The smart card uses an uncommon size battery CR2412 . The first card I received had a dead battery and I tried all the usual sources... HEB, Walgreens, CVS, Frys. No luck. Ended up at the (penske owned) Lexus dealer and it cost $10. Bought some on Amazon to have on hand. Thin enuf to put in my wallet without adding bulk.
It would really suck to have jumper cables and battery pack in the truck and not be able to get to them. However, as far as I can remember, I've always been able to get in my cars via fob even when the car battery was too weak to start the motor.
I've been reading that you can start the truck with a dead fob battery. Decided to try it with the smart card. It worked with absolutely no hesitation. Impressive! Unfortunately, it won't open the door.