I am going to pick up a new battery for my 2001 3.0 X5 tomorrow. In reading online, I see that the 4.4 has a bigger battery than the 3.0. Looking at the batter in my 3.0, I see that there is plenty of room for a wider battery.

Can I buy the battery for the 4.4 and put it in my 3.0? Is this a good idea?

Also, what is in the big heavy round thing that is under the spare tire and over the battery?