System does not hibernate automatically in Windows even though Windows hibernate timer is set - ThinkPad General
The user modifies power settings in the Windows Control panel to set an automatic hibernate timer after a defined period of inactivity. When running on battery, the system will not hibernate from suspend mode, even though the timer has expired. When running on AC, the system behaves normally and will suspend & hibernate exactly as defined in the power configuration. However, when on battery power, the system will automatically suspend at the defined time, but will not go from suspend to hibernate.
Any ThinkPad system running Microsoft Windows Me, 2000, or XP.
This function is by design. When a system hibernates from suspend state it must first resume to full power mode. Since it is battery mode, the system may be in motion, or packed in a carrying case. If the unit were to power on, the hard drive may sustain damage if the system is in motion. Also, the system may wait at the password input screen when resuming from suspend. The system will remain in a full-power-on state until the user either inputs the password, or the battery is completely drained. To prevent these situations from occurring, the hibernate timer has been disabled if the system is in the suspend state while operating in battery mode.
If the battery reaches critical low power, the system still has the ability to hibernate.
This function can be disabled in BIOS by pressing F1 at boot-up time. To disable this, select Config, then Power, then Timer Wake with Battery Operation. Set this to Disable.