The average lifespan for a BMW battery is about 4-5 years. If your battery is still original, I would play it safe & replace it. These have a nasty habit of being fine one day, and then the next day one of the cells will intermittently short out and cause a no-start. I've seen it too many times....You can check the age of the battery by checking the production date on the negative post; it will have the week & year of production(26/01, 26th week of 2001). To check the charging system, just put a voltmeter between the jumper lug on top of the driver's valve cover & the ground lug on the passenger shock tower. Check it with the car running. It should be around 13.7 to 14.2 volts. Anything lower or higher could show a faulty voltage regulator...
Last edited by bmwtech6367; 12-30-2012 at 03:43 PM.