Have you tried resetting the Power Manager?

From the Service Manual:

Reset the power manager if:

• The battery and power adapter are proven good, but the

computer will not power on.

• The computer will not reset after a system crash.

To reset the power manager in the PowerBook 500 Series,

1 Remove the AC adapter and the battery.

2 Let the unit sit without power hooked up for 3–5

minutes.

3 Simultaneously hold down the Control, Command, Option,

and Power On keys for 5–10 seconds.

4 Reinstall the battery and, if necessary, reconnect the AC

adapter.

5 Turn on the computer.