Save as unpack.bat 

@echo off
REM Assumes (x64 7zip on x64 OS or x86 7zip on x86 OS ) and (default 7zip installation path)
REM Unpack all RAR files in and below directory (X) with 7zip into specified subfolder
REM Example: C:\unpack.bat "D:\some\directory\or another\" "extract"

REM store variables
set target=%1
set destination=%2
REM remove quotation marks
set target=%target:"=%
set destination=%destination:"=%
REM switch to target drive for command context

REM switch to target directory
cd "%target%"

REM check target directory and subdirectories for RAR files to extract.
@echo on
"C:\Program Files\7-Zip\7z.exe" x *.rar -o"%destination%" -y
for /d /r "%target%" %%X in (*) do (cd "%%X" && "C:\Program Files\7-Zip\7z.exe" x *.rar -o"%destination%" -y)