I've had my car for 5 months now and use the fob only. I still have not replaced the battery and the car still responds to it. I probably should change the battery just to be sure that it will continue to work though. I have not had to do the fob as a "valet card" yet.I use that for for back up leaving my phone in car without thinking is easier than accidentally leaving fob clipped to my belt loop. Now if my Samsung g3 watch would open the door that might be a great solution . Thanks for the suggestion but fob batteries should last longer than a month. And lithium cells they go dead instantly... Very little drain down time hence little warning. A tech guy told me you can open car with dead key by placing it under the pillar camera and then pressing the rear door handle several times. Keep a spare battery in cari guess. Haven't tested that yet but for s150k car this is not classy