Transmitter, Integrated Key Fob: Specifications: Specifications: Battery
WARNING: This page is about a different car, the 2012 Jeep Liberty. However, it is still accessible from the selected car via links, so may be relevant.
The transmitter has two 3 volt batteries, which can be removed and replaced without special tools. Insert a dime in the side slot of the transmitter case and twist. The halves should separate and the batteries are stacked on top of each other. The batteries are available at local retail stores. Recommended batteries are Panasonic CR 2016 or equivalent. Typical battery life is about two years.
CAUTION:
Do not handle the batteries any more than necessary. Hands must be clean and dry.