Have a look to see if the battery is really charged. Even though the PowerBook can tell you this, there is a manual way to do it also. You can do this by pushing a little silver button on the battery. There are green lights that will light up to show the amount of charge in the battery. If the lights are all lit, call Apple and explain about the new hard drive install and the battery problem you are having.
If none light up - your battery is not being charged. Try a PMU reset and re- charge the battery.
PowerBook G4 (15-inch 1.67/1.5GHz), PowerBook G4 (17-inch 1.67GHz)
PMU reset sequence:
If the computer is on, turn it off.
Disconnect the AC Adapter and remove the computer's battery.
Press and hold down the power button for 5 seconds and then release the button.
Reconnect the battery and AC Adapter.
Press the Power button to restart the computer. Charge the battery and see it that rectifies the problem.
If that doesn't work, a call to Apple would be in order.
aussie_geek