(03-05-2012, 04:55 PM)rriley Wrote: The 12v marine batteries and inverter are not a very elegant solution to providing backup power. A better way to do this is to use a pure sine wave converter such as the CyberPower CP1500PFCLCD Pure Sine Wave UPS. They come in various capacities and prices, the most powerful is 900w and costs $188 at the "big store" online. Plug in your CPAP to the Cyberpower, plug the Cyberpower into the wall and you are done. Easy to move around, enough power for the night, no tangle of cables to fool around with. It stays charged all the time, makes no noise and automatically supplies power if your AC goes off. Simple.
Maybe I'm missing something? It's not the
wattage of a backup power supply that matters, but the
amp-hour rating of it's batteries. Please see my post
HERE on figuring how long a lead-acid battery will last and how to figure what amp-hour rating on a battery system you're going to need. From what I've read, this UPS you mentioned is powered by two 8.5 amp-hour batteries, correct? Most CPAPs without humidifiers draw about 1 amp per hour. Add a humidifier and that can go up to 3 amps per hour or more (depending upon several factors).
I'm assuming those UPS batteries are maintenance-free sealed lead-acid like other similar UPS backups.
Those two batteries add up to 17 amp hours and the formula is amp hours x 90% / hourly amperage draw, which will give you your max hours that will run your battery down to zero percent charge... but you should never run a lead-acid battery that far down (60 % max). So with this Cyberpower UPS, the formula would be:
8.5 AH X 2 batteries = 17 total amp-hours
Then, it's 17 AH x 90% (Peukert's Equation factor) = 15.3 AH remaining. Now if your CPAP draws 1.0 amp per hour, that would give you 15.3 hours of operation time, but there's a problem - that will take your battery down to zero percent charge... you can really only use up to 60% of a lead acid battery, otherwise you'll have a dead doorstop if you do that a few times. So, that supposed 15.3 hours has to be reduced to give you the maximum safe discharge level... 15.3 X 60% = 9.48 hours in operating time... okay for one night on backup power, perhaps, now you have only 40% charge remaining. While doing this will not kill your battery immediately, it will drastically reduce battery lifespan... Ideally, for optimum battery lifespan, it's recommended that you don't discharge a lead-acid battery down below a 70% charge. This means that you really only can use 30% of your total capacity if you want to extend battery life. This means that with the above system, you can only run your 1 amp-hour CPAP draw for 4.59 hours without risking poor battery lifespan. Now, add to this factor a heated humidifier, and it gets worse... assuming you have a heated humidifier where both the CPAP and humidifier draw 3 amps per hour total, your ability to use this backup source for powering your system goes down to 1.45 hours... not a lot of time at all.
So, even a high quality UPS like the one you wrote about cannot power a CPAP with heated humidifier for an entire night of sleep, unless you're willing to destroy the batteries within the UPS. Alternatively, a standard marine deep cycle battery like you get from the local Wal-Mart will power a CPAP easily for several nights before a charge is necessary... many of them are rated at over 100 amp-hours.
Now if you purchase one of those very expensive CPAP backup power systems that use a Lithium Ion Battery, then those are designed to handle a deeper discharge with no risk to battery lifespan... but I'm pretty sure most UPS systems come with lead-acid batteries, not Lithium Ion batteries.
I dunno, maybe I'm wrong here.
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