EXFO FTB-2 - Manual (Page 273)

Maintenance
FTB-2 and FTB-2 Pro 263
Replacing Batteries
3. Install the new battery as follows:
3a. Place the battery so that its poles are facing down and that they
point towards the back of the battery socket.
3b. Slide in the new battery, and push it toward the back until it stops.
4. Repeat steps 2 and 3 with the other battery if it also needs replacement
(FTB-2 Pro only).
5. Position the battery compartment door so that you can see its two
screws.
Battery poles