Chaging issues

[Mattyboy80]

Well guys I have an 02 WS6 and am having some really bad charging issues. So far I have replaced the alternator twice, and the battery about 5 times in the last 4 months. The battery keeps getting a dead cell. Just replaced the battery again with a optima red top and its already down to 12 volts. The car charges while driving but when I am at a light it is way below charging at around 9-10 volts. It is obviously overcharging due to the dead cell but cant figure this out. Any help would be greatly appreciated.