We were on vacation last week. We left one of the Model X keys in the trunk on the driver side as that's a "Safe" area. They key ended up dying anyway (but the car was locked the entire time). This was on a Wednesday. On Sunday we finally got around to getting the odd spec'd battery. We put it in...nothing. Tried another battery. Nothing. Tried a new battery in the other fob...it worked. So we knew it wasn't a battery issue. We called the SC and they said bring it in. Turns out if the key fob is dead too long...it can't be saved. They had to replace it. Odd. We will be leaving spare batteries in the glove box going forward.