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Originally Posted by ptanner
Thanks to all for your advice and guidence, this is why I use this forum. Being technically challenged I can see a lot of challenges ahead. Luckily we have a good local dealer and also a couple of RV shops like Camper Land close by where I can go to have proper installs done.
To answer a couple of your questions:
1. The CPAP in necessary but not "life threatning" for short trips, except the DW treatens me for snorring so loud :>)! BUT it sure improves my quality of life. It does not have a "warning system" but that is not really a need at this time.
2. Presently, we camp in with a tent although we have had pop ups, a TT, and a class C in the past, pre CPAP. Because of our power need we use only campgrounds with 110 service but would like to be able to add places like NPs and SPs that have no power, to our travels.
3. We have ruled out solar because we prefer forest settings like the eastern Appalachians that would not provide the needed sunlite.
4..For now all trips of any consequence will be during the summer months, lasting up to a month or more so the need for an a/c is paramount.
Thanks again for all you help you have given me a lot to ponder.
pete
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The increasing frequency of threads (like this one) regarding CPAP machines...and my scientific curiousity...caused me to do some googling today on both Sleep Apnea and the use of CPAP machines to treat the symptoms of Apnea. It looks like CPAP machines are a real growth industry...there are a bunch of manufacturers. And a fair number of them have accessory devices (inverters) to allow their use when 120V AC is not available. But seemingly most (all) of these inverters require a steady 150 to 180 watt DC power source. And they also specify having a battery that has at least 100 amp-hours of capacity available each night. I didn't find any CPAP machines that could be operated on as little as 3 amps of 12V DC current...most seemingly need 1 to 2 amps of 120V AC current to operate their compressors. Even 1 amp at 120V is 10 amps at 12V with a 100% efficient inverter...and most inverters are only 60 to 70% efficient...hence the requirement for 150 to 180 watts of DC power.
And there's the problem. 180 watts of 12V DC power is about 15 amps of steady current drain. Multiply 15 amps times even a 6 hour night and you're at 90 amp-hours. As Bob correctly notes above, a deep-cycle battery's nominal amp-hour rating is measured at roughly a 5 amp discharge rate. Raise the discharge rate to 15 amps and the battery's rated capacity drops a lot...maybe as much as 30%. So now you really need a battery that can provide ~120 amp-hours at its rated discharge rate. Add in a reasonable discharge buffer (i.e. don't discharge below 30% of capacity) and you're really going to need about a 200 amp-hour deep cycle battery (or, realistically, pair of batteries)
just to power a typical CPAP machine for one night. And you'll still need another battery to meet the trailer's power requirements.
And finally there's the need to recharge all this drain...if the CPAP pulls 120 amp-hours out of its dedicated source battery(ies)...as Bill suggests...and the trailer's house battery needs another 30 to 50 amp-hours to replenish the current used for lights/heat/water pump/etc., you'll have to put around 170 amp-hours back into 3 (not one, not two, but three) batteries. Even with a high efficiency converter (say one that's rated at 60 amps of charging current), you're looking at at least 3 hours, more likely 4 to 5 hours of generator run time each and every day since battery recharging can't be continuously done at full current nor is it a 100% efficient process (especially at high recharging currents).
And, finally-finally, there's the issue of where to put 3 large & heavy deep cycle batteries. That's about 200 lbs of lead...which is both bulky and will seriously affect the front/rear load balance of the trailer.
Now, I'll be the first to admit these concerns are overblown if the CPAP machine in question is truly able to operate on only 3 amps of 12V DC power (a miniscule 36 watts) but I seriously doubt that's really the case. But if the CPAP machine will need something closer to a more-realistic 1 amp of 120V AC power, then all of the above are very real issues.
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