EXFO FTB-2 - Manual (Page 268)

Maintenance
258 FTB-2 and FTB-2 Pro
Recharging the Batteries
I
MPORTANT

Batteries are not charged at the factory. You must fully charge
them before using the unit for the first time. The batteries are
fully charged after a few hours or when the battery LED
indicator stops flashing.

The time required to charge batteries depends on various
factors such as the type of modules currently in use and the
ambient temperature.

To ensure that batteries function or charge properly, keep them
in temperatures between 0 °C and 40 °C (32 °F and 104 °F). Store
them between –20 °C to 60 °C (–4 °F to 140 °F).

Do not leave a battery discharged for several days.

Depending on the way the unit is used, after a while, the charge
status icon may no longer correspond to the actual power level
of the battery (for example, the icon indicates that the power
level is sufficient, but the unit turns off because the battery is
too weak).

After 300 cycles (approximately 18 months of use), you may
want to replace the batteries with new ones to maintain
optimal operation conditions. Otherwise, the operating time
might be reduced.