The last time I took my Z3 to A BMW dealership, for a battery that failed, they refused to replace it under warranty .
Their reason, I didn't drive the vehicle enough .
They would sell me a battery, for $195, plus tax and labor to install it, total price came to around $300 to replace the battery .
I went to NAPA and got one of their house brand AGM type batteries for $95, that was 7 years ago, hasn't quit yet .