Replacement batteries can be expensive, especially if you need a high-powered battery such as the battery in a BMW 3 Series car. Getting a mechanic to replace the battery just increases the cost, so the best way is to do it yourself. If you have a basic tool kit in your garage and about half an hour to spare, save yourself money and replace your BMW 3 Series battery yourself.
Open your BMW 3 Series boot. Lift the panel on the floor of the boot using your hands and remove it from the boot. There's a plastic strap in the centre of the panel to make it easier to lift. You'll see the battery cover on the right side of the boot, once the floor panel is removed.
Remove the battery cover. It has three screw fasteners holding it in place. Use a suitably sized flat head screwdriver and turn the screws 90 degrees counterclockwise. The screws pop up slightly, once you have turned them. This releases the battery cover, so lift off the cover using your hands and place it to one side. The battery is held in place by a metal plate across the length of the battery, connected to two bolts either end.
Loosen the two bolts holding the metal plate onto the battery using a wrench. Slide the metal plate to one side using your hand. You don't need to completely remove the bolts and the metal plate to remove your BMW 3 Series battery.
Look at the battery and make a note of which way the battery terminals are before removing the battery, so you put in your replacement battery correctly. One terminal is labelled "+" for positive and the other "-" for negative.
Use both hands, one each side of the battery, and lift it straight up until the base of the battery is clear of the battery housing. Carefully remove it from the boot, ensuring you keep the battery upright at all times.
Put your replacement BMW 3 Series battery in the battery housing, ensuring the terminals face the correct way as you noted earlier. Use both hands and gently lower the battery into place.
Reconnect the two battery cables. The clamp on the end of the red cable attaches to the terminal labelled "+" while the clamp on the end of the red cable attaches to the terminal labelled "-." Place the clamps over the terminals and then tighten the nuts to secure them using a wrench.
Slide the metal plate across the battery so it centrally positioned. Tighten the bolts using a wrench.
Replace the battery cover. It can only fit one way so you can't get it wrong. Line up the three screw fasteners on the battery cover to the three holes on the battery housing.
Use a flat head screwdriver and push down on the three screws so they connect to the fasteners then turn the screws 90 degrees clockwise. Replace the floor panel by sliding it into the boot, then lowering it into position using your hands and the plastic handle. Close the boot of your BMW 3 Series.