Siemens SIMATIC PG 720 P Manual page 33

Note
Whenever possible, avoid running down the battery too far. Switch off the
unit when it is not in use and remove the battery connector if it will not be
used for some time (weeks).
Before the battery is completely discharged, the red power LED lights up
and a warning signal is heard (see also Section 4.2).
PG 720 P Programming Device
C79000-G7076-C721-02
Getting to Know the PG 720
3-19