The 2015 Mercedes Benz B250e has a warning “replace battery” on the key fob. I took out the battery and sure enough, it’s measured 2.99V instead of 3.3V as a new one. Yes, with 2.99V, the car still drives. The battery size is CR2025. I got 10 LICB batteries from Amazon for $6. The battery measured 3.35V which is higher than the Maxell.
I didn’t need the screw driver or anything to remove the battery cover, just remove the key on the bottom of the key fob, use the blade to press inside the case at the correct position and the cover pops open.