Lithium ion battery pack to power C-PAP machine for off-grid (backpacking) use

[RobertWJones]

Sorry, if this is too far off-topic, I understand if it gets promptly moved to the bit bucket. I'm working to put together an economical Lithium battery replacement pack to power my C-PAP machine during backpacking to replace the two $200+ each battery packs I purchased initially, only several years back (they didn't last long at all). My current attempt at a DIY replacement is using 4 X 18650 UltraFire BRC 6800mAh batteries in series, since 13-14V minimum is needed to keep the machine running. However, initial attempt only ran machine for 20-30 minutes before depleting the batteries to approximately 12V total (under the required voltage to power the machine). I must admit, I didn't purchase a charger to top off the batteries, initially, but they measured about 3.7V each, so I figured they may not need to be charged. Commercial Li-ion pack, I initially purchased, is rated 14.4V, 5200mAH, 75wH, and ran the CPAP machine for approximately 3 nights when it was new, before requiring recharge. The commercial AC adapter for the C-PAP machines is spec'd at 18V DC / 1.67A, although I suspect machine is using well under the 1.67 amps. In reading, it appears that Samsung, Panasonic, or LG Li-ion 18650 batteries may be much better and perhaps up to the task, but I don't want to purchase them if there isn't a reasonable chance that they will work much better. Maybe I need to add additional batteries in parallel? It does not seem like the commercial pack is physically large enough to contain more than 8 cells, in total, however. Any thought or recommendations would be greatly appreciated.

 

[madmax]

The ultrafire got you. 6800mah is more Ike a pipe dream.
You probably want a 4s3p to be as close to possible to original spec.

 

[zaps]

You aren't by any chance talking about the Somnetics Transcend Auto CPAP?

 

[RobertWJones]

You aren't by any chance talking about the Somnetics Transcend Auto CPAP?

Yes - the original Transcend - user manual linked to

 

[LanduytG]

I have the same unit, one for home and one in the truck. I was always going to see what the amperage was but never did. I have the mobile charger that I'm using in the truck and found out you should never leave it plugged in all the time. I had to send that unit back to the manufacturer to have it reprogrammed because of a surge when I started the truck. Now I leave it plugged into the P8 battery and it acts as a surge protector. When I need a new battery I will be building my own for a fraction of the cost.

Greg

 

[John Frum]

from the manual

"Mobile power adaptor (optional) - MPA1
Mobile power adapter input: 13.0 VDC nominal. 10 to 15.5 VDC, 7.5 Amp
Mobile power adapter output: 19.2 VDC, 2.6 Amp"

Suggest you use a boost converter to get regulated output.
probably a ~10% overhead.

Although you can probably get away with a 3s explody chemistry pack.
Personally I wouldn't want to sleep in a nylon tent with a diy "vent with fire" battery though.

 

[RobertWJones]

The ultrafire got you. 6800mah is more Ike a pipe dream.
You probably want a 4s3p to be as close to possible to original spec.

Yep - they got me, sure as shootin'
Thank you for pointing me at the 4s3p battery configuration. Even though it is intuitive (after the fact), I was unaware that there are stock li-ion battery configurations like that. Now I see that there are even commercial sources I can check into - thank you, many times, for that extremely valuable tip!

 

[RobertWJones]

from the manual

"Mobile power adaptor (optional) - MPA1
Mobile power adapter input: 13.0 VDC nominal. 10 to 15.5 VDC, 7.5 Amp
Mobile power adapter output: 19.2 VDC, 2.6 Amp"

Suggest you use a boost converter to get regulated output.
probably a ~10% overhead.

Although you can probably get away with a 3s explody chemistry pack.
Personally I wouldn't want to sleep in a nylon tent with a diy "vent with fire" battery though.

Thank you for the valuable insights! I have been attempting to get several different boost converters to work. Maybe I have defective ones, but they work momentarily, then seem to be current limiting several seconds into powering the C-PAP. But then, again, it could be ignorance on my part as to getting them correctly adjusted pertaining to current limit. And you also bring up a very good point, I have also been concerned with pertaining to the makeshift li-ion battery packs, from a safety standpoint - thanks, again, for the interchange!

 

[madmax]

You can do a lifepo4 pack if it makes you feel better. Even cheap packs have some sort of bms. You can get 3.7v bms that prevents over and under voltages for single cell protection. Luckily I don't think your use is high drain, which should decrease the possibility of fire even more.

 

[RobertWJones]

You can do a lifepo4 pack if it makes you feel better. Even cheap packs have some sort of bms. You can get 3.7v bms that prevents over and under voltages for single cell protection. Luckily I don't think your use is high drain, which should decrease the possibility of fire even more.

madmax - you've been very helpful and given me some great ideas to chew on - thanks, much!

 

[Sporadic]

I'm curious as to how this DIY went. I'm also looking to put together a DIY battery pack for my Transcend Auto Mini CPAP. I have 2 P4 batteries that have served me well for over 3 years now. I don't want to spend too much on replacements down the line so a DIY pack is my favored option. The specs of the P4 are straightforward and I can imagine the battery build inside without taking it apart. The P4 is a Li-ion 14.4v, 2600mah battery pack. Which leads me to believe that the it is a 4s configuration of 2600mah 18650 batteries. You gotta scratch your head though cause the Transcend CPAP wall adaptor is rated at 18v 1.0A. Though a 4s configuration should range from around 14-16v depending on the charge level. With battery tech these days I should be able to build a P8 capacity battery but at the size of P4.

 

[Rocketman]

A few years ago just when I was starting on lithium batteries, we were going camping with my brother and sister in laws, they needed to power a cpap.

For my first lithium battery I used eight 25ah lithium fortune cells, overkill bms and put it in a harbor freight pelican case. I made it 50ah for other future uses, four cells would power their cpap through an inverter just fine.

We ended up staying at full service RV parks- so my battery was not needed- but it has been great for other things

 

[Tomthumb62]

FYI for anyone still reading this thread, don’t buy Ultrafire cells. Pure junk, way way over rated in capacity (highest 18650 capacity available is about 3500mAh) and a potential fire hazard. Lithium ion by its nature is already a potential fire hazard and super poorly made corners-cut cells like Ultrafire increase your chances. Buy only if you like to gamble with fire!