I have just had my BMW 520d Touring serviced at 33,000 miles by a local dealer, preceded by a warranty check, which found a leaking exhaust manifold and a faulty EGR. When I took it away, I noticed that there was a small warning symbol displayed on the dashboard, but assumed this related to another display which advised the need for a brake pad change. On getting home, and checking, the message "change battery" came up. Is this usual for BMWs of this model and age, or could there be another reason?