I just replaced an alternator and a battery on my 1998 XJR, I drove the car around the block and parked it, but the next morning, to my extreme surprise, my battery was dead. guys please I really need help with this car.

 

This is just a shot in the dark. Are your trunk lights staying on? I've noticed they do not affect the rest of the interior lights and can remain on without your knowledge.

 

The first thing I would do is test the parts your replaced. Its very common for new alternators and batteries to be defective.

 

the new parts were tested and proven to be functioning great and i forgot to mention this's my 3rd battery on this car. out of desperation I put a video camera in the trunk to record whether the trunk lights stay on, but the video showed they remained off for a couple of hours.

 

Well... There is obviously a draw somewhere. This would be tedious but will narrow the problem down. You could put a voltmeter on the battery and start pulling fuses on different systems until the draw goes away.

 

Only, I would use an ammeter, either clamp on or in-circuit. You should have 45 mA or less after shutdown. You have a draw greater than 1 amp I would GUESS.

 

Have you checked the actual battery.

I only ask because I once had a defective main power relay -- battery was fine but no power.

 

battery is good.

 

aftermarket radio?

 

well it turned out that there was a loose connection in the engine bay. the plastic around the contact point begun melting, I replaced the bolt and made sure that it was a tight connection. The generator output measured at the battery terminal seems right where it should be and even the starter cranks with vigor!!. so far so good......thanks for your support guys !!!

 

Awesome. I know you are breathing better now lol. I hate electrical issues. For some, they are easy, but I am not for sure.