Originally Posted by Cleveland
You are making the exact same arguments now, that I did in the beginning. I was convinced that I had figured out exactly why the BMW electrical engineers had designed a 800 CA 70Ah battery for my car, and how I could successfully trim that down by a substantial margin with no ill-affects.
I don't think anyone here has ever assumed that reducing the reserve capacity of the battery would have no ill effects. BMW has made a design choice based on assumptions pertaining extreme usage patterns. If you are willing to be on the other end of the usage "distribution", you can reduce the reserve capacity and weight. However, you have to be cognizant of the fact that you have made that choice and indeed stay with the required usage pattern to make things work.
Now, this discussion is really about understanding how far that approach can be pushed. We have the stock battery at 90 amp/hr and we have the Braille battery at 31 amp/hr (I understand you are questioning the methodology for arriving at the latter spec, which is perfectly appropriate). As an outcome of this exploration, we will most likely find out where that line is, and if the Braille battery is pushing things too far or not. A workable solution might very well end up being another battery, say perhaps at 60 amp/hrs with less weight reduction. But, the point stands: one can reduce battery size and weight if one is willing to make certain compromises in usage behavior. By definition, the stock battery cannot be at the very limit of any kind of nominal operational range.
Thanks for providing the information on the issue of some modules not shutting down properly. That information not only informs but complicates this consideration.