something is robbing my XK8 of battery power as soon as I shut the car off. Overnight dead battery after a full charge is common. Brand new Bosch OEM Blue Top battery installed just a month ago.

 

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First, even new batteries can fail. Check the voltage with the engine off and the lights on. It shouldn't be below 12.4v and shouldn't show any signs of dropping over several minutes. A quick test by an auto parts store will confirm that you have a good battery.

Leaving the key in the ignition switch keeps the security system 'awake' and provides a higher level of battery drain. Similarly, the drain continues if the spring loaded key slot cover does not return to a closed position with the key removal. This should not affect a good battery overnight but will over a few days.

Beyond that you will need to identify which of a large number of circuits continues to drain current with the engine off. A way to do that is to remove the negative battery cable from the battery and measure the current flow between the battery post and the cable. Something like 30 ma is normal after a couple minutes. Then you have to start removing fuses to find the offending system.

Let us know what you find.

 

Thanks, will try it today