I'm looking for a method to rename a bunch of files from BIG5 to Unicode in a batch on Windows.
NOTE: BIG5 is a special Chinese encoding.
You can't rename a file from one encoding to another. On NTFS, filenames are handled as native Unicode strings and so do not have any byte-encoding associated with them. (Internally, they are stored in the UTF-16LE encoding, but that's an implementation detail.)
Do you mean the filenames are rendering wrong at the moment due to a previous encoding misinterpretation? If so, what do they look like at the moment?