Sounds like you may have fried the Power Supply Card (Apple P/No 661-22286).
If the cable you refer to connects to an oval shaped object just under the DVD drive then that IS the backup battery.
The following is from the troubleshooting section of the Apple Service Source for the Pismo:
Power
Computer won’t
power up
1. Restart computer by pressing the reset button on the back of
the unit. Wait five seconds and press the Power key.
2. Try known-good power adapter.
3. Try known-good, firmly seated, charged battery. Repeat.
4. Connect power adapter and restart computer in 3-4 minutes.
5. Disconnect internal keyboard completely and try powering up
unit. If unit powers up, replace keyboard.
6. Verify microprocessor card is firmly seated.
7. Replace sound card.
8. Replace power supply card.
9. Replace microprocessor board.
10. Replace I/O logic board.
If you haven't already, you could also try removing the battery and disconnecting the internal cable, leave the computer for several hours, then reconnect the internal cable, plug in the power supply, leave for about one hour, then try to start it up - do not attempt to use the main battery though - if you can borrow one off someone else and try theirs.