I was in the opposite situation. The AGM H8 battery I found was listed at C/20 95Ah. The AGM battery options cover 90 and 105Ah. I consulted with the battery manufacturer and they recommended I choose the 105Ah option, assuming of course that the BMW capacities are indeed C/20 capacities. Given the options and the relative standard use of such capacities throughout the industry, this seems very likely.

Not knowing exactly how the BMW IBS works and the exact strategies used, mathematically, at worst, using a battery with a higher capacity than the system expects means your battery will be chronically undercharged and that would certainly lead to a shorter life span. Likewise, a battery that is of a lower capacity than the system is set up for - as in my case - may experience a shorter lifespan but for the opposite reason. Ultimately, the actual effects will depend on the battery design and construction.

ETA: I find it interesting that the OP replaced his battery with a Interstate MTP-49/H8 100Ah battery and stated that it was the same type that the dealer had installed in the past. 100Ah is not an option so I wonder what the dealer did? Probably nothing special or extra and just picked the closest value.

ETA2: I looked in Carly and the battery capacity options are the same as those in the first couple of posts of the thread. Another interesting facet is a note in the battery registration window in Carly that says to choose the next lowest capacity if your exact battery capacity is not shown. I'm not sure of the source for that guidance. The Ah value is probably used to help determine state of charge (SOC) more than define precise charging profiles. I'd hope that an INTELLIGENT Battery Sensor would use voltage, temperature and current indications to determine when the battery needed charging and when to terminate that charge. For that reason, I'm probably going to choose the 90Ah option for use with my 95Ah rated battery.

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8/2011 X5 xDrive35d Sport, Black Sapphire Metallic, Black Nevada Leather, Fine Burr Walnut Trim
2/2001 M5, Jet Black, Exclusive Complete Black Walk Nappa Point Heritage Leather, Black Cubic Trim