All modern laptop batteries have intelligent controllers. Amongst
other things, these ICs have a "gas gauge" function that is reported
to the laptop and the operating system using a serial protocol.

Some of these ICs are supposed to compensate for the natural aging and
reduced capacity of the cells in the battery. Some are more successful
at this task than others. When the "gas gauge" is out of calibration,
unexpected shutdowns are what happens, as the controller's safe
minimum operating voltage is exceeded. To protect the cells from
possible excessive discharge, the controller disconnects them from the
computer and it shuts down.

Booting an operating system that doesn't support power management
(such as DOS), turning off all sleep or standby options in the system
BIOS and running the system from batteries until it stops operating
*may* fix the problem.

You may also wish to see if Compaq has an updated BIOS for the system
that may address battery issues.

William