EXFO FTB-500 - Manual (Page 286)

Maintenance
278 FTB-500
Recalibrating the Batteries
Recalibrating the Batteries
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 batteries (for
example, the battery LED is green and not blinking, which normally
indicates that the batteries are fully charged, yet the battery gauge
indicates that they are only charged at 85 % of their capacity). A complete
calibration cycle will be necessary.
You can perform a recalibration with the battery calibration utility:

The utility will charge the battery until it is full.

It will then discharge the battery completely.

You will have to let the battery recharge completely.
The whole calibration process can take several hours. You can stop the
process at any time, but the battery will still need calibration and could be
empty.
If the batteries still do not have the expected autonomy after a complete
calibration cycle, it probably means that they are worn out and they need
to be replaced by new ones. New batteries can be purchased from EXFO.
To recalibrate the batteries:
1. From the main window, tap the Utilities button.
2. Tap Battery Calibration.