It deals with many amusing things, from keyboard backlight, charging, fans, etc.
SMC controls a host of important functions in a Mac.
Safe Boot feature is included since Mac OS X 10.2. When everything is ready, boot your Mac with peripherals reconnected. If your MacBook stuck on login screen because of battery power, the battery could be thoroughly depleted or malfunctioning.
How to Fix Mac Stuck on Boot Screen after Update? Fix 1.