It was a vital victory for Charlton, whose poor form at home this season has been a worry for manager Alan Curbishley and this win puts plenty of distance between themselves and the strugglers.
Birmingham must wonder how they failed to get something out of the game having had a large part of possession and plenty of chances as their dismal away form again haunted them.
They have managed to score only six goals in 11 away matches and have failed to put together back to back Premiership victories for over a year in a run which has now reached 39 matches.
City manager Steve Bruce was able to play virtually his strongest side this season with the return on Matthew Upson, Muzzy Izzet and Mario Melchiot and a debut for former England striker Chris Sutton, signed on a free transfer form Celtic.
Charlton boss Curbishley kept faith with all but one of the team that had beaten Sheffield Wednesday in the FA Cup with skipper Luke Young returning in defence.
Jiri Jarosik had two early opportunities for the visitors, shooting wide and then firing a low shot across the face of the goal and against the run of the play. Charlton took the lead from virtually their first attack of the game.
Dennis Rommedahl's corner was headed home by former Birmingham midfielder Bryan Hughes. Charlton could have increased their lead when Matt Holland fired over the bar and Emile Heskey was also wayward with an effort for Birmingham.
Charlton might have increased their lead within a minute of the restart when Holland fired across the face of the goal, while Charlton keeper Thomas Myhre kept his side in the lead in making a fine save from Heskey's point-blank range shot.
Birmingham were having plenty of possession with Sutton, Jarosik and Heskey all going close.
Charlton finally made sure of the three points with a fantastic goal from England hopeful Darren Bent.
Myhre had again made a great save to deny Sutton when Charlton broke clear in injury time, Bent racing 40 yards to outpace Melchiot and rifle home his 14th goal of the season, his 11th in the Premiership, to send the Addicks fans home happy.