EXFO FTB-500 - Manual (Page 277)

Maintenance
FTB-500 269
Recharging the Batteries
To recharge the batteries:
Connect the unit to a power outlet using the AC adapter/charger (four-slot
units) or a standard power cord (eight-slot units). The charge cycle will
start and end automatically.
I
MPORTANT

Remove the batteries if the unit will not be used for more than
two weeks.

If you need to store the batteries for an extended period of
time, place them in a cool dry place, and ensure that they are
charged at around 50 % of their capacity. Every three months
during the storage period, verify the battery level. Recharge the
batteries when necessary, so that their charge level remains
around 50 % of the total capacity. This will ensure that you get
the optimum performance out of the batteries.

Li-Ion batteries that are not used for a long period of time (over
three months) will not be damaged, but might need to be
recalibrated.