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Key Fob Battery Life

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Woke up this morning to find that my MX would not respond to either key fob. I had to use the mobile app to unlock the car and start. Called service and was told that batteries for BOTH key fobs had to be replaced? Something about a known problem with the key fobs. Sounded a bit off that both would go kaput at about the same time. Took my MS over to the SC and they replaced both batteries (they actually wanted me to drive the MX over using my mobile app..LOL...they seemed to think it was something with the car). Got home and Voila, the car worked perfectly. My guess is what is happening is that unless the key fob is out of range, it is still in communication with the MX (this was the problem with the driver side door opening and closing, it was in contact with both key fobs even though the second was in another room) causing battery drain. I suggest that when parked, both key fobs be located as far from the MX as possible or placed in a metal box (Faraday cage) to eliminate the RF link.
 
So now we know what the drawback to bluetooth (Model X) vs radio controlled (Model S). I wonder why they switched over? I thought my S key was better in terms of range. I am not real happy with the change but there must be some benefits or they wouldn't have changed the technology...
 
So finally got warning to replace my fob battery after 19 months and 18k miles. I am technical, so how to replace battery on my own? Is 18 months reasonable time for fob life?
18 months sounds very reasonable to me. If you purchase the yearly service plan, Tesla changes the batteries for you every 12 months.

I couldn't find the batteries at most retailers, so I just ordered CR-2354 from Amazon.