The handset battery has a thermistor in it that prevents the battery from overheating and causing a thermal runaway condition. If the phone is left charging but is near a heat source, windows with sunlight, PC fan output, heat registers, etc, then it is possible for the phone to get too warm to continue charging. Since the act of charging generates heat as well the combination of an external heat source and the phone charging can cause the phone to power off.
If this is a consistent problem first ensure you are running the latest software for the 8002, version 130.009. If you are already running this version of software you will need to open a support ticket as there may be other issues occurring and any specialty software fixes that may be available will be provided only via Polycom support.
Justin Borthwick Manager, Wireless Escalation Support Wireless Escalation Support Services