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Best way to ac power in wilderness w/out gen

Discussion in 'General Electronics Discussion' started by kj724, Sep 5, 2011.

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  1. kj724


    Sep 5, 2011
    Hello. I need advice for powering a cpap machine while wilderness camping. I have sleep apnea and I can't go more than one night without cpap. I tried plugging into a/c adapter and car battery only lasted 4 - 5 hrs. I can't afford a generator at this time. So I'm thinking of getting a deep cycle battery, a charger and a converter. The cpap electrical info: 100-240v 50/60 Hz AC 1.70 A max. Not sure what that all means. I know of volts, AC and herz but don't know how they mean relative to my question. I will be needing power on weekends and maybe for a two week stretch down the road.

    My questions: 1. Does my plan sound ok? 2. How much battery do I need? 3. How much converter do I need? 4. How often can I recharge the battery effectively? All the converters I've found have cigarette lighter plug. I need to hook up to battery. What do I need to do that?

    Thanks in advance. Sorry if these are simple questions, but I am seroiusly challenged when it comes to electrical stuff.

  2. jackorocko


    Apr 4, 2010
    How long do you sleep? 8 - 10 hours? Hook up another battery in parallel with your car battery and you will double the capacity. I believe a alternator is over built for a car so I don't think charging another battery should be that difficult. Just make sure you wake up before you run the battery dead or you might be camping till someone comes and picks you up.
  3. daddles


    Jun 10, 2011
    You need to measure the actual current of the machine while it is running; a good tool for this is a Kill-a-watt meter ($20-$30 online) -- or use a digital multimeter if you are familiar with the dangers of making line current measurements. But we can make some back-of-the-envelope estimates.

    Assume it uses 1 A at 120 VAC. OK, that means it's slightly more than a 100 W load. Note it's twice this value if the operating voltage is 240 VAC.

    You never want to discharge a lead-acid battery more than about half of its capacity, as doing so can reduce the life of the battery. You'll get better life if you only discharge it to say 70% of capacity.

    Now, 100 W will be roughly 10 A from an inverter running at 12 VDC from a battery. If you want to run that for 8 hours of sleep, that means you'll consume 80 A*hour of charge from the battery. Thus, at a minimum, you need a 160 A*hour battery. You'd want to bump that rating up a bit to be conservative. I haven't bought RV batteries in a number of years, but I'd imagine such a battery would be in the $200-$300 range.

    Next, you've got the problem of charging up the battery the next day. You typically don't want to charge such a battery at more than 10-20 A, so you can see you have a basic problem -- that's 4 to 8 hours of charging time. You might as well use a generator instead. Then you've got the obnoxious noise of the generator (which torques off other campers) unless you get one of those real quiet units.

    Frankly, these CPAP machines aren't designed with energy conservation in mind. To me, your only real option is to get a generator and put up with its inconveniences.
  4. kj724


    Sep 5, 2011
    Thanks for the input. The alternative to a generator may be cheaper, but not real world realistic, with having to recharge daily. I appreciate your help. I will either have to camp by myself at the electrified sites or deal with the sleep apnea problems.

    thanks again
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Running the generator by day to recharge the batteries (that you use by night) is probably more friendly to other campers.

    I presume you've checked to see if there are sleep apnea devices designed to run on 12V?

    my quick googling suggests that the inverter option is the common solution. At least one supplier describes what you need.

    However there is one supplier in Australia that supplies 12 volt devices for camping etc. From the look of the images on the web site the devices come from the US ::)
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