Lots of comments and guesses from everyone. Regarding the charging function, the stator and the rectifier, all of these can be quickly and easily tested using a home voltmeter. The testing procedures and expected results are clearly described in the service manual. Have these tests been done yet? It seems to me that this should be the first thing to do when trouble-shooting this problem. If the stator and rectifier perform as per the service manual, and the battery voltage is within spec when not running, at idle, and at 5000 rpm (again as per the service manual), then the charging circuit is working fine. But until these tests are performed, and functionality confirmed, it's impossible to trouble shoot.