As threatened/promised above, here is WRITTEN Step-by-Step Procedure for Battery Replacement, along with other things to check while battery is out, & Diagnostics using INPA.
Suggest that you NOT ASSume that proper Type/Capacity setting has been selected for the battery that is currently installed. Prior owner or his Mechanic may have changed Type and/or Capacity of the battery previously installed, and NOT reset Code or Setting. INPA can quickly tell you the current setting. Use NCS (ALSO in BMW Standard Tools with INPA) to select DIFFERENT Type/Capacity if necessary.
The following is the procedure I used for my 3/2007 build E91 328xi, with IBS (Intelligent Battery Sensor), and Old-style BSD Connector, WITHOUT connector on TOP of Rear Power Distribution Panel (RPDP). Your trim panels and battery cover may differ. An H8 is the largest 12V battery commonly available (AFAIK), and fits in the battery well, using rearmost battery hold-down clamp threaded hole in floor.
REMOVAL of OLD BATTERY:
Remove Mat (covering carpet) & Bin Cover
Remove Trim Panel: 2 plastic screws 90 degrees rotation; lift bottom edge, slide inboard & disengage top tabs
Remove Plastic Cover: Rotate 2 Top Fasteners 90 deg; slide tabs outboard to disengage
IDENTIFY BST @ (+) TERMINAL & IBS @ (-) TERMINAL; HANDLE ALL ATTACHED WIRES WITH CARE!
Negative Terminal & IBS: Loosen T-bolt 10mm Nut 1 turn, wiggle gently & remove terminal from post
Remove Large Blue Connector to RPDP, from IBS, 2 sockets outboard of Small Red DME Cable
Remove S-Brace Over RPDP/Battery: 2 10mm bolts
Blue IBS Connector over Battery (-) Post: Disconnect from chassis harness
DME Cable: Small Red cable, unsnap Connector from RPDP; 1st socket outboard of Large Red JB Cable
Lift/Rotate Inboard edges of two Nut Covers, Red plastic RPDP; remove two 13mm nuts & RPDP
RPDP = Rear Power Distribution Panel; press down metal band & pry lower edges, side of battery
RPDP Lift UP one side at time while prying metal band catches away from battery; Remove upward
Positive/BST Terminal Removed: 10mm T-bolt Nut; loosen one turn wiggle & lift gently
IF you have a BST Trigger Connector plugged into the RPDP, disconnect that Connector
Rear Battery Hold-down Clamp: 10mm bolt, remove; careful of BST trigger wiring & harness clip
Gray Vent tube to Elbow/grommet by Transfer Points Removed from front of Battery; plug in Rear Vent Hole also removed from old battery for use in New
Lift out battery using handles on battery
WHILE BATTERY OUT:
Note the two "Transfer Points with round black plastic covers just above floor of Battery Well
Snap open the round faces of the covers and inspect the Transfer Points for rust. Oil Lightly or apply a thin coat of dielectric grease to slow rust/corrosion.
Suggest taking photos of ALL components mentioned, including the three B+ supply cables, so you have record of HOW components are located and their function. LARGE Red B+ cable connected to Outboard Transfer Point is B+ Cable for Starter & Alternator, and is connected to (+) Battery Terminal via the BST (Battery Safety Terminal) which disconnects from battery in frontal collision. Smaller Red Cable at Inboard Transfer Point is B+ supply to VVT Relay in E-box under hood.
B+ Supply to the JB (Junction Box) Fuse Panel is the Medium-sized Red Cable that runs INSIDE the car and does NOT pass through a Transfer Point. If you EVER have Power Supply issues, you need to understand what EACH of those 3 B+ Cables serves.
INSTALL NEW BATTERY:
Install Vent Tube Elbow on Front Vent hole of New Battery, by (+) Post; Install Plug on Rear Vent Hole by (-) Post
With (+) Battery Post forward, Lower battery into compartment & ensure FRONT lower edge is under Bracket to hold-down front
Install Battery Hold Down @ Rear of battery, 10mm 1/4" Drive Socket
Positive/BST Terminal Installed, 10mm Socket to tighten T-bolt Nut
Rear Power Distribution Panel Installed: Two (2) 13mm Nuts; Large Red JB Cable under Inboard Nut
Small Red Cable for DME snapped into RPDP in 1st socket outboard of Large JB Cable
Negative Terminal w/IBS Installed: 10mm socket to tighten T-bolt Nut
Blue IBS Connector attached to Chassis Harness over (-) Battery Post
Black IBS Wire Sheath w/large Blue Connector to RPDP, 2 sockets outboard of Small Red DME Cable
Reinstall S-Brace (2 10mm Bolts)
Reinstall Plastic cover with 90-degree twist fasteners
Reinstall Trim Panel with twist fasteners
Reinstall Bin Cover & Mat
INPA to Check current Battery Type & Capacity Setting:
INPA > DME > F5 Status > F5 Power Management Menu > F3 PM InfoField 2;
NOTE the "Battery Capacity in Ah (Amp-hours)" = Batterie Kapazitat in Ah value appears in the 7th row, Left Column of that screen. THAT SETTING is what affects HOW your battery is charged, and NOT either the "Registration" or "Histogramm" Reset discussed below. On my E91, that value was 90 Ah at the factory and is still set to that with the Walmart H8 installed. That H8 is NOT AGM, but simply conventional FLA (Flooded Lead Acid).
You can use INPA to "Register" the Replacement Battery (DME > F5 > F5 > F5 Batterietausch Registrieren = Register Replacement Battery. ALL that does is enter the current Odometer reading (in km) in a text field in another screen when you press the last F5 function key. It is an INFORMATIONAL Field ONLY, and has NO effect on how the battery is charged. THIS entry is NOT important. If you keep your Battery purchase receipt for warranty purposes, & record the Date of Installation and/or have a date sticker on your battery, this step is redundant.
If you have INPA and you are ANAL(ytical ;-), you can Reset the Histogramm which is simply the last 5 days of SoC (State of Charge). At the DME > F5 > F5 Menu, simply press "F6 Histogramm Reset". If you don't have INPA to do that, NO BIG DEAL, as in 5 days that Histogram will be overwritten, and you and your battery will be same place as if you HAD pressed F6 Reset.
So WHAT is IMPORTANT is knowing what your Battery Capacity Setting is, and if NOT correct for the battery installed, use NCS to correct that setting/code.
I have NOT used NCS, so will leave that to someone who has. In three+ years on the Forums, I've NEVER seen anybody give a simple Step-by-Step explanation of HOW to change that setting. Anyone who has used NCS Dummy, please provide.
George