Just initialize the key! It has a rechargeable battery inside that charges through induction (electromagnetic radiation conducted wirelessly by a coil inside the ignition to a very small coil inside the key, just like a cordless charging toothbrush). Whenever it is in the ignition, it is charging! Go for a quick drive and your key comes back to life.
Unfortunately, your problem (and mine occasionally) is that once the key gets dead, even after it has been re-charged, and even though the charged key throws the signal correctly once again, THE KEY AND THE CAR DECIDE NOT TO TALK! (If you don't believe me, go to an autoparts store, they have keyfob checkers that light up if it 'hears' the fob throw an IR signal.)
Luckily, we can simply 're-pair' the car to the key, this short process to fix your problem is called 'initialization':
Up to 4 keys can be initialized, but one after the next. 'button 1' in the locking button on the center of the fob. 'button 2' in the unlock button (NOT the trunk button closest to the keyring hole).
1. get in and close all doors and have all keys available (which can just be 1 key)
2.Using first key (or only key), turn ignition to KLR (so dash lights up, but don't start the engine), then switch off within 5 seconds and remove key
3. within 30 seconds of turning ignition off, press and hold button 2
4. all while holding button 2 down, tap button 1 three times within 10 seconds
5. release both buttons
6 if done correctly, all doors auto-lock and un-lock to indicate a successful initialization, good job.
if you have other keys, repeat steps 3-5 withing 30 seconds.
Cheers