The battery is an essential component in any car, providing the initial power the engine needs to start. After the engine is running, the alternator takes over keeping the electrical systems working, and the battery charged. Batteries wear out over time and, if an electrical load is left running with the engine off, they can soon discharge. If you turn the ignition key in your BMW 325i and nothing happens, it's a simple and straightforward task to replace the battery.
Open the boot. Locate a plastic cover at the back of the boot on the passenger side. Remove the two small plastic screws that secure the cover with a slotted screwdriver. Lift the cover off the battery compartment.
Remove the terminal clamp from the negative (black) battery cable with a 10mm open ended wrench. Pull the clamp off the battery terminal.
Remove the positive (red) cable from the battery in the same way.
Attach an extension and a 10mm socket to a socket wrench. Reach behind the battery to its base and remove the retaining bolt and retainer. Hold the battery with both hands, then carefully lift it out of the compartment.
Purchase a fully-charged battery with the same dimensions, amperage rating and cold cranking amp rating as the one you removed. Place the battery into the compartment, then install the retainer and retainer bolt.
Attach the positive (red) cable clamp to the battery terminal and tighten the nut.
Attach the negative (black) cable clamp to the battery terminal and tighten the nut.
Replace the compartment cover and install the plastic screws. Close the boot, then start the engine to test the new battery.
When removing the battery cables, always detach the negative cable first and hook it up last. A battery produces hydrogen gas. Wear protective eyewear while working around the battery and keep naked flames away.