Brendan, in the E89 and G29 it must be charged under the bonnet and not directly connected to the battery. Something do with the Intelligent Battery SystemIn a similar thread on this forum (unfortunately I cannot relocate it) a member wrote that when using a trickle charger one had to do "it through the car's charging system under the bonnet" and not just to connect the trckle charger to the battery terminals.
I’ve used a CTEK on all of mine without any issues ranging from a week to months