what's the code for the backup battery ?

There are no real secret, semi-secret or published codes to activate any non-existent backup/reserve battery, if that's what you mean.

Other than that, the code (type) of a replacement battery for the 6630 is: BL-5C

Are you sure ??
'cause i've heard it somewhere long time before i got my cellphone. But i cannot remember the code. i recall that it lets you talk about 30 minutes.