For the regular key fob the owners manual says to use the key occasionally to keep the battery charged. Did not say what happens if the battery dies but sounds like if it does you are off to the dealer

 

If the battery dies, you simply cannot use any of the store or transmission functions. The car will simply revert to defaults if it does not get the programming options from the FOB.

The car supplies the power when the FOB is plugged into the docking port.

For comfort access, the the car will remind you when the battery gets low. The battery in non-comfort access keys is a litium ion and gets charged up each time you dock it. Not sure how you can replace the battery in the non-comfort access FOB. The ones I have seen appear to be sealed from the factory. Only the comfort access FOB hsa a removeable door to replace the battery.

As far as replacement costs go, these are cheap compared to some of the other car manufacturers. Lexus wants $750 to replace my key for my IS if I lose it. The reason being is they actually replace the computer as the computer and key are matched and cannot be re-programmed once set. Supposed to help prevent theft.

 

You can hide a key with comfort access in the car. You just have to remove the battery.

If you leave a comfort access key in the car with battery installed, you cannot lock the car.

I took the battery out, stored the fob and battery in a small zip lock bag, and removed the key. Ground down some of the plastic so it fits on my keyring, and hid the fob and battery in a hard to find location with a little tail made from duct tape so I don't have to remove the panel I hid it behind to get to it.

Since the battery is lithium, it should be good for several years before it needs replacing. Now my only problem is not misplacing the one complete fob, because I can't just grab the second one and go.

I shudder to think of the replacement cost of the CA fob if the standard fob is $350.

I'm thinking of getting a copy of the key part (which shouldn't be too expensive since it has no circuitry), and hide it somewhere on the outside of the car. Since you can't start the car with just the key there's not too much danger of theft (except maybe the contents of the car, but I should be able to find a well disguised location).