As you might imagine, the Battle Bridge is a scaled down version of the main bridge and all functions that apply to the main bridge apply equally to the battle bridge.
The Battle Bridge is situated in the stardrive section and is the place where the stardrive section is controlled from and where the senior officers are stationed in circumstances where it is necessary to separate the saucer section and the stardrive section. In such circumstances, the Captain will remain in command of the Saucer Section and the First Officer will be "captain" of the stardrive section.
Like with the main bridge, the battle bridge is served by a turbolift and also a smaller conference room. It again has a view screen and a Ready Room.
Although its main purpose is to control the Stardrive section, the battle bridge can also control the entire ship should the main bridge be damaged or controlled by third parties. In the latter case, however, it would be necessary to transfer all commands to the battle bridge.
There is also a second turbo lift giving direct access to the main bridge.