Jag xj8 3.2 v8 charge light staying on

The light means the alternator is not charging the battery. Once the charging circuit is completed, the warning light looses its ground connection.
You may need to double check the circuit going from the alternator to the battery, or the warning light circuit going to the alternator as it could be shorted to ground.

The alternator is probably junk, it happens; should charge 13.5 to 15.5 volts

Not good to pull cable off batteries to test alternators an yes could be alternator or u may not have power to the alt.meaning the wire on top of alt or may havy bad plug wire boyh easy to check

The alt should charge at around fourteen volls , any ways, yogi need to put another battery. A weak alt will kill a good bat.and vise versa

You need to measure the charging voltage and current using a multimeter while Rev'ing the engine. If there is no clean flow to the battery then you need to start from the alternator, the output ac and the charging regulator section which rectifies and controls the charging. The alternator can be taken out and checked individually , if there is a steady AC flow. Check the battery ground and also the charging regulator which is usually an extra unit- trace the lines of the alternator to this unit.
Please find some tips below .

http://www.fixya.com/cars/r5824824-draining_car_battery_charge

Have charging rate checked at the battery. Should be between 13.6 to 14.2. Above 14.2 is not good and will overheat your lights and cause burnout. Under 13.6, will cause alternater failure by overheating the alt.

First tell me why you don't think it's charging.
Because of the voltage reading you are getting at the battery terminals? The range you want is between 13.6v to 14.2v. The higher the voltage the lower the amps. and true the other way, the lower the voltage the higher the amps. If you get a reading of 13.6v or lower, the alt is nearing or at max output. An alt can only run at maximum for around fifteen minutes and it will start slinging solder from overheating. It's a good indication the battery is either totally dead or junk. Charging above 14.2 the alternater is running wide open and is only putting out high volts which will cause the battery to boil , in turn destroying it. This is a bad alternater. All alternaters have integrated voltage regulater with the exception of ver few of the earlier ones. You cannot test an alternater by removing a battery cable to see if the engine keeps running without taking great risk of damaging the alt, regulator. So tell me why you feel it isn't charging. Remember an alt is not made to charge dead batteries. Their sole function is to maintain the battery charge. Without a battery, the alt won't last 10 seconds.
Hope to hear from you and good luck.

change your alternater

Have you checked the battery voltage with the engine running?

Mazda Mpv 2000 I also have the same problem, the alternater is testing good- 14.12Amp, so what is the problem the battery light will not go off.