PSC PT2000TM - Manual (Page 132)

Batteries
122 PT2000 & Top Gun
If the message does not disappear or occurs other than at initial use after
prolonged storage, then the backup battery may no longer be accepting
a charge from the battery pack or adapter. Contact a PSC dealer.
Use the PT Dock to recharge the lithium backup battery in the portable.
The battery will be recharged while the
READY light is on. (Make sure
the switch in the portable’s battery compartment is in the correct
position for the type of batteries being used.)