In the v0.3.9 release, mining when not plugged into power has been disabled by default. This setting can be changed in the preferences page if you run into any issues.
Due to popular demand an additional power mode has been added to fine tune control on the mining vs power load. Maximum has been introduced as a rebrand of High power and High power has been tweaked down to fit in-between Medium and Maximum.
For more information on the debate for terms in-between Medium and High visit StackExchange: Term for an importance level.