Is there a way to leave the brightness of the screen the same when it is on battery power like it is on AC power?
There sure is! Go to:
- Apple Menu
- System Preferences
- Energy Saver
- click on the "Battery" tab
- deselect the "Slightly dim the display while on battery power" box
This is how it's done on OS 10.8.2. But it should be relatively similar on older OS versions (if you are running an older OS version).
HTH,
- Nick