Any battery that has a pro-rated warranty is designed to fail before that warranty is up. It brings the customer back in for what amounts to a minor discount on the price of a new battery.

It is relatively easy for battery manufacturers to design to a specific life. To me, anything after five years is borrowed time, and I would replace it, due to the inconvenience of a failure when you least need it.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue