Info from my WIS:

The 2 battery power supply is a complex system. It comprises the main battery (G1) in the trunk, the auxillary battery (G1/7) in the engine compartment, the battery control unit (N82), and the auxillary battery relay (K57/2).

If a problem is sensed by N82 in any of the above items (plus the alternator) it is able to reduce power to selected consumers in two stages, 1 and 2. Cut-out stage 1 is when G1 voltage is below 11.0v dc, and cut-out stage 2 is when G1 voltage is below 10.5v dc. Normally it will be about 14.2v dc when the Alternator & VR are working OK.

The warning message (in white at the cluster) seen by the OP was given because: "The charge level of battery G1 is too low". In that particular case I would have suspected either the alternator or the voltage regulator.