This has already been answered in detail on SO. Check out this thread, It should answer all your questions:
Executing a subprocess fails
I’ve tried it myself with this code:
batchtest.py
from subprocess import Popen
p = Popen("batch.bat", cwd=r"C:\Path\to\batchfolder")
stdout, stderr = p.communicate()
batch.bat
echo Hello World!
pause
I’ve got the batchtest.py example from the aforementioned thread.