Coincidence that you should put this up today @msinger
I've just spent part of this morning modding my P3 Pro controller so that can use any of my 2-cell lipos eternally to run it via a longish lead. I can also easily balance charge the batteries as normal, using one of my lipo balance chargers or set them accurately to correct storage level when not in use. 18650's are now also another option for powering the controller.
I did it because after 5 years, the original 2S, 6000Mah battery was starting to show signs of its age. If I charged it and left it for a month or so it would discharge to no lights showing, so I figured better to do the mod now, rather than later... ;-)
And boy - with no battery inside, the controller is so much lighter now!
Good post msinger. Hopefully save folks here some future grief. And explain what went wrong to others.
Remote batteries are Not smart. Maintain or lose. The only way to drain the Phantom remote, to safe storage level, is to use it. I have several so a log is necessary to keep track of charges.