EXFO FTB-500 - Manual (Page 279)

Maintenance
FTB-500 271
Replacing Batteries
Before you go out in the field, make sure you install batteries in the unit
unless you have an adequate and reliable power source.
I
MPORTANT
To avoid losing test data that you have not saved, make sure at
least one battery is always present in the unit if it is not under
alternating current (AC).
I
MPORTANT
When changing batteries, make sure that the unit is off or that the
AC power is connected.