I purchased a 2007 XKR with only 19k miles. I knew it needed a new battery. I installed a high quality, fresh battery on Tuesday. I went to start it this Sunday completely dead, alarm was off, could not open the doors or trunk with the clicker had to use the key. Any thoughts ?
Thank you for your time...

 

Although you bought a new battery and installed it on Tuesday, it probably wasn't fully charged. You don't know how long it was sitting on the retailer's shelf discharging before you bought it. It's also possible that it's defective. It's also possible that the battery cables need to be tightened or that you don't have a good ground.

is your XKR garaged? Do you use a battery maintainer, like a CTEK? Do you always lock the doors? Did you leave a door open with the engine not running? If you don't lock the car, convenience mode stays active and doesn't shut itself off for around 15-20 minutes (I'm not sure of the exact time) and that will deplete the battery.

Fully recharge the battery using a conventional battery charger (not a maintainer). If it doesn't take a full charge, it's defective and you should return it. Otherwise, buy a CTEK MUS4.3 battery maintainer from Amazon and use it every day.

Search this X150 Forum for "dead battery". There are more threads than dogs have fleas.

 

Stuart hit the nail (battery) square on the head. Follow his advice.