2002 XJ8 Sometimes I can't start because the battery is down. The idiot light doesn't come on to show that the alternator is not charging, but when I disconnect the battery the car dies. Is there any other reason than the alternator is no good that would happen. Is there a fuse or fuselink to the alternator.

 

First have the battery load tested. Most auto parts stores can do that. Second if the battery test good make sure to clean the posts and terminals bright and shiny.
If the battery is good then most likely you have a parisitic drain from something not shutting down and crating a drain on the battery. Since you are not getting an indicator that you have lost alternator output.
Do you always lock the vehicle so that the electronics will power down?
One last quick check, when you remove the key does the ignition look like the little door has closed over it. The little door is the signal for the electronics to power off. If it doesn't look like it is closed then give a shot or two of electronic cleaner and insert and remove the key a few times to free up the door.

 

With a correctly charging alternator you should read between 13.5 and 14.5 volts on the large B + terminal post while spinning . This is a bit above a fully charged battery hence a current flow .

Any auto parts store will test your alternator on a spin up machine . Or if you can drive it there , while on vehicle .

There is a fuse that powers the regulator before the alternator as a whole can produce electricity . Must have electricity to make electricity in this example .

Fuse # 2 / 5 amp Engine Compartment fuse box

See page 51 :

http://www.jagrepair.com/images/Auto.../jagxj2002.pdf

On the removable ignition switch attached on the key barrel you should see a change between pin 4 to pin 5 as you pull the key in / key out . This starts the security system to start the shutdown sequence before going to sleep , if not the car stays alive if the 4 to 5 doesn't change . This is one of the things Avern1 referred to as a parasitic drain .

 

never disconnect battery with any car running as a general rule,it will burn up the alternator

 

And even much worse could happen - the voltage spikes from the alternator, in the absence of the voltage smoothening out effect provided by the battery, could fry all your electronics (ECU, modules etc.).