FWIW the original Bosch battery in my 325e lasted 10 years. I replaced it with a BMW battery ($140, made by Exide, I believe), which has lasted 5 years so far. A German made Bosch battery I bought ($90) for my 2002 was 8 years old and perfectly good when a moron (installing an air conditioning condenser) drove a sheet metal screw through it and the acid leaked out. But on a strictly cost-per-year basis, a regular Exide from Kmart must be the least expensive. About $40 when on sale, and they last 4 years. The trouble is if the installation requires a ventilation hose (such as on a 325e), generic batteries may not have a hose attachment point, so you're foced to buy a purpose-made battery.