Ryobi to power my CPAP?

Yes, search eBay for a ‘DC Buck converter’. These are inexpensive devices that convert a higher voltage to a lower one, efficiently. Get one that is rated for more amps than you require.

You need to understand the capacity of your battery. a 18V 5AH battery has about 90 Watts of capacity. That means it can deliver approximately 18V at 1 Amp for 5 hours or 18V at 5 Amps for 1 Hour. When you convert the voltage to 12V, it will still deliver near 90 Watts, so you can get 12V at 1 Amp for 7.5 hours or 12V at 7.5 Amps for 1 hour.

If you look at your CPAP it will require 12V at some current level. As you can see, if it needs more than 1 Amp, a single 18V 5AH battery will only work for 7.5 hours. You can parallel the batteries together for nearly double the capacity.

Batteries do not supply voltage at a perfectly linear level, meaning they supply a higher voltage initially, and drop down lower as they discharge. Your DC Buck Converter will fix this problem, as it will supply a constant voltage output. Also, battery capacity lowers, as the current draw increases.

This kicks butt. I have a bunch of Ryobi batteries for all my yard tools (mower, blower, edger) and it’s nice to be able to drop any of them into this for 120v power, light and USB charging. Excellent deal. I got mine for $80. I suspect the prices are up because of the recent hurricanes. Here’s a link to the Ryobi inverter: https://amzn.to/3CKMVUV