Here's what the options book says about the EE3:
"The 115-V socket is located in the lower part of the center console and serves to power small electrical
devices with an output of up to 150 W, such as mobile phones, cordless electric screwdrivers, laptops,
or flashing lights. The system is comprised of a DC/AC-converter with overvoltage protection and a
power socket. The socket is only supplied with power if a plug is recognized. For vehicles without additional
battery, the power socket is functional via the KEYLESS-START power supply when the ignition is
set to position 1. Vehicles with an additional battery have a run-on function that allows for devices to
be charged for up to four hours after the engine has been turned off. A light on the socket indicates
when it is operating."
Reading between the lines ... is that "keyless-start power supply" the dinky 1.2 amp hour battery that's under the driver's seat? (and is disabled by pulling fuse #32 if you're disconnecting power for a long time).
And it would appear that the 115v socket shuts down after 4 hours even if it's powered by the 100 AH House Battery.
--dick