2000s Jaguar XJ8 Problem Resolution: NO START, WON'T CRANK

It's winter, and my 2003 Jaguar XJ8 (99k miles) has been reacting badly to the cold. First it did not properly auto unlock the other day at the gas station, locking me out. And then I came out the other morning to start it up and it was dead - no crank, no start. After spending some time charging the battery, thinking that either it was failing in the cold weather or there was some other electrical issue, I discovered the key I had been using - my spare key - had mysteriously stopped working. This is the first time I've ever had a failed security KEY be the reason a vehicle would not crank. The keys have embedded security features, but apparently this one has gone bad. Maybe other Jaguar owners will be helped seeing this. Luckily, I had two keys, so I can still drive it, but no fun knowing I'm down to just one key.
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