You can easily replace BitLock's battery by following the instructions below. Please note that you have to first release the U-bar in order to access the battery compartment. If the lock is in locked position and you cannot release the lock, you have to first power it externally through the Micro-USB port ( read the instructions here on how to power the lock externally).
Your BitLock is shipped with two hex keys. Pick the smaller Hex key and unscrew the hex screws at the the end of the BitLock as shown below:
After removing the end cap, use a flat head screw driver to unscrew the battery cap that is inside the lock:
After removing the battery, place in the new battery with the positive terminal facing toward the battery cap. The screw back the battery cap using the flat head screw driver and put in the end cap and tighten the set screws back in place.
Please note that the battery is not a regular AA.The battery type used in BitLock is CR14505. For more information regarding the battery type, look at this article.