I don't believe it is just about the spec. Many of the systems on modern BMWs have a learning mode whereby the computers learn the behaviour of a component or subsystem. That needs to be cleared when the component is replaced. I have read that a risk with not coding the new battery is that the charging system has adapted to the old battery, and can over charge the new battery to the point of failure. We are past the old days of simple voltage regulators.

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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