Charging issue

[Pete]

I just got a 2000 Gz 250 for my wife as a starter bike. Brought it home and found out that the battery discharges while riding. I charged up the battery, tested charging and it is not charging. Tested the stator as per guidelines, it is out putting over 65 AC volts on all phases. Output from rectifier was 8 volts. I replaced the rectifier and still no charge to the battery. Disconnected the rectifier battery connection and tested voltage there, it show 13.5 volts. However, voltage at same plug when connected drops to 11.5. I’m not sure what could be causing the charging voltage to drop below the required voltage when the rectifier is plugged back in. The bike stalls immediately if the negative terminal is disconnected from battery. Any help is appreciated.