Some people may raise an eyebrow on this one, but I have a Walmart Everstart 41-4 in my M3. It has worked fine for the last 2 years, the PO had the same battery in the car when I bought it. One day shy of being one year old the battery died. Took it to Walmart and they gave me a new one (pro-rated the difference in price). This battery has 810 cranking amps and 650 cold cranking amps. I believe the price was ~40 dollars or so. Need your help on something here guys... I've tried to jump start it and it was fine while it was warming up (about 30 mins) but after I shut the engine off, the engine wont turn again. Could it be the starter or just the battery ? Any help is appreciated.