Panasonic Rechargeable FOB Battery Vertical 

 Compatible with BMW / Mini

BMW E39 / E46 keys (Commonly Known as Diamond Keys).
BMW E90/91/92/93 aswell as many BMW key fobs from the early 2000's, Mini

Technical Info:

Voltage: 3V
Capacity: 20mAh
Diameter: 20.0mm  Thicknes: 2.0mm 
Chemical Compound: Vanadium Pentoxide Lithium
PCB Mounting: Vertical

Note:
This is a replacement Genuine Panasonic battery, It may not look like the one originally installed but it will serve the same purpose.