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
Voltage 12 V
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