My X5 never had any battery drain issues. But now that it has close to 50k miles, I wonder if I should replace the bettery for peace of mind. How long should X5 battery last? If replacing it (not with warranty cause it's working fine), how much should I expect to pay?

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2011 X3 35i
2008 X5 4.8i
2003 330xi -sold-
2001 X5 4.4i -sold-
2001 M5 -sold-
2001 M Roadster (SCed, Highly modified) -sold-
2000 M Roadster -sold-
1995 325is -sold-