BMW 3-Series automobiles are European in design, and repairs are made differently yhan on American cars. The battery, for example, is located in the boot, rather than under the hood like in most other vehicles. Finding and getting to the battery might be a bit tough at first, but once you find it, connecting it to a battery charger is simple.
Open the boot. Lift up the rear of the floor using your hands to reveal the spare tire and a plastic battery cover located on the far right of the well. Remove the screws holding the plastic battery cover down using the flathead screwdriver. Lift it off of the battery.
Pull the plastic terminal cover off of the positive terminal on the battery. Clamp the alligator clip on the red wire of the battery charger onto the positive terminal on the battery using your hands, then do the same with the negative terminal and the black alligator clip on the charger.
Adjust the settings on the battery charger. If you want a faster charge, set the amperage rating higher. If you want a slower but more thorough charge, set the amperage rating lower. Connect the extension cord to the plug on the charger, then plug the other end of the extension cord into a wall outlet. Allow the battery to charge for at least an hour prior to driving the car, although the longer it charges the better the battery will be.