It is false to say that a controller already has what it takes to "be a charger".
Even a Sorenson PSU costing thousands of dollars, which is compatible with world power AC as input, and can precisely output a huge range of volts and amps independently of each other,
does not qualify as a battery charger, if the owner must watch the process, judge when the battery is full and manually terminate charge.
A dedicated purpose built unit like a Satiator is really the target to reference here.
Now, it is the case that some controllers can take the "motor as generator" current, and feed it back into the battery.
I guess, **if** you can feed it a similar current at similar voltages, then it could do the same thing from another source.
But getting from **mains AC** to whatever that input needs to be, rectifying / converting / regulating everything to stay within those input bounds, is obviously not a trivial project, in fact afaict nearly as difficult as creating an all in one "controller / charger".
Just because some functionality, some circuits and sub-modules share some components does not make the project a" "easy no-brainer"
The whole concept of designing it to do **both jobs** as well as we expect what is now separate units to do, is a huge design challenge just technically, and even bigger from a holistic business POV, ROI, marketing etc.
Look at the Victron, Mastervolt and Magnum "combi inverter / charger" products. How long after inverters were common did it take for those to emerge?
Fine products now, but still cost thousands and not exactly portable (yet).
Given this "industry" is still in its infancy, not exactly a great opportunity for a big player to consider investing in fast-results R&D, no wonder we haven't seen such devices in the marketplace. Yet.
I bet Grin could do it. Think they haven't thought about it? No, it will just have to wait until the lower hanging fruit is being harvested at volume first.