The Midland club slumped to their third successive defeat and already
the alarm bells are ringing at St Andrew's.
It was a sweet victory for Alan Curbishley, a former Birmingham player, as it marked his 600th game as Charlton's manager.
Bent, a £2.5million signing from Ipswich, has been in outstanding form for Charlton as he demonstrated his value with his fifth goal of the season which he scored after 14 minutes.
But the outcome of the game revolved around the early departure of Emile Heskey. His prospects of an England recall were quickly dented as he limped out of the game after just eight minutes.
He suffered a leg injury after only two minutes following a challenge for a high ball with Chris Perry and six minutes later he was replaced by Walter Pandiani.
This was yet another injury blow for Birmingham who have already suffered badly this season with the absence of several key players.
Manager Steve Bruce had been looking to the Heskey-Mikael Forssell partnership to spearhead a Birmingham victory but these hopes were quickly destroyed as were their hopes of dominating the early proceedings.
But worse was to follow for the luckless Blues in the 14th minute when they again conceded an early goal.
This has been their problem this season and once again the defence was badly at fault.
Jerome Thomas produced a cross from the left and Bent was given a free header to notch his fifth goal of the season. Maik Taylor, who kept a clean sheet in midweek against England, had no chance of saving the close-range effort.
In the 17th minute Charlton had a let-off when goalkeeper Stephen Andersen handled outside the area. Referee Mark Halsey took a lenient view of the situation and only issued a yellow card.
Birmingham's defensive problems were repeatedly exposed and in the 19th minute Dennis Rommedahl raced down the centre to lob the ball over the advancing Taylor only to see the ball go just over the bar.
Forssell, who scored his first goals in 12 months when he grabbed a hat-trick for Finland in mid-week, attempted to inject some life into Birmingham's attack but his long-range efforts were sadly off-target.
The indifferent Birmingham play was indicated by the fact it was not until the 30th minute that Pandiani managed to get a powerful drive on target.
From the corner Matthew Upson's header appeared to be handled by Chris Powell but referee Halsey did not see any infringement.
With Danny Murphy enjoying the midfield battle, Charlton were able to stay very much on top. Thomas and Rommedahl both appeared to have plenty of space to exploit as Birmingham continued to frustrate their fans.
Only Jermaine Pennant matched the Charlton attack with his confident runs but they were generally unproductive.
The half-time response from Bruce was to send on Julian Gray for Mario Melchiot with the former Crystal Palace man operating on the left wing in an attempt to give Birmingham a better balance.
As a result Birmingham launched countless attacks with Charlton, who had never previously lost at St Andrew's, dropping back to defend in depth to successfully protect their slender lead.
Man of the Match: Danny Murphy – The mastermind behind this latest Charlton performance. The former Liverpool man put himself back into the England frame with this performance.