As with any other automotive part, vehicle batteries periodically need replacement. A drained battery is unreliable and can leave you stranded. You can replace the battery on your BMW 3 Series yourself instead of taking it to a mechanic, saving time and money. Battery replacement is a relatively generic procedure with all vehicles. However, the 3 Series battery is in the boot instead of the engine compartment. Replacing the battery in your BMW 3 Series requires a few basic tools and should take you about half an hour.
Open the boot of your BMW 3 Series. Wear safety glasses for protection.
Lift the floor panel up and out of the boot. The battery is located on the right side of the boot under a plastic battery cover with three fasteners.
Loosen the two fasteners on the top of the battery cover by turning them counterclockwise using a flathead screwdriver. Loosen the other fastener along the right side of the battery cover using a flathead screwdriver.
Lift the battery cover off and remove it. You may need to pull the side trim panel in the boot up to extract the battery cover.
Remove the two bolts securing the protective bar that covers the battery using a socket wrench. Remove the protective bar.
Loosen the nut on the negative battery cable. Remove the cable from the battery terminal and push it out of the way so that there is no chance of it contacting the terminal. The negative cable is usually black and has "Neg" or "-" on it.
Loosen the nut on the positive battery cable. Remove the cable from the battery terminal and push it to the side. The positive cable is usually red and has "Pos" or "+" on it.
Lift the battery straight out of the boot. Be careful, as the battery is relatively heavy.
Place the new battery in the battery compartment. Connect the positive cable to the positive battery terminal first. Tighten the nut on the battery cable.
Connect the negative cable to the negative battery terminal. Tighten the nut on the battery cable.
Reinstall the protective bar on the battery and replace the battery cover.
Dispose of the old battery properly by taking it to a battery recycling centre.
Clean the battery cable terminals using a wire brush and battery cleaner if there is any corrosion.
Be careful when handling a battery. Don't shake it around, turn it upside down or drop it.