Can I run a 24V / 3.75A CPAP off my 75A/hr AGM?

[BigT]

For the longest time I’ve had a 12 V CPAP machine, and I’ve had no issues running it off the 75 A/Hr AGM battery in my little van.

(I’d run a larger battery, but this is a tiny little van, and there just isn’t much room in here. A 75 Amp Hour AGM is the biggest I could fit under the hood).

Now, however, I’ve been given a 24 V, 3.75 Amp/Hr. model, and I am unclear as to what sort of issues I might face with the higher voltage.

I bought a 12V power cord that boosts the voltage from 12V to 24V, and I’m a little concerned that this is going to wipe out my little 75 A/Hr AGM battery faster.

I do have a 200 W solar panel on the roof, so I know, given enough sunlight, that I can recharge whatever I use, but should I be concerned over the increased voltage of this machine?

 

[jimindenver]

Ignoring the conversion losses 3.75 amps at 24 volts is 7.5 amps at 12 volts. That's 60 amps in 8 hours out of 37 usable amp hours of battery.

 

[BigT]

Ignoring the conversion losses 3.75 amps at 24 volts is 7.5 amps at 12 volts. That's 60 amps in 8 hours out of 37 usable amp hours of battery.

Hmmmm.  That makes it sound as if I can't run it.  

I just checked the old 12V system I've been running in my van for the last 4 years, and it shows 12V / 6.67A.

I've been running that 12V CPAP off my 75 A/Hr. AGM for years without a problem.  

What am I missing?  (Sorry, I can be a little slow).   :blush:

 

[jonyjoe303]

you should be able to run it, the 12 volt cpap uses 80 watts, the 24 volt cpap uses 90 watts, there will be some losses in the conversion from 12 to 24 volts, because of the higher wattage it won't run as long.
As long as you fully charge the battery every day, you should be able to run it several hours a day.

 

[bullfrog]

The determining factor that is missing is how long do you run it in a 24 hour period and realizing you may be using more of the battery power per hour of use with the new 24 volt machine. The deeper you discharge the battery the shorter the battery life will be. We keep 12 volt systems and 12 machines because converting from 12 volts to higher voltages almost always results in more power usage. How deeply you discharge your battery and how long it takes to get it fully charged again will determine the battery life. Will it work? Yes, but how long will the battery continue to work if you run it so long it deeply discharges the battery is the important question.

 

[BigT]

you should be able to run it, the 12 volt cpap uses 80 watts, the 24 volt cpap uses 90 watts, there will be some losses in the conversion from 12 to 24 volts, because of the higher wattage it won't run as long.

80 vs 90?  That's it?  Weird.  I usually run the 12V 6-7 hours a night, then the van and its 200W panel sit in the sun all day, then I run it again the following night.  
I usually do this 5 nights in a row.  4+ years later, and the battery seems to be doing fine, though admittedly the system hasn't seen a lot of use during the pandemic, since nobody's working.  

I don't generally run the CPAP during the day, though sometime I do for a couple hours while on my split.  During the day the 200W panel helps to offset the power used.  

This morning I noticed the transformer on the cord was warm, so I imagine that's pulling a bit of power off the battery, too.   

I'm going to see if I can find another 12V unit.

 

[Scorpion Regent]

This morning I noticed the transformer on the cord was warm, so I imagine that's pulling a bit of power off the battery, too.   

I'm going to see if I can find another 12V unit.

By its very definition a transformer loses power just being plugged in.  Transformers are like middlemen in a bad gangster movie, they always take a cut even if nothing is happening.  So to conserve power only plug it in when you are actually using it. 
You have the right idea finding another 12V system.  The money you spend now will save you a lot down the road.  At their core CPAP's are incredibly simple tech that only require a basic power source.  In theory you could build one yourself out of a aquarium pump, but that's a big ask.  Try to find something that is as primitive as possible, no remote controls, blue tooth, bells lights or whistles.  Now days that's asking a lot, everything seems to have a CPU built in to do some function that could be done with a abacus.      
Good luck.

 

[BigT]

.  Now days that's asking a lot, everything seems to have a CPU built in to do some function that could be done with a abacus.      

Exactly.  My old, 12V model was just a pump with a SIM card to save data.  
The new one has no card, but instead uses cellular data to keep track.  I even have an app on my phone so I can check it.  

I get that it's convenient and means I don't have to go to Kaiser to get my annual printout for the DOT, but it's just more fluff I don't need.  
I did just fine with the old, simple system.  

There's been a recall of most units, and I'm supposed to mail mine in for repair (replacement?).  I'm hoping they simply fix mine and send it back to me, but I'm not holding my breath.  

Thanks for all the replies.  

 

[BigT]

Exactly.  My old, 12V model was just a pump with a SIM card to save data.  

Sorry, that's SD card, not SIM card.  

 

[Scorpion Regent]

Sorry, that's SD card, not SIM card.  

Yeah, I know.  I think I have the same model you had.  I was at Kaiser too, but not now.  Their sleep lab took good care of me, but I could only walk in at a certain 2 hour interval one day a month and that got really old.  I just use the machine now and buy parts on line.  I really have no idea how well it is doing, only that I sleep a lot better with it.  The mask took getting use to.  Now it's like going into space travel hibernation.  The pump starts up and in less than three minutes I'm out.  I got a batch of N95 fabric to make mask liners from.  Turns out it makes a great exterior dust filter for a CPAP machine.  I just snip off a little rectangle, tape it on with gaffers tape and replace it every month or two.  It really helps with allergies and my other filters last longer.

 

[DLTooley]

The power rating is the maximum use. The biggest way to cut power consumption is NOT to use the humidifier.

Also, the various pressure flex techs in newer machines reduce power usage.

My relatively high pressure setting consumes less than 1 amp at 12v, again without a humidifier.