I have a 2018 Hyundai Elantra that I bought new in May of 2018. I have had to replace the spare key fob battery 4 times since then. I have not had any issue with the primary key fob that I use all the time. Both key fobs are kept on the dining room table. The car is either parked in the driveway or on the street when not in use. I mentioned the issue to the Hyundai service manager who told me that he had never heard of such an issue like that and it was likely that the fob was constantly communicating with the car and draining the battery but that doesn’t explain why the other fob is OK. He seemed somewhat dismissive when I asked about a replacement. He told me I would need to bring it in and they would look at it. I’ve done some research online and the two main issues seem to be either the circuit board is faulty or the buttons are making contact even when they are not pressed. I don’t know how they are going to be able to test a key fob circuit board or even the contacts. If this continues to happen after they’ve looked at it what can I do to convince them I need a replacement?
Since you said it’s your “spare” key fob, why not let it sit without a battery, and only put one in when you need to use it?
Bring it in to them and let them look at it under warranty. When you pick it up, insist that they put fresh batteries. If it goes dead again you bring it back to them with their batteries installed. They will then have records on how long the batteries lasted.