Welcome to these Forums, Cap'n!
If you haven't yet tried a drive lens cleaner disk, I'd give that a shot. If it doesn't work, the drive is probably history and may need to be replaced. Before you do that, make sure it's properly hooked up; perhaps something got dislodged when the HD was replaced (does it show in System Profiler?).
Edit: Oops, I see you mentioned it did, which renders loose wires less likely.As to the presence of a backup battery in your PB, that depends on exactly which model 15", 1.5GHz you've got. See
Apple Portables: Identifying the right battery--US for battery ID numbers. The 2004 PB5,4 model used a 922-6007 backup battery, while the 2005 PB5,6 model sported the 922-6714 battery. For the latter PB model
Mactracker doesn't list a battery at all.