A problem with the aux battery triggers a red battery warning just after you start the car. If you have no other malfunctions listed, then the red battery warning and "drive to workshop" msg is likely to be a bad aux battery (or, maybe less likely: bad aux battery relay).
If you have the "some convenience functions disabled" msg: This message is triggered if the BCU measures the systems battery voltage to be less than 11V. This triggers the cars power saving mode and the blue message in the mfd. There are two stages of consumer cutoff:
Consumer cutoff stage 1, when the voltage of the systems battery drops below 11V the following functions are deactivated: Heated rear window, memory, interior blower (reduced to 69 %), headlamp cleaning system, seat adjustment front passenger, seat heating/ventilation, multicontour backrest, entrance lamp, exit lamp, warning lamp, steering wheel air conditioning, dynamic seat, stationary heater
Consumer cutoff stage 2. If the systems battery voltage drops below 10.5V, this stage is activated in addition to stage 1 to disable the following functions: Parktronic, trunk lid remote closing, washer fluid hose heater, washer nozzle heater, mirror heater, reading lamp, passenger compartment lighting, footwell lighting, load compartment illumination, rear/load compartment socket, front cigarette lighter, trailer socket, interior blower (reduced to 53 %)
If you have problems with the systems battery for a longer period, you may get warnings also from the aux battery (red warning). Reason: The BCU will not charge the aux battery if the systems battery voltage is below 13.5V. If the systems battery voltage exceeds 13.5V, the BCU will energize the aux battery relay and charge the aux battery if needed. The BCU will only charge the aux battery for a maximum of 20 minutes per ignition cycle
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