Had a chance to pull the Lithium battery from a F82 M4 over the weekend, quite a bit of weight was shaved off the previous AGM version:
Part: 61.21.9.312.759
Price: Not yet listed in system
Specs: 12V 69Ah 770A/860CCA
Lithium
Original
30lbs shed just from the battery alone.
System uses a different IBS connector and vent tube than the E9X M3, doubt a swap would be suitable for previous M3 models. However, the footprint is the same, keeping AGM/Lead Acid batteries an open possibility for budget-conscious as lithium type batteries can be $$.