Hi, Where does your friend want to use the C-Pap ? Cheers Wantago.

You can't get round the fact a CPAP machine runs on electricity and that power needs to come from somewhere. If AC power isn't available then it will have to be a battery of some sort. However, it's not difficult to work out what size that battery needs to be. Ask your friend to check the specs. for his CPAP machine for what the typical power draw is in watts. Divide this by 12 for 12V DC and that will give you the current draw in Amps DC. Then multiply this by the number of hours he runs it each night, and that will give him a figure in Ah or Amp Hours. This is what a battery will need to supply, and considering normal practice of not discharging a battery below 50% capacity, multiply that figure by two to get the size of battery in Ah he needs.

Then he has to consider some kind of charging system to replenish that battery each day - solar, car, generator, mains charger etc. which will need to be capable of providing the same amount of energy back into the battery, plus another 20% or so to account for losses.

If that kind of battery system is too big for him, then he'll have to consider a CPAP machine with a smaller power requirement, they are getting more efficient all the time. My current CPAP typically draws 6A DC so by the above calculation I would need an 100 Ah battery. However, there are new 'travel' CPAP machines available which draw as little as half an Amp DC, which by the same calculation would only need an 8 Ah battery, which takes it into the realm of a small portable battery pack. Still need a way of recharging it though!