After 2 years of using the same key fob I started seeing the message “Vehicle key Battery Empty” upon startup of the car. Around the same time I noticed my fob would occasionally not open the trunk or unlock doors etc. when the buttons were pressed on remote. I swapped the battery from my other fob and got the same error message. I then purchased a brand new Energizer battery and to my surprise the error message was still popping up. I searched around the bmw forums and found a few members saying the same issue happened to them and they discovered Panasonic batteries solved the issue. So I went ahead and followed their advice and got Panasonic batteries. Voila! my fob was no longer causing the error message and my car was happy again. Seems like some battery voltages may be slightly under the threshold for the sensors in the car.