Battery replacement is simple. When I first got my 300h I started keeping the key in a Faraday pouch because of concern about relay theft. I then discovered that If the lock button is held down and the unlock button is pressed twice quickly the led indicator would blink and deactivate the key eliminating the need for the pouch. Pressing any key reactivates the fob. As well as enhancing security this measure may prolong the life of the fob battery?