(08-28-2014, 10:48 PM)APAAW Wrote: I admit I don't understand why the direct connection of a nominal 12V battery to an xPAP would have issues for either the battery or the xPAP. (or two 12V batteries in series to feed a ResMed, I suppose.) What am I missing? Thanks much!!
The power supply that comes with your machine is regulated. This means that output is whatever voltage your machine requires, even if the input voltage swings between roughly 90vac and 250vac.
A battery is not regulated. A battery starts off at it's raged voltage (generally a little above that), then drops slowly until it hits around 75% of it's capacity, then drops quickly, so your 24v CPAP machine could easily be fed 18v or 15v or less.
I have absolutely no idea how the machine would react to this, maybe it would be OK, maybe not.
Also, there's the human factor. If you hook your machine up directly to the battery, backwards, you'll probably destroy the machine.
Neither of these mean that a direct connection won't work. People do it all the time. It's certainly more efficient, but as with most things in life, it's full of compromises and there's still no free lunch.
A (good) inverter supplies it's rated voltage until the battery drops too low, then it just turns off.