From Halopedia, the Halo wiki
The cell is also a Ghost's primary weakness. One well placed shot can take out the Ghost when it is hit in the fuel cell, making it of the utmost importance to aim for. It was only lethal to the driver of a Ghost in Halo 2, as in Halo 3, it does not function the same way it did in Halo 2. An explosion big enough to destroy the Ghost can be triggered if it is hit with a single bullet from such weapons that are able to cause a headshot.