You need an external tool, in example: SoftInterface.com - Convert XLSX to CSV.
After installing it, you can use following command in your batch:
"c:\Program Files\Softinterface, Inc\Convert XLS\ConvertXLS.EXE" /S"C:\MyExcelFile.xlsx" /F51 /N"Sheet1" /T"C:\MyExcelFile.CSV" /C6 /M1 /V