I have comfort access and I was told by my sales rep that the key does charge when pluged in the car but only need to be done once in a blue moon
I actually once got the same light come on my car, but never came back...

 

Yup I confirmed this today when I was at the dealer and did charge my key and it got rid of the message. CA keys are also rechargable. Atleast in 07

 

I have CA and got the low battery warning. My SA told me that the battery could be re-charged by putting it in the ignition every now and then. I tried but it didn't help. He wound up replacing the key under warranty. If the battery is replaceable and not rechargeable it would appear that all I needed was a new battery. He obviously thought it was rechargeable. Oh well - it didn't cost me anything.

 

CA batteries are not rechargeable (and won't recharge if placed in the ignition) regardless of what the salesman says...

Salesmen don't RTFM, they just sell cars.