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I suspect that if you register the battery every year in the DME regardless if the battery is replaced, you would get more life out of the battery since it appears that BMW charges a new battery at a lower rate than an old battery. Lower charging rate = longer battery life as long as the battery is maintained in a fully charged state. Higher charging rates = accelerated battery aging. Contrary to what I have read on the forums, older batteries do not need to be charged at a higher rate than new batteries even though it appears that is what BMW does. All of my other non-BMW vehicles I have owned typically get 5 to 7 years or more out of the batteries before needing replacement and they do not alter the battery charge rate regardless of battery type or age. High temperature is the main driver of accelerated battery aging and with BMW placing the battery in the trunk rather than the hot engine bay, 5 to 7 years out of a battery should be no problem. Maybe this is BMW's way of failing the battery after 3 years is up. So, now that I think about it, maybe there is a benefit to registering the battery so the car always thinks the battery is within the 3 year warranty period regardless of the battery age and keeps the charging voltage at the lower end of the range.
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