Compaq 239158-999 - Mini Tower - Power Supply 250 Watt Hardware Reference Manual page 54

Battery Replacement
C-2
CAUTION: Static electricity can damage the electronic components of
the computer or optional equipment. Before beginning these
procedures, ensure that you are discharged of static electricity by
briefly touching a grounded metal object.
1. If you have locked the Smart Cover Lock, use Computer Setup to
unlock the lock and disable the Smart Cover Sensor.
2. Shut down the operating system properly, turn off the computer
and any external devices, disconnect the power cord from the
electrical outlet, and remove the computer cover or access panel.
It may be necessary to remove an expansion card to gain access to
the battery.
3. Locate the battery and battery holder on the system board.
4. Depending on the type of battery holder on your system board,
complete the following instructions to replace the battery.
Type 1
a. Lift the battery out of its holder.
Removing a Coin Cell Battery (Type 1)
Hardware Reference Guide