A 12 W7 requires a lot of power. What amp are you running on it? If you are running a jl audio 1000/1 and that is it, you can prob get by with a optima yellow or red top (yellow is better, more Cold Cranking Amps and recharges quicker) and a good capacitator (preferably 5 to 15 farads). The alternators in BMW's are good cause bmw's come with a lot of electrical features, and the car needs a high amp altanator to run on a normal basis. Anyways... dont get an altanator till u get the other stuff installed, if it doesnt sound good, you lights dim a lot, then you will prob need to get an altanator. But i definitely recommed getting the yellow top and good Capacitator.