My wife's Toyota Prius key fob seems to hold a battery forever but mine was dead after 3 months and I couldn't figure out why. Then, after research I found that these things can be influenced by cell phones and your computer. Mine was right near my cell phone when it was charging and through the wall, 5 feet from my computer and wi-fi router.
So, I bought a Faraday bag from Amazon (not Ebay as stated) for $9 and it has worked great as I store the fob in the bag until I need it and no more battery drain.
How do you know if the bag works, put your cell phone in it and call the phone. No ring if it works. Also put your key fob in the bag and try to open or run your car. Won't work.
This may also keep your car from being stolen.

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