I'm writing a small program to save my laptop's battery, and I can now switch between power schemes using PowerSetActiveScheme.
The next step is to control the battery saver in Windows 10. Though I can read the state of it using GetSystemPowerStatus, I can't find a way to enable/disable it programmatically. Are there any functions in Windows API to do this?
Most probably you can do it Linux-way, by calling a system app named PowerCfg through ShellExecuteEx():
powercfg /setdcvalueindex SCHEME_CURRENT SUB_ENERGYSAVER ESBATTTHRESHOLD 100
powercfg /setactive scheme_current
This means that the energy saver is activated even when the battery percentage equals 100%. SUB_ENERGYSAVER and its sub-GUID ESBATTTHRESHOLD are described here.
@hidefromkgb's answer is pretty much correct. The only missing part is that to disable Energy Saver and prevent it from turning it on, you need to do:
powercfg /setdcvalueindex SCHEME_CURRENT SUB_ENERGYSAVER ESBATTTHRESHOLD 0
powercfg /setdcvalueindex SCHEME_CURRENT SUB_ENERGYSAVER ESBRIGHTNESS 100
If you do that, and go back to the control panel Battery Saver section, you will see that the first checkbox is now disabled (although it still shows 20% but it is grayed out so it should be ok). Also the second checkbox (lower screen brightness) will be unchecked.
You seem to be out of luck. MSDN docs show no API through which the battery saver could be controlled. Examining SettingsHandlers_OneCore_BatterySaver shows that only GetSetting is exposed. Even SetPowerState in WMI Win32_Battery is not implemented -- I know this is not exactly what you need, but it shows that Microsoft has not gotten around to exposing the battery-related functionality. At this point, instead of reverse-engineering the button click, your best bet is probably to emulate it with something like AutoHotKey, however beware of the pitfalls with that.