The problem with my Lexus battery was sorted by a Lexus Centre yesterday and the problem was a dead cell. It was explained to me that the ‘starting’ battery has only that function, starting the engine, - after that the computer and other batteries take over the management for running the car .... this includes charging the main ‘starting’ battery.
Lexus uk stated that they had posted information on their blog as to how to ensure that the problem of risking a flat battery could be eliminated by starting the car and letting it run for about an hour in a stationary position. Unfortunately, unless you went on to their blog then you would have been none the wiser, since no general information was received, - well not by me anyhow.
The Lexus centre have been magnificent, whilst Lexus Uk were not really helpful.