There seems to have been some interest in my question, but no replies so far, so I thought I would finalise it myself. I went to a Mercedes dealer a couple of days back and asked the service department the same question. The man there said just to connect the charger negative lead to the battery negative terminal! Of course, this is exactly what the owners' handbook says not to do. Anyway, I did this, connecting the charger negative lead to the earth strap - everything went OK, and the electronics did not self-destruct. I use a Ctek MXS5 smart charger. I have used Ctek chargers for a long time and my batteries survive a long time. For instance, the battery on my Harley I replaced a few months ago, and it was 19 years old - I regularly charged it with a Ctek charger.