Battery does not charge on my AE92

[robotype]

battery led was red from a couple of days then last night finally the car stopped while it was running...the battery was gone...so today I went back to pick it up with another half/charged battery and it started but after 15 minutes the battery led was red again...so it was not the battery that was damaged, I am thinking about the alternator? But why the car stopped while it was running? Actually not suddenly, it was going slower and slower and after switching the lights off I could ride for 5 more minutes before complete death...any clue?