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Minimum DC Voltage to Operate CPAP?
#11
RE: Minimum DC Voltage to Operate CPAP?
I used an in-line power analyzer (watts, amps, amp hrs, time, etc) over several nights to arrive at my figures. I don't use a humidifier.
My amps drawn vary with the load on the machine, as you stated (inhale, exhale, idle). It draws an average of 0.55 amps. Not much at all. The 4.4 amp figure is the total amps drawn over the 8 hours.
I tried but never got ahold of Respironics so I'm going to experiment to find a ballpark figure myself. I'm going to run the CPAP off battery power until it cuts off. Planning on letting it run freely which draws about 0.95 amps. Will be using an 8Ahr battery so it won't take too long.
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#12
RE: Minimum DC Voltage to Operate CPAP?
(07-08-2020, 01:28 PM)khw210 Wrote: I used an in-line power analyzer (watts, amps, amp hrs, time, etc) over several nights to arrive at my figures. I don't use a humidifier.
My amps drawn vary with the load on the machine, as you stated (inhale, exhale, idle). It draws an average of 0.55 amps. Not much at all. The 4.4 amp figure is the total amps drawn over the 8 hours.
I tried but never got ahold of Respironics so I'm going to experiment to find a ballpark figure myself. I'm going to run the CPAP off battery power until it cuts off. Planning on letting it run freely which draws about 0.95 amps. Will be using an 8Ahr battery so it won't take too long.

Got it.  Smile  Best of luck.
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#13
RE: Minimum DC Voltage to Operate CPAP?
(07-08-2020, 11:24 AM)Sleepster Wrote: 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.


That is exactly how I tested my portable battery pack set up I made.
With mine I can get a little over 16 hours run time with the humidifier.. but I gave up using the humidifier even at home because I found in better off without it personally.

I figure I can do 3X 5 hour nights in my tent before needing to ride my motorcycle to recharge the battery pack (set alarm for 5 hours sleep with CPAP, wake up and turn it off and then go back to sleep without it.. that keeps my compliant to my driving licence conditions)

Not doing much camping with the current situation re Covid-19 lately though.
- They are not spelling/grammar errors.. I live in Australia, we do it differently Down Under  Big Grin -
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#14
RE: Minimum DC Voltage to Operate CPAP?
(07-08-2020, 01:28 PM)khw210 Wrote: I used an in-line power analyzer (watts, amps, amp hrs, time, etc) over several nights to arrive at my figures. I don't use a humidifier.

Are you on the 120 volt side of the power brick, or the 12 volt side? A significant amount of loss occurs in the power brick.

Quote:My amps drawn vary with the load on the machine, as you stated (inhale, exhale, idle). It draws an average of 0.55 amps. Not much at all. The 4.4 amp figure is the total amps drawn over the 8 hours.

0.55 amps times 8 hours equals 4.4 amp-hours of charge, not 4.4 amps of current.

Quote:I tried but never got ahold of Respironics so I'm going to experiment to find a ballpark figure myself. I'm going to run the CPAP off battery power until it cuts off. Planning on letting it run freely which draws about 0.95 amps. Will be using an 8Ahr battery so it won't take too long.

But that won't simulate the conditions under which you'll be using the battery. I guess that will answer your original question about the minimum voltage needed to spin the blower motor.
Sleepster
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#15
RE: Minimum DC Voltage to Operate CPAP?
No need to use the power brick for the test. Running directly from the battery to the machine.
I know that my test will not mirror actual use. What I'll do is run the battery down until the machine stops - Allow the battery to recover for a few minutes. Then I'll put on the mask and restart the machine to draw down the battery under actual use until it stops. I expect allowing the battery a few minute to recover will be enough to run the machine a little longer. If not, I'll put it on charge for a few minutes. 
Thanks for your thoughts. All good.
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#16
RE: Minimum DC Voltage to Operate CPAP?
Sleepster is on point with his questions above in Post #14.  Clarification is needed on those points.

Might I suggest another source of help.  Go online and find a local Amateur Radio club.  Contact them and tell them that you are trying to determine the cutoff voltage of your CPAP device.  We (KB6xxx) Amateur Radio operators are geeks by nature and most members in their group will probably have variable power supplies.  And they will be more than willing to help.  You could simply run a controlled test on your CPAP machine with/without humidifier.  Slowly drop the voltage and see what level the CPAP machine ceases to function properly.  Many variable power supplies will have built in volt meters and ammeters.

Keep in mind these are regulated power supplies.  So when the load varies, the power supply will maintain a regulated voltage up to the maximum current it can deliver.  In a an unregulated battery based power supply, the battery voltage will sag under higher current loads and bounce back up a little when the higher current load subsides.  As in a humidifier heater cycling on and off and air pump motor cycling up and down with varying therapy pressure demands.

We techno-nerds in the Amateur Radio field would jump at the chance to help someone with questions as you pose here.  Why?  It's fun.  And it helps someone out.

Here are a few links, may be dated, but still useful, of ResMed products - battery sizing and current requirements of various ResMed xPAP machines.

ResMed Battery Guide

ResMed Technical Services Battery Guide

Another ResMed Technical Services Battery Guide

These are good reads and great information.  They still won't tell you how much the battery voltage will sag under load and how long an unregulated battery will last.  But you can conduct experiments to determine that.

This is interesting.  Please post your findings.
RayBee

  
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#17
RE: Minimum DC Voltage to Operate CPAP?
Thanks for the links. I'll check them out. I happen to be a Ham myself, K4*** but don't have a variable power supply. My radio club may be of help - once this pandemic has passed. Good idea!
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#18
RE: Minimum DC Voltage to Operate CPAP?
(07-09-2020, 08:56 AM)khw210 Wrote: No need to use the power brick for the test. Running directly from the battery to the machine.
I know that my test will not mirror actual use. What I'll do is run the battery down until the machine stops - Allow the battery to recover for a few minutes. Then I'll put on the mask and restart the machine to draw down the battery under actual use until it stops. I expect allowing the battery a few minute to recover will be enough to run the machine a little longer. If not, I'll put it on charge for a few minutes. 
Thanks for your thoughts. All good.

Running the battery right down will kill it quickly.. 

There are charts that show what the minimum battery voltage should be drained to (as a rough rule of thumb 12 volt deep cycle batteries should only be taken down to 12 volts, normal cranking batteries - car starting batteries - would probably been similar) 

I only ran mine until the machine fan slowed noticeably, and I did not restart it to completely drain the battery to fully flat.
- They are not spelling/grammar errors.. I live in Australia, we do it differently Down Under  Big Grin -
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#19
RE: Minimum DC Voltage to Operate CPAP?
Thanks. I've taken a look at charts which reveal, of course, that there are a lot of variables. i want to get a baseline for my particular equipment with my particular usage profile. Hence the actual experimentation. I know it's not good for the battery but I'm weighing risk vs benefit. When I use my CPAP off grid, I don't plan on routinely draining my battery any further than necessary and will attempt to charge it after each use but I want to know how far I can drain the battery before the machine shuts off so I'm not sleeping in the dark  Smile . Remember, I'll be using solar to charge the battery and can't depend on an endless supply of sunlight.
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#20
RE: Minimum DC Voltage to Operate CPAP?
(07-12-2020, 12:01 PM)khw210 Wrote: Thanks. I've taken a look at charts which reveal, of course, that there are a lot of variables. i want to get a baseline for my particular equipment with my particular usage profile. Hence the actual experimentation. I know it's not good for the battery but I'm weighing risk vs benefit. When I use my CPAP off grid, I don't plan on routinely draining my battery any further than necessary and will attempt to charge it after each use but I want to know how far I can drain the battery before the machine shuts off so I'm not sleeping in the dark  Smile . Remember, I'll be using solar to charge the battery and can't depend on an endless supply of sunlight.

You might want to add a dedicated volt meter on your battery.  If you keep a periodic log, you can use it to roughly estimate how much time you have remaining to run your equipment for a given charge.  And you can also get an idea of when the battery is getting tired and in need of being replaced.  I think you mentioned that you would be recharging it with solar cells.  Well, on cloudy days, you may not have a good idea of how much of a charge your battery really received.  A dedicated volt meter would help.

Not sure how sophisticated you want to get, but I'm sure there are plenty of solar battery management systems that would do a much better job.  But it's just a matter of money and how fancy you want to go with it.
RayBee

  
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