battery going flat after washing

[ard]

(Reminds me of a trip to Aus where I drove from Sydney to Perisher and back over a weekend to ski in July...)

Your issue sounds like you have an underlying issue wherein your car may be straining the battery all the time, and shortening the life...then as the battery reaches the end of life it is susceptible to this behavior where a mild use can cause it to die.

There are faults where a module 'wakes up' periodically, causing the entire system to come on and draw a lot of current. This can happen constantly through the night, days on end, weeks on end...a battery that should last 5,6,8 years will deplete in 2 -3 years.

You want the dealer to LOG the voltage draw overnight and see if this is the problem. BMW has an SIB (service bulletin) for this.