My nephew in Lower 48 was driving my 2007 Jag XK along without problem with fully charged relatively new battery when it suddenly lost ALL power. After getting it home he found the battery completely discharged. He has scheduled service with my usual local mechanic in about 2-3 weeks (he has lots of business). He noticed no dash warning lights but then he wasn't looking for them either. I've searched previous posts to no avail.
Out of anxiety, until the mechanic gives his diagnosis, I wondering what this could be. I'm thinking (hoping) only maybe a dead alternator, though possible "main" fuse. Nephew says belts are good.
Just glad it didn't happen on our recent 6-700 mile road trip which was completed without problems. Any comments or thoughts. I'm not nearly as knowledgible or experienced as those here.

 

If the battery was dead and the engine was still running, then the alternator would be the entire electrical supply for the car. One hiccup with that supply and blam-o, zero anything. So yeah, a new battery probably and something alternator related.

 

The battery was good when he left the house. Driving down the road and the vehicle just died. Later the battery was tested and out of useable power/amps.

 

How do you know it was a good battery? Was it load tested and at FULL charge and capacity prior to leaving the house? Just because the battery started the car, doesn't mean it was a good battery. You say the battery was tested AFTER it died and determined not to be usable. Batteries don't go from 100% to 0% just like that. Likely it was already done and gave its last gasp.

 

I would have hooked up a small charger asap to maintain the computers. Followed by testing the battery and finding out it was dead. I think they can have internal shorts.

 

Agree with the above and battery is a common issue. You've already determined it's not usable so purchase one for an auto parts store and have them charge it up the night before you come in. Once installed and before you start the car, check the voltage at the battery. Fully charged it'll prolly read about 13 v +-. Then start the car and check the voltage at the battery terminals. You should be seeing 14 volts. A cheap OBD II dongle with Torque Pro is your friend here. You can see the voltage when the car is running and also set alarms on the app. This is your cheapest and easiest way to eliminate the alternator from the battery. You can get a dongle for $20 and you can get a free version of Torque Pro on google play. If you're an apple owner, they have equivalents.
Next morning, before you start the car, check the battery again. Anything less than 12.6 volts and you likely have a quiescent drain. Also, FWIW give your ground points the once over.

 

A battery will keep working as long as the car stays running even if the battery is no longer able to charge, but won't crank back up no more once you turn the car off if it was the battery.Now If the battery is not getting any charge at all and the car just lose power and dies,then it's like everybody said... it's the alternator and it happened to me in my 2007 XKR back in 2016 when i had it.I'm surprised you had your 2007 XK this long without needing an alternator replacement.The base XK must not be as hard on an alternator like the XKR is and that's probably why your alternator survived this long

 

Thanks for that @Barry Leftwich …. I’m still on the original generator.