Originally Posted by Efthreeoh
I'm not sure the lineage of your battery, but the OEM battery in the 330i was a white regular, flooded-cell, lead-acid battery, not an AGM. Whoever installed the replacement battery after the original one that came with the car from the factory, seems to not know what they were doing. I'd suspect the IBS is damaged (it is easily damaged) and that is what is causing your battery problems.
Attached is the TIS battery replacement procedure.
Hope all this helps. You can't order from real OEM.com but you can find all the part numbers and then order the parts on line. I use getBMWparts.com because they are a large stocking dealer and usually have most everything in stock or two-days shipment from BMW USA. Just get all the parts from them, or at least use all BMW OEM parts, the prices or not unreasonable.
Wow thanks for that info. I didn't realize the OEM battery isn't AGM. I fixed the IBS by connecting the CAN bus connector that wasn't connected. I now suspect previous owner changed battery at some indy shop that didn't know what they were doing judging by your info regarding the flooded cell battery. He probably also installed a battery with wrong specs because I purchased a replacement with those specs and it's not what the OEM specs are. My car apparently only needs a minimum of 650 CCA to crank but even with this 700 CCA one, crank is rough and hard. However, if I connect my car to a friend's car with jumper cables, crank is always smooth, probably either a voltage drop or a lack of amps issue. Either way, I'll go and put back in a factory 850 CCA battery in once I get the chance. Higher amps should reduce the voltage drop.