Ok, the end of my rope is near...
2005 xk8, 77k miles, Southern California car so corrosion is not an issue.
Battery is 1 year old, resting voltage shows 12.4vdc. Alternator appears to be functioning voltage at battery when running 14.2vdc.

Symptoms: All electrical system shuts off. No lights, to window drop, no trunk release, no start. The vehicle presents as having the battery disconnected.

When: This has happened at various times. Several times this occurred after stopping the vehicle, turning off the engine and by the time I reached for the door latch...nothing. No window drop, no electrical functions when key reinserted in ignition. It has also happened after unlocking the vehicle with the key fob, opening the door with window drop, then nothing. As if the battery was disconnected.

The Fix: remove and reinstall battery ground cable at battery. Then everything works as it should. At least once I did nothing except walk away and then back and everything worked fine. On several occasions my wife called and said car was dead, when I arrived it worked fine. Yeah, that is not the way to marital bliss But to be clear it has done this to me personally too.

What I have done so far: removed battery connections, cleaned and treated with Deoxit. Removed and treated with Deoxit engine compartment relays. Removed and treated ground connections adjacent to battery. Sprayed a little Deoxit into ignition switch. Just a note, none of these electrical connections appeared compromised in any way, shinny metal on all connectors.

I also want it known that I have not uttered any harsh words at or about the car, I can't say the same about my wife who has been stranded several times by this!!

Any ideas would be appreciated.

 

You need to have the battery tested correctly, you cannot do this with a volt meter, most auto parts store have the correct equipment.

The battery being a year old is no guarantee of serviceability.

.......FWIW, I've muttered " I brought you back to life, I treat you well , feed you properly, keep you clean and dry and you, you repay me with this........!?' On more than one occasion

 

Have you checked the ground strap at the right rear of the bell housing to frame?

 

...and the connections to the High Power Protection Module behind the spare wheel, if you haven't already been there. See this post from GGG:
https://www.jaguarforums.com/forum/x...2/#post2251816

12.4V resting is a shade on the low side.

 

The battery cables themselves have been known to fail. Are they warm to the touch after a drive? These cables then go to a power block that can work itself loose over time.

Best of luck, keep us posted.