So - I've had my 2000 XK8 for 3 years, worked all the kinks out, upgraded this and that, and love it for better or worse.... Except - the cars electronic " self protection " I've been round and round about this one... Got a brand new Sears Diehard AGM battery, been tested - its AOK. Problem.... if I run the battery down even a little while the car is off, it won't even " try " to start. Voltage reader reads 12.7 - 12.8 lights are bright, and horn loud, except maybe I left the interior on for 10 mins or so...whatever - if the battery is anything less than 100 % the cars electronics " protect " it from even trying to start... other cars would " wah wah " nope, not the Jag. 2ndary problem then is, if you try to use a jump pack, either the full size battery kind, or the mini hand held kind....with other cars - when hooked up, and the car " tries to start " these type units, then trow in a " boost " to start the car....Not with the Jag...Electronics of car, see a " imperfect " battery, and won't send a " try " charge to the starter to turn over, thus - jump packs don't see a " try " to then add a boost, so do - nothing...Even when AAA sends out a guy with his " Pro " jump pack - it doesn't see the " try " by the starter, so isn't triggered to jump in and kick the battery.... I try to explain he needs to do a " hard wire " " manual " start to his engine....." this thing'll start a tractor trailer at ten below " he says....yea - well it won't start a Jag... Isn't there ANYWAY to over ride, or bypass this " protection "..... I even bought a 2nd battery, hooked the two together to form a double " parralel " unit.... still - if it gets run down even the slightest, the car refuses to even " give it a try " It's embarasing, people are always ribbing me, " whats wrong with your car ? "...." maybe you should get a new battery " - I got two I show them... Let me try my mini-jump pack - it'll start anything....maybe - but not a Jag..... There's gotta be a way to over ride this.... Help !!!

 

Maybe not battery so much as connection of battery. Have you checked all the associated positive and ground connections for corrosion and tightness etc.

 

All connections are SUPER tight, new nuts and bolts all around, all cranked down hard...Thanks tho

 

Definitely not normal.

Re-tighten the connections at the mega-fuses at the rear of the trunk and the heavy cable connecting each fusebox in the vehicle.

Also check if the crimps at the battery cable are heating up when you have the car running...they have been known to go bad.

There might also be a problem with the key transponder if the above does not resolve it.

Let common, but there could also be a bad relay in the starting system.

 

When you park the car do you engage the security system? I was led to believe that this keeps battery drain to a minimal and worked for me just saying

 

Jags do not have the self-protection circuitry or devices you describe. Don't know where you heard otherwise. They will crank the same s any other car when low.

Voltage alone is not a reliable indicator of energy available. Could be your battery is simply toast.

 

I had the same problem several years ago on a VW Karmon Ghia. I know, a VW with the simplest electronic system on the planet. Bought a new battery, and unless I kept it on the charger overnight it wouldn't turn over. It turned out to be the battery cable. The cable had become corroded on the inside of the insulation. The insulation looked nice and shiny, but the wire inside was corroded and brittle. Replaced the cable and no problems.