Iphone 3gs with 6.15.00 baseband requires a custom firmware to restore. If you restore it through apple original firmware 6.15.00 baseband gets broken. Itunes will not recognize and will not charge it thats normal.
What you can do. To make sure it has 6.15.00 baseband download redsnow.
Try to charge it through a strong wall charger (ipad charger is the best).
Do not connect to itunes. Connect to redsnow to see if 6.15.00 is the problem.
If not let me know.
It may be a different issue..
Good luck ...