Could be the voltage regulator. I don't know if on your Jag it has a intergrated voltage regulator on the alternator or not though.

 

Felicia, first, what you will need is a multimeter capable of going up to 10 amps (most multimeters will do this). Now, with your negative battery cable disconnected (DO NOT!!!! us the positive battery cable for this test, can lead to other issues), using some electrical tape, tape the red lead from the multimeter to the negative battery cable and then tape the black multimeter lead to the negative battery post. UNDER NO CIRCUMSTANCES WILL YOU START THE CAR!!!!!! DAMAGE WILL OCCUR TO THE MULTIMETER. With the multimeter set to the "A" scale (amperage scale), what is it reading? If you are reading over 0.2 (amps), then you have a definite wiring problem, follow what I say below. If you are under 0.2 (amps), then you want to move the dial on the multimeter over to the "mA" scale. Are you under 50 (milliamps). If yes, then your car more than likely just sits to long and if you have a car alarm or other small electrical load, it can drain the battery if let to sit for long periods of time. A basic car alarm can kill a standard battery in a few days in some cases. If you are over 50 (milliamps), the follow these directions.

If you are having an excessive current problem, first disconnect the multimeter and place the battery on a charger and get back to 100% (ie, battery voltage of 12.6 VDC). With the battery at full charge, remove all fuses and relays from any fuse box in the vehicle. Note which fuses go where (ie, slot one has a 5 amp fuse, slot 2 a 10 amp, etc). Now, reconnect the multimeter as described above. Are you under 0.050 amps/50 milliamps? If no, then you have a wiring problem with your main wiring, write me and I will give you further direction. If you are under 0.050 amps/50 milliamps, install a single fuse into its correct location. Does the multimeter still read under 0.050 amps/50 milliamps? If no, remove this fuse, note the fuse and what is on that circuit. Continue. Install 1 more fuse. Still under 0.050 amps/50 milliamps? Repeat this process of installing 1 fuse at a time, checking the multimeter between each fuse, verifying still less that 0.050 amps/50 milliamps. Note all the circuits that cause the current to exceed the limits I specified. Now install all the relays one at a time, checking the multimeter between each relay. Again, exceed the limits, note the circuit, remove the relay. Continue adding the relays back in until all relays are either installed or noted as being removed due to excessive current. Once you have the list of bad circuits, let me know and we can give you more specific information on troubleshooting those circuit(s).

Hope this helps. Like was mentioned, it could be a bad regulator on the alternator. You can verify this by connecting the multimeter across the battery (set to VDC) and with the car running, it should be reading somewhere between 13.5 and 14.4 VDC. If you are under 13.5 VDC, then you have a bad alternator or a problem associated with the alternator. This check should probably be done prior to starting the fuse pulling bit.

If you need any more help, just hollar.