I recently load tested the Varta battery in my W202 C220 CDi. It showed up as ‘weak’ despite being regularly conditioned with a CTEK charger and was only about three and a half years old. I suspect it has a faulty cell. I replaced it. It is no surprise that the car now starts in a more spirited fashion. However, unless I am imagining it: the tick-over is now smoother: it revs much more enthusiastically and has more torque. Even the radio works properly again without periodically turning Radio 4 into techno. So what is going on? Is common rail injection voltage dependent to that extent? I suppose the other point to make is beware that CTEK type chargers can disguise a faulty battery. Arrange for a load-tester to be in your stocking this Christmas!