According to the Driver Capability List,
powerSource is an enum with four values: "battery", "dc", "mains", or "unknown".
However, in Rule Machine, it appears that when you wait for events, you can only wait for the states "battery" or "mains".
Shouldn't all four values -- or at least the three known ones -- be supported? I'm currently working around it by waiting on the device to change when it's already on battery, and I know the device can only change to DC. But this seems like an odd gap.