When mine was starting to die I got the 3 pack from radio shack and put the 2 extra in the center consol for later use.

 

Yeah all you need is to change the batteries on these suckers and your good to go.

When my key fob was dying my initial reaction was to get pissed that I had to go buy a new key from the dealer ( I came from a 2003 BMW 330i and the only option you have when your batteries die is to Replace the whole key!!!) I Then realized that I had a Lexus now and that I had a NORMAL key fob with removable batteries so I just went down to the local auto zone and problem solved lol

 

If the battery has any juice in it, holding the key fob directly over the start button and doing the normal startup procedure should work. Or if you like wheezing every last bit of juice out of it, leave a spare in the armrest.