[Qsilver7]

BMW warns against using the 12 volt (cig lighter sockets) because they are on a relay and will shut down after a few minutes. If you're using the 12v sockets to charge...then the charger will also be cut off from doing it job.

The car will have a dual battery setup (aux battery) based on options. The aux battery usually is installed if you have active integrated steering or its used to helpw with EPS (electronic power steering). If you have an auxiliary battery, it is maintained by either a PCU (power control unit) or a charging controller.

Use the dedicated jump start terminals in the engine bay for maintaining the car's main battery. The PCU or charging controller will take car of the aux battery.