We use only one key in each BMW vehicle. The other sits in the home safe. I try and remember to take each of them out and use them every 6-9 months or so, for a day or two. Never had a flat battery in a key in seven years. Maybe we are just lucky.

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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