This is the procedures to replace the battery for a 2009 E Series 328i convertible.
Expect 20-30 mins for replacement.
I replaced an OEM BMW battery with a Walmart EverStart Maxx-H8 (900 CCA). Cost was $116 USD.
As in other DIY procedures, there is no cross brace on this vehicle over the battery (missing??)
1. Prepare & Remove NEGATIVE battery terminal
1.1. Remove cable management (push tab away from battery and lift. Take note of orientation)
1.2. Loosen nut (10mm socket) enough to pull off terminal connector
1.3. Remove BLUE connector from Red power block (push tab down and pull)
1.4. Tie back cables.
2. Remove RED power block
2.1. Lift Cover & Remove nut (13mm socket)
2.2. Pry loose metal clamps
2.2.1. Closest clamp you can get with a flat head screw driver
2.2.2. Farthest clamp you’ll have to work at to pop off.
2.3. Lift cable out of channel & CAREFULLY push small blade flathead screw driver at tab and separate connector
2.4. Lift Red power block away from battery
2.5. Use a blanket to hold the power block over the trunk space away from battery space.
2.6. Remove bottom half of cable management (take note how it’s oriented and attached)
3. Remove POSITIVE battery terminal & vent tube
3.1. Lift Cover & loosen nut (10mm socket) enough to pull off terminal connector, tie back out of way
3.2. Pull vent tube from battery (not seen in photo)
4. Remove battery hold down bolts with phillips head screw driver.
4.1. (Not seen) hold down clamp can be lifted out.
4.2. ** there are two small washers under hold down clamp. Re-align them over the holes when re-installing
5. Verify you have a clear path to remove battery.
5.1. Battery is difficult to remove
5.2. Lifting “bumper” side of battery out first worked best for me.
6. REVERSE procedures to install new battery.
Good Luck. Pay attention to how things disassemble. Now to the TOUGH part…. re-installing all the molding.
Also, you should register the battery. At least check using OBD software.