Cleveland: Useful and valuable anecdotes. You may be right that those of us trying to get the Braille battery to work may never get there. However, this may not be entirely apples to apples with your experience and efforts. It sounds like you were trying some really small batteries (motorcycle batteries).
Can you confirm:
1. Did you ever try this particular B3121 battery?
2. Ah rating of the OEM E36/E46 batteries?
3. Ah rating of all the batteries you tried?
4. Also do note: I do not have an alarm, neither does lucid and I don't have satellite radio.
No one here (well at least lucid and myself) believes they can outsmart BMW/BMW engineers. Well you clearly can with regards to performance... but that is another topic. What part of the design for worst case scenario principal don't you understand? Let me repeat for the 3rd time: The OEM battery needs to handle various combinations of very cold weather, lengthy use of accessories with the engine not running and weeks of idle time. Cold weather in particular is a huge tax on a battery both for starting (CCA) and reserve. It sounds as if you do not believe the OEM system has a tremendous amount of extra capacity for warm weather, no extended use of accessories with engine off, and a mere couple of days between uses. Can you tell us your thoughts on this particular point? You just can not argue that the OEM battery both has no significant extra reserve but at the same time it is the smallest that will work reliably when not heavily taxed.
Did you miss that member ButterzM has had this battery for 3 weeks with no issues? If it can work for 3 weeks it can work for years as I see it.
My problem in particular has nothing to do with being able to start the car. It is not and has never died completely, it is just chronically a few tenths of a volt low. Again, the voltage does not fluctuate much, after a short drive, after a long one, after one night or after several. The voltage is just about the same - and is just a hair too low. All of this tells me there is a problem not with the battery's capacity but with its ouput.
Can you also expand on this point, "Modern alternators are not designed to deal with charging AGM batteries."? Do you mean the voltage regulator or alternator? The charging voltage on the battery is from the voltage regulator and it maintains a fairly constant voltage well above the zero load battery voltage, hence charging it. Alternators today do what they have done for decades.
Do you have any idea why for you a 33 lb battery seemed to magically work when a 20 lb one wouldn't? 33 lb was surely significantly lighter than the OEM battery. Where do you draw the line?
808MGuy: The voltage you measure at the battery terminals when the car is running is the alternator voltage, not the battery voltage, and as I mentioned at idle this is something like 14.5-14.8.