NHK News
NHK news spent 15 minutes on this issue.
Passenger reported a very quick decent with a newspaper falling off a seat and coffee cups sliding forward.
Pilots apparently received several warning messages in the cockpit including generator issues and backup system issues. Smoke was detected in the forward battery /electrical compartment. A burning smell was also noticed.
Battrey was found to be discolored and leaking fluid. Japan air safety authorities said the battery temperature may have been extremely high.
Takamatsu controllers noticed smoke after landing and the pilot then initiated emergency evacuation
Rgds.