I just replaced the battery in my Honda fob, and found a very helpful video on line that was made by some Honda dealer in the mid-west. Worth googling to see what you can locate for guidance if you want to do it yourself.
Check your user's manual for instructions on how to replace the battery, including which battery you need. Google the battery number with your zip code. Lots of local sources should come up.
Yeah, I got a 2 pack of 3v lithium Duracells from Amazon for less than $3.50.
I took mine to the car dealer. Nissan dealer charged $3.50 to change them. For the BMW they did for no charge.
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I used to purchase this at Radio Shack but they are no more. They would replace it for me which was helpful.I think it's a coin cell lithium battery.