you want to use SQL authentication. this would be the user and password that you use in Store Administrator > File > Configuration.
What I do with my batch files is I backup to the local C:\POS_Backup directory. then i do a xcopy command to copy the backupfile to a network drive.
The following script assumes the following... User: sa Password: Password1 Database: RMS
del C:\POS_BACKUP\Database\Friday_POS.bck osql -U sa -P Password1 -Q "BACKUP DATABASE RMS TO DISK='C:\POS_BACKUP\Database\Friday_POS.bck'" xcopy c:\POS_BACKUP\Database\Friday_POS.bck "e:\" /Y xcopy c:\POS_BACKUP\Database\Friday_POS.bck "\\POS02\POS_BACKUP" /Y xcopy c:\POS_BACKUP\Database\Friday_POS.bck "\\BackOffice01\POS_BACKUP" /Y exit