AASTRA DT590 for MX-ONE User Guide EN - Manual (Page 96)

Installation
DT590 96
Replace the battery
If the standby time for the cordless phone becomes too low, the
battery should be replaced by a new one. Please contact your
system administrator or your Aastra certified Sales Partner for more
information.
After replacing the battery with a new one, the
Battery icon can
indicate an empty battery even though it is fully charged. Placing
the phone in the charger for a while will remove the problem.
Note: To avoid fatigue of the phone´s battery connector,
use the charger instead of repeatedly replacing an empty
battery with a new one.
To replace the battery:
Open the battery compartment and remove the battery
Disconnect the battery cable, and replace with a new battery.