I'd have to check whether the 12vdc is present when the vehicle is off, but I do remember that they are limited to about 2A output (don't remember the exact number, but it's not all that much). Another thing to consider is that the 12vdc battery in the i3 is essentially a small motorcycle battery, and if it is not up high enough, the car won't work, and it will take it a bit longer to even start to charge since the 12vdc is required to run the logic on charging and the circuits that power the displays and other controls. If that battery is discharged and you try to charge the vehicle, there's a backup circuit that will charge the 12vdc battery by itself until it can power up the logic, then it switches, charging the main batteries, which then use a DC-DC power supply to recharge the 12vdc battery.
