I'm trying to avoid having to run my CPAP machine off my generator on an upcoming camping trip.  The various small battery packs I have won't deliver near enough run time, but I suspect the 48V 13Ah pack that powers my ebike will be up to the task.  Maximum power draw is 90W, but the average is likely to be lower, so I suspect it'll get me through the night, and I can recharge it off the generator in the daytime.  (Update: It was.  I tested it before leaving with a fully-charged pack.  It started out at 54V and after a full night's sleep was down to 47V, which would suggest a bit more than half of the available charge was used.) 

I started out buying this 4-pin plug, hoping it would fit.  The pins were too close together, though, so I used a hacksaw to cut out two of them and designed this plug to hold them the right distance apart.  Solder a 2-conductor cable onto the ends of the pins, press the pins into place with whatever is at hand, and close up with a couple of M3x6 screws.  Make sure you get the polarity right when you plug in.  The cable I'm using has separate red and black wires fused together, and the connector on the battery has small “+” and “-” markings.

I was originally going to use a DC-to-DC converter to knock the 48V output down to 24V for the CPAP and an appropriate barrel-connector pigtail to plug into it, but the power indicator only blinks briefly when first plugged in and it won't power up…some anti-hacking chicanery on the manufacturer's part?  I found I could get a compatible DC-input power supply for not too much and then connect it to the DC-to-DC converter with a lighter-outlet pigtail…they're supposed to arrive overnight, so I think all of this can be considered sorted.

I'd recommend printing this with stronger settings than usual.  I used 4 walls and 50% infill, and it was strong enough to hold up while pressing the pins into place (they should be a tight fit).  Overture orange PLA was already loaded into my AM8, so I used that.

OpenSCAD source is provided, with variables at the top that you can tweak to adjust the plug to your battery pack and whatever size of pins you need to use with it.