I'm hoping somebody can help me understand this. I use a Resmed S9 VPAP ST-A/ S9 COPD which according to documentation at my settings draws a current of 1.4A.

I use a Resmed Powerstation 2 battery. The ratings on the back say 97Wh output 24V 3.75A.

I wanted to calculate how long this battery would last. I know it lasts for around 10hrs but i'd like to know how to work it out.

I started by working out the amp hours by dividing 97 by 24 = 4.041 which I thought was the amp hours. I then divided this by 1.4 which gives me 2.8hrs which I know is not right. I'm no expert on this sort of thing and obviously missing or not understanding something. Please can somebody help! Thanks in advance

  • 2
    \$\begingroup\$ What voltage does the S9 VPAP draw its 1.4A at? If there's a DC-DC converter lurking somewhere, that would throw off your sums. Or maybe the 1.4A is a very generous maximum, or perhaps a high starting current, and the long term run current is actually much less? \$\endgroup\$
    – Neil_UK
    Commented Apr 23, 2018 at 12:48
  • \$\begingroup\$ Hi thanks for getting back. I'm 99% sure its 24V. This is the info I got from the net. Product Treatment pressure* (cm H2O) Current draw at 12 V DC (amps) Battery size for 8 hours use (amp-hours) 1.4A. \$\endgroup\$
    – R Thomas
    Commented Apr 23, 2018 at 13:33
  • \$\begingroup\$ Sorry I dont have anymore info \$\endgroup\$
    – R Thomas
    Commented Apr 23, 2018 at 13:36
  • \$\begingroup\$ Your calculation appears right for the numbers you have given. \$\endgroup\$
    – user16324
    Commented Apr 23, 2018 at 20:51

2 Answers 2


As Brian said, your calculation is right: P=UI, so Ph = UIh. If you rearrange, then you get Ih = Ph/U ~= 4Ah. This, divided by 1.4A will give you ~2.8 hours.

There must be something wrong in the specs of your values (I think your COPD does draw less than 1.4A).


97WH battery means it can power 97 Watt load for 1 hour at rated voltage of 24 volt. if you take lesser load which in your case is (at 1.4 A continuous) it will last for approx 3 hours.


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