I know the remote is charged through the ignition but does anyone know if the key is charged with the car off?

The reason I ask is that my spare key is dead and was wondering if I could just leave it in the ignition overnight or if the car needs to be running.

__________________
2008 BMW X5 4.8i (E70)
Alpine White, Burl Walnut, Nevada Black, Sport Package, Cold Weather Package,
Premium Package, Technology Package,
Rear Climate Package, iPod adapter