Any device used for mobility with a battery of over 300wh MUST be checked (counter checked or gate checked) and the pilot must be notified.
I have flown with a Mini Pro or my Segway Elite+ on more than one airline. The first time I flew with the MiniPro, I did not know all the FAA rules (and neither did local airline personal at the gate!). They told me that I could take my MiniPro on board, and the flight attendant even helped me put the duffel bag that held it above with carry-ons.
The last minute before they closed the cabin door, the top local airline official came and told me that I could not fly with it (despite having cleared all required hurdles with notification of their disability department and pre-inspection!) Fortunately they held it for me until my return flight and did not make me get off the flight.
I wrote to the airline and was fortunate to find someone who was able to straighten this out for future flights.
There is no FAA lithium-ion battery size limit for someone who is mobility challenged - if the battery is not detachable and the device can be securely turned off.
Reference: FAA 49 CFR ยง175.10 Illustrated - (See Page 10)
Link:
https://www.faa.gov/about/office_org...s_and_Crew.pdf