The high-voltage system remains on for at least 30 minutes after a door, the frunk, or the hatch is unlocked or opened. The high-voltage system also remains on for a while after an i3 is shut off (not sure how long). If the 12 V battery's voltage is below a certain limit, the DC-DC converter will turn on to charge the 12 V battery during these periods. So the 12 V battery can be charged after an i3 is shut off without the need for an additional maintainer circuit.
2014 BMW i3 Arravani Grey, Giga World, Tech + Driving Assist, Parking Assist, DC Fast Charging, JuiceBox EVSE