I finally had to change the battery on my 2000 BMW 528i. I bought the car on Dec 30, 2003 and haven't changed the battery since. That makes for a minimum of 11 years and 5 months in service. The battery was still starting the car but I had to be careful not to leave it for more than a few days without starting. The battery I replaced was a NAPA Legend (I think) with 880 CCA. A very good run in my opinion. Contrary to my usual practice, I had the battery tested when I had to have the car boosted - in March 2009. It was fine - just a bit discharged. It cost $48 to have the battery and the charging system tested. I got over 6 years of additional life out of that battery - making those $48 well spent dollars! Since then I put a CTEK charger on it if the car isn't started for a week or more, and once over a week-end during the winter for good measure. To be fair it's a huge battery that lives in the trunk. And our climate isn't very hot in the summer and, while the winters are cold, the car is kept in an unheated garage and is driven almost daily. But countering that, the BMW has lots of electronic stuff that will run the battery down if the car sits.