale and exhale between 7 and 12 watts, I'd say the average is about 9 watts.

9/12 = 0.75 amp continous current draw. So my previous calculations are good. With the machine connected directly to a battery, the loss in the power brick is gone, so that will mean that it will use even less power from the battery. The 0.68 amp current draw that someone else measured is probably right.

Good catch Archangle. We have to keep the values streight in our heads to get the calculations right.

[Yooperman_57]

I too have done more testing as well. I connected my multi-meter in series with the 12V cord to eliminate any loss through the power brick. My meter reads directly in amps to 2 decimals. The results are approximate because the load changes constantly. I tested my machine both with and without the humidifier.

Without the humidifier the inhale readings were from .80 to 1.60 amps, but it seemed to be in the 1.20 range most of the phase. On exhale it was more stable, but ran from .20 to about .40 amps, it seemed to hang at about .25 mostly. So if you average the inhale readings you get 1.20 amps and .30 amps for exhale or .75 amps average for the entire cycle. So for an 8 hr period of sleep I would use 6 amp hours of power. Since I have battery rated at 105ah (obvious over kill), I could theoretically get 17 nights, even with a 40% safety factor it would still be about 10 nights.

With the humidifier attached the average cycle rate was 3.85 amps.

Wow, were my original numbers off! This has been a very interesting and informative exercise. Thank you all for your comments and contributions.

[archangle]

So far, I've measured 10 watts average on PRS1 without humidifier. 30 watts average on PRS1 and S9 with humidifier. I'll try the PRS1 without humidifier some time.

--- Edit --- I'll try the S9 without humidifier some time.

My pressure is around 16, EPR or Flex at max, Auto CPAP, no big unintentional leaks.

I'm measuring AC power with a Kill-a-watt meter. Used for 4 or more hours. I'm going to guess that the "real" readings might be plus or minus a few watts.

[Burkebang]

This has been a really good thread, we have togeather documented the PRS1's need for power very well. This makes it much easier for people to decide what size battery they need

[cosmo]

Since I have battery rated at 105ah (obvious over kill), I could theoretically get 17 nights, even with a 40% safety factor it would still be about 10 nights.

2 weeks of sleep on a battery that cost under $100? I'll be lugging home a marine battery soon

The total cost with parts is not too bad. I'm ordering a portable CPAP battery pack this weekend and will order the stuff for a marine deep cell battery too.

[archangle]

I did finally try the S9 without humidifier and it was around 10 watts as well.

[cosmo]

I was seeing 10 watts or so average with my PRS1 setup as well. Haven't tried my S9 yet.



I have one of these Belkin power monitor but it doesn't record, just live power usage or monthly/yearly cost can be shown on screen. I need to get one with memory that saves the data.