I wrote a small shell script for avoid this:
if [ "0" = $connected ] then
setterm -blank 1 > /dev/tty0
echo "`date` - Battery, enabled screen blanking"
setterm -blank 0 > /dev/tty0
echo "`date` - Connected, disabled screen blanking"
This cannot be used in a crontab (the original idea) but is useful, enjoy :)