Well, I just got a CPAP machine, and it’s amazing, I can sleep through the night now. Yipee. Now I want to be able to use it if I’m away from power (or the power goes out) for a night or two, so I’m building up a battery pack to power it.
The respironics batteries are about $300, so I wanted to build my own.
Here are the specs needed:
The power supply is marked with 12V @ 5A. However, using an oscilloscope & .165Ohm piece of wire, I measured the actual absolute peak current at 7.2Amps, and therefore peak power at about 7.2 * 12 = 86 watts. By using a Kill-a-watt and the stock power supply, I measured about 40 Watt-Hours per night.
The connector is the same as an HP laptop power supply. I found one on ebay for $1.99. If that link doesn’t work, search for “DC Power Supply Adapter charger Tip Plug Connector Cord Cable for HP compaq pin”
I wanted to use a pack of Li-Poly or Li-Ion batteries to keep it small and light, so I bought a set of 8 18650 batteries on ebay for about $13. I also had some LC2596 DC-DC converters around, but they definitely can’t handle the 7.2 amps required, and sure enough, it shuts down with the 7.2A motor startup current.
I also ordered a Mobile Power Bank that includes a charger & dc-dc converter on ebay for $20 here, but I’m afraid it’s going to shut down with the 7.2A requirement too. Oh well, I’ll just use that as a backup USB bank I guess.
Next step: find a much beefier DC-DC converter. I really liked the power bank plan because it has a charger built in. With the dangers of Li-Ion (“rapid dismantelment” as the data sheet calls an ‘explosion’), I don’t really want to build all the circuitry myself.
Maybe I’ll just grab a 12V gel-cell. It’s heavy, but it’s simple, robust, and doesn’t need a DC-DC converter, just a charger. And it’s much safer.
|Model:||Respironics System One REMStar|
|Power used during an average night for me||40 W-H|
|Average current consumption||0.41A|
|Peak current Consumption||7.2A on startup for about 100ms|
|Power connector||HP/Compaq laptop, but at 12V|