Easy way to keep second key charged is to place it on one of those rechargable toothbrush stands. Remove the toothbrush obviously, and then rest the key where the brush would sit. Its an induction loop i believe and should charge the key easily!
My spare key went flat over the weekend. Just had to manually open the door and start the engine while the alarm was doing the warning beeps and it cancelled the alarm. Then it charged up on the way home.
