Within the Windows Defender options page, there are no choices to prevent scheduled scans while running on battery power or to restrict scans to running while the computer is at idle. However, Windows Defender uses simply the built-in Windows XP (or Vista, I assume) Task Scheduler. The Windows Defender scheduled task is hidden, so you might not realize at first that it is adjustable.
Go to Scheduled Tasks in Control Panel, and select Show Hidden Tasks from the Advanced menu. The Windows Defender item is "MP Scheduled Scan" with the run command:
"C:\Program Files\Windows Defender\MpCmdRun.exe" Scan -RestrictPrivileges
You can adjust settings for the scan to run only during idle or AC power as you like. Using the command above, you could also make a shortcut to run a quick scan whenever you like.
