A mobile computer that is running Windows 8 is connected to battery power.
The Allow wake timers option is disabled for Battery power and is enabled for AC power (Plugged In).
A mobile computer that is running Windows 8 is plugged into an AC power outlet.
The Allow wake timers option is disabled for AC power (Plugged In) and is enabled for Battery power.
In these scenarios, Windows 8 may wake the computer if there is no ACPI Wake Alarm system device present.
Note If the ACPI Wake Alarm system device is present, it will appear in Device Manager and will have a hardware ID of ACPI000E.
To determine whether the ACPI Wake Alarm system device is present on your computer, start Device Manager, and then examine the devices that are listed under System devices.
Windows 8 automatically disables the Allow wake timers option for AC power (Plugged in) and Battery power when no ACPI Wake Alarm device is detected on the computer, on all in-box power plans, for mobile computers. Additionally, Windows 8 automatically disables the Action Center / Automatic Maintenance setting Allow scheduled maintenance to wake up my computer at the scheduled time on mobile computers.
All these settings must be changed from their default values for this issue to occur.
The ACPI Wake Alarm device can distinguish the current power source when the device wakes the computer. If this device isn't present, Windows 8 programs the wake event in the Real Time Clock (RTC) before the computer enters sleep or hibernation if either of the Allow wake timers options is enabled under the power plan advanced / sleep option. This behavior helps make sure that a wake event occurs even if the power source changes while the computer is asleep if either of the wake timers was manually enabled.