i am on the verge of breathing life into my series 3 109 after replacing all the perishable engine bits i need to put a new battery on it and here lies my quandery there is a battery tray under the bonnet in front of the airfilter with all the leads but there is also one under the passenger seat i would prefer to use the one under the seat as its much bigger but why are there two the battery is under the bonnet at the mo but does not look big enough to start a watch