Is the key dead or is the battery in it just run down? It recharges as you drive the car. If you can get the car to start with that key take it for a long drive to charge it. At some point they will not hold a charge and you have to buy a new key. I don't think you can buy just a new battery for it. Good Luck. N4S
As mentioned, the older square keys have user-replaceable batteries. The newer diamond keys are "sealed" and have a rechargeable battery; a loop antenna in the keyhole does the charging while the engine is running. However, all rechargeables eventually die. You can either purchase a new key (only from the $$stealer, eBay generics won't start the engine) or try to replace the battery yourself. See here:
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