If it's original then it uses a rechargeable battery that is soldered onto the board. There are replacement units that can be soldered onto the board or you could purchase an aftermarket key that uses a replaceable battery.
i replaced mine the other day , took about 30 minutes of messin around to separate the fob, replaced the battery and done works like a charm (i replaced my battery with a CR2016)
i replaced mine the other day , took about 30 minutes of messin around to separate the fob, replaced the battery and done works like a charm (i replaced my battery with a CR2016)