As I understand it, the key fob battery is required for normal operation. If the battery is exhausted, then alternative methods are used:

To unlock the door: use the mechanical key stored within the fob.

To start the car: take the fob out of your pocket and hold the fob up to the start button so the car can read the key (using technology similar to passive dog ID chip).

So, without the battery, the fob is much less convenient, but still usable.

 

It's interesting that I got the warning only once, about a week ago, but haven't seen it since.
Does the power level of the battery in the fob recharge, just like the car? (that was a joke)

 

At just over 2 years, the distance from which I could lock/unlock had dropped noticeably, so I replaced the battery. I never did see any "low battery" warning messages, but the new battery works much better.... And get the new battery from your local drugstore as the dealer may charge "dealer prices" for them!

 

What's the distance the fob has to be from the vehicle for the fob to go to sleep?

 

At a little over 2 years and 40,000 miles, I started getting the "low key battery" warning from my FOB, but not my wife's. I drive the car almost exclusively, and she is a passenger about 50% of the time. She keeps her FOB in her purse, I keep mine in my pocket. At night, her purse is just inside the house from the garage where the car is parked, I would say about 10 feet away from the car. At night my FOB is about 30-40 feet away from the car. I keep my cell phone in a different pocket from the FOB, she keeps both FOB and her cell phone in her purse. Her cell phone charger is kept right next to her purse (and FOB) at night, my FOB is no where near the charger at any time.

At first, the message was intermittent, sometimes I would go for days without getting a message. Then as time went on, it got more frequent, somedays I could not open the door without using the mechanical key, but the car would start. I purposely wanted to see it get to the point where it wouldn't allow the door to be opened, or the car to start, to test the emergency procedures. The little mechanical key worked, but it is hard to use as it is so small, so daily, it would be a PITA. I would imagine in a cold climate it would be very difficult to use, especially with gloves. Without her FOB anywhere near, holding my FOB up to the Start button did exactly what the manual says, and the car starts just fine. I bought 2 batteries from FRY'S Electronics for somewhere about $1 each. WAY less $ than the dealer, or any other source I found localy (Walmart, drug store, Home Depot, Lowe's, etc.)

added: BTW, I almost NEVER use the buttons on the FOB to unlock or lock the doors, so I can't comment on the activation distances getting reduced. About the only time the FOB comes out of my pocket, is when my wife leaves her purse in the car, and I have to use my FOB to lock and then unlock the car. I ALWAYS forget, and get that annoying beeeeeeep when I press the little black button on the door handle, and then when I return, of course, I forgot that I used the FOB to lock the car, so it won't unlock by grabbing the door handle...That's when I get a sideways look from my wife....sigh... I guess husbands are not perfect! I ALWAYS remember her birthday and our anniversary!

 

Never thought about it, but I keep my key near the door which is near the garage, so maybe it stays up all night!