How can I start my BMW Motorrad if the radio-operated key battery is empty or I have lost my radio-operated key?

If the battery of the radio-operated key is empty, you can start your BMW Motorrad by placing the spare key in the ring aerial under the seat. 

Proceed as follows to start the motorbike:

1. Insert the vehicle key into the keyhole on the rear left-hand side of the vehicle and turn the key clockwise
2. Raise the rear passenger seat and release the vehicle key. If you have seat heating installed, disconnect the plug connection
3. Then lift the rider's seat at the rear and disconnect the seat heating plug connection if necessary
4. Lay the rider's seat with the cover side down on a clean and dry surface
5. Place the spare key (2) in the ring aerial (1) – as you can see in the figure. The Pre-Ride-Check is then carried out
   
   Note: The position of the ring antenna may vary from motorbike to motorbike. The position of the ring antenna can be found in the manual for your motorbike. 
6. Now reconnect the plug connection of the seat heating to the rider's seat
7. Insert the rider's seat into the bearing block on the left and right and place it loosely on the motorbike
8. Push the rider's seat slightly forwards at the rear and then firmly downwards
9. Start the engine within 30 seconds, otherwise it must be unlocked again
10. Only if seat heating is fitted: Connect the plug connection to the passenger seat
11. Place the passenger seat in the front of the rear frame and press it down at the rear until the seat audibly clicks into place

Here you can see how you can check the battery charge of the radio-operated key.