http://www.instructables.com/id/Make-a-Battery-Protection-Circuit-low-voltage-cut-/ something like this should do the job of low voltage cutout,
I have added some further pictures of the adapter in place with a 20VMax battery and my 18V impact driver.
feels a little bit loose, so will adjust...
I've been drawing up a design for 3D printable 20Max to 18V adapter.
Issues regarding low voltage I am considering as has been suggested a low voltage cutoff which would disconnect power at the adapter.
the design and circuitry will be available as open source, non commercial.
some pics of the...