Minimum DC Voltage to Operate CPAP?

[khw210]

Hi All,
i've done my due diligence by searching the internet and this forum but haven't found the answer I need. i'm planning an off-grid camping trip and will be operating my Respironics System One Series 60 via 12 volt battery power. At my settings, my CPAP draws a total of 4.4 amps during 8 hrs of use. Therefore, my 16Ah Lithium Iron Phosphate (LiFePO4) battery should give me a good 3 nights of CPAP use. A 25 watt solar panel should charge it up during a sunny day.
My question is... if i have to tax the battery further than I anticipate, what is the minimum voltage at which my machine will operate? My LiFePO4 battery will cut off at 10.0 volts. I expect the CPAP machine would cut off prior to 10V but would like to know what that minimum voltage is. I could call Respironics but don't want to endure the pain in getting the right answer from the right person.

[SarcasticDave94]

There's been discussions similar here on AB, but not targeted to what voltage is needed. Most of the other discussions are like yours, I'm taking ABC device, that uses X amps and Y watts, and feeding it Z power from some sort of battery. And most of these other discussions conclude with something like you'll end up using a lot more watts and/or amps than you think. I'm not good at the math aspect and all that's involved, but most do miss the mark somehow, possibly by way of under-estimating how much power is really needed. Anyway, on the voltage question, I'd say you're machine's voltage tolerance is probably going to demand to be pretty close to that rated 12V, 11-13V maybe.

FWIW if the tolerance from the 12V device such as a CPAP were +/- 15%, that gives 10.2-13.8V. And I do caution you that the 15% I mention is for example purposes only. I don't see a rating on voltage tolerance for any CPAP.

[khw210]

I’m pretty good with the numbers. I used an ammeter over time and have confidence in the power requirements of my CPAP machine and the capacity of the battery. The number I don’t have is the voltage tolerance of my CPAP machine. By the way, “voltage tolerance” is a great term. Thanks!
It just so happens I’m on hold for a technical representative at Respironics. Been on hold for over an hour.

[SarcasticDave94]

Good luck with the project. And I hope you have unlimited minutes.

[khw210]

I’m still on hold with Respironics. Now, I’m more interested in how long it’ll take them to pick up the phone then my original question!
While on hold, I tried the chat option on the Respironics website but a chat representative was not available.
I also sent them an email. Not holding my breath!

[SarcasticDave94]

Ew, not very good customer service there. Holding your breath while trying to discuss apnea issues would not earn you a gold star.

[mesenteria]

Voltage range tolerance...

[clintber]

Hi khw210,

How did you arrive at the 4.4 amp number? Was it directly measured or computed from the wattage figure given by the mfr?

Amperage is an instantaneous measurement, not cumulative, so 4.4A multiplied by 8 hours is 35.2Ah. A 16Ah battery will provide 4.4A for only 3.6 hours.

You question about minimum voltage is a valid question and unfortunately one I cannot answer for your machine.  As the battery dies the voltage will drop below a level that can push the needed 4.4 amps through the resistance of the machine and it won’t work properly.

The best thing to do is get battery that can provide 4.4 amps for more than the required number of hours and recharge it well BEFORE it wimps out, but first be sure you’re really drawing a full 4.4 amps.

I connected a volt ammeter directly to my battery that provides BOTH voltage and the EXACT amperage being drawn so I have the best measurement to use when computing how long the battery will last . My machine, with humidifier uses between 4.5A and 6.8A as the BiPAP pressure changes during exhale and inhale. Without the humidifier it draws 1.5A to 3.0A.

This is the meter I use
[Commercial Link Removed] Search amazon for, "WATERWICH DC12-24V/0.1-10A Dual LED Digital Multimeter Amp"

This is the battery. I recharge it after one night w/o humidifier. I don’t let it die completely during the night.
[Commercial Link Removed] Search amazon for, "ML35-12 - 12 Volt 35 AH SLA Battery"


Clint

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[Sleepster]

I recommend you experiment with it before your trip. Charge the battery and sleep with it while you're home to see how long it lasts.

Also, you didn't mention humidifier use. The heated humidifier draws a very significant amount of current. You can turn it down to zero and still get passive humidification.

[SarcasticDave94]

Agreed with Sleepster on the humidifier. It's best to turn it off if operating on a battery. Include no heated hose as well.