Brief component description: Alternator
WARNING: This page does not describe the selected car, but rather 7 other vehicles, including the 2017 BMW X3, 2016 BMW X3, 2015 BMW X3, 2014 BMW X3, and 2013 BMW X3. However, it is still accessible from the selected car via links, so may be relevant.
With the engine running, the alternator generates a variable charging voltage for battery charge. The variable charging voltage is influenced by the power management depending on the temperature and current by the DME/DDE raising the engine speed.