Comparing old battery to new - Something I forgot to consider

[Robert Moulton]

I was going on a four day canoe trip and knew my single three year old battery was not going to last that long.  I generally got two nights out of it when new but it had reduced to not being able to give me two full nights anymore, just one and a fraction.
 
I have the old Medistrom Pilot 24, no longer available, so I got a Medistrom Lite.  I figured it would give me two full nights and the older battery would give me one full night and some fraction of another.
 
Well when I went camping things didn't work out as expected.  The old battery gave me one full night (and then I used the remainder to charge my phone).  Then I switched to the new battery and it behaved about the same as the old.  I  didn't buy the battery at my local CPAP store because the prices there were over 10% higher than buying online.  They refused to price match so I got it online (pricing is already crazy enough online without tacking on another 10%).  Then on the trip when it conked out early I figured I had been sent a dud and now I was kicking myself over the battle I saw ahead of me getting a refund from an online company.
 
I reviewed my times at home and saw the new battery gave me 11:13 operating time on the trip.  I thought maybe I hadn't fully charged the battery or something, so I made sure to fully charge it and tried it again at home.  This time I got 11:15.
 
Then I started checking this board and it twigged me to the important fact I had forgot:  When I initially started using CPAP I was set at 7 cm H20 and that is how it was on my first canoe trip when I got the two full nights (I actually started on CPAP two days before the trip).  Then soon after I was moved to a new setting of 9 cm H20.  I didn't go camping again till a full year later and that is when the run time on the battery sunk below two full nights.  I attributed this to the aging of the battery.  But I realize now the real issue was the increased pressure.  I recharged my new battery again and then went for two nights at home using a value of 7 cm H20 again and the time I got increased to 13:22.
 
So that was the answer, the new battery was good.  I just wasn't accounting for the increased pressure I have been on for the last three years.  I checked against this Resmed document and it says to expect about 10:20 at 8 cm H20, so I am right in line with expected performance on the battery (leak rate would account for some of the variance).
 
https://www.resmed.com/us/dam/documents/articles/198103_battery-guide_glo_eng.pdf
 
Anyways the takeaway for me in all this is:  when going camping, temporarily lower pressure on your machine to get the run time you need.  I would rather run at a lower pressure for 100% of my sleep time than at full recommended pressure for less than 100% of my sleep time (and so do the rest of the people in the tent).  Of course the amount you should reduce is limited by the resulting increase of AHI.  For me dropping from 9 to  7 in my testing resulted in no significant increase in AHI value (but of course this is only data for a few nights).
 
 
And one thing I don't like on the new battery model vs the old:  The old battery starts beeping continuously when charge gets low, and wakes you up so you can turn off the CPAP machine.  The new battery just gives a tiny single quiet beep that won't wake you up and then shuts off, so you wake up not getting air thru the hose.

[ardenum]

The easiest and cheapest is to make your own pack out of li-ion 2170 cells, like Sanyo or Samsung. That will not only give you more kWh's but also be cheaper than getting a premade stack. With the difference in $$ you could even get one panasonic solar panel to charge the thing, extending its use even further.

[SarcasticDave94]

I’m not sure how heavy the batteries are, but I’d rather carry more batteries rather than dropping pressure settings and risking reducing therapy. But that’s just me.

[OMyMyOHellYes]

Check AH of whatever battery you are using. Fully charge it. Reduce the AH rating by a third. Compare to the current draw on your machine (Resmed publishes that data). That's what Resmed does in their Battery Guide. You will probably be safe with that unless the battery is in poor condition/has not been maintained and won't hold much charge...