battery does not charge enough?

hi all. the charging voltage on top of the battery posts is between 12.5 to 13. when i start the engine, then goes down to 12.15 when idle or revved. changed alternator, no difference. still the same. brand new battery fitted. don't know what to do. disconnected radio and boot light, no change. any suggestions please will be highly appreciated.

baz
June 2010