GM-GMC: Battery light. discharge, alternator diodes, zero volts

Question
The battery light illuminated on my 1999 GMC Jimmy.  while I was driving. The charge gauge  dropped to 11 volts. I was able to drive the Jimmy home. I checked the voltage of the battery when I arrived home and  it had dropped to 11.38 volts. I connected a battery charger to the battery and set the charger too a 2 amp trickle charge rate.  After leaving it on charge for 2  hours the charge complete lite on the charge  came on. I once again checked the voltage of the batter after  connecting it to the vehicle and it had  fallen to  zero. Then I removed the batter from the vehicle  and took it to my shop and  attached the charger once again. With -in  5 minutes the charge complete light illuminated and  the voltage  had recovered to 13.25 volts. I then once again tried to re -install the battery in the vehicle  and when attaching the  cables to the battery  sparks flew  when I attached the neg  cable indicated then  nust be a large load on the battery.  There was nothing in the vehicle selected on and the key  was in the off position. What could  have caused this large drain and why would the battery light on the instument  panel come on? Why also now is there zero volts  on the battery when I connected  it since when I first arrived home the was 11.38 volts available. Could putting the battery on charge while it was still  connected to the vehicle caused this?
This is a 1999 jimmy Descente.  

Answer
It is probably a shorted diode in the alternator.

Since about 1986, GM alternators will not charge a battery if its voltage is too low.  You proceeded correctly by first trying to charge the battery back up.  The rapid drain indicates a short and alternator diodes are known to cause such problems.

The easiest way to proceed is as follows:

I would recharge the battery and then take it to an auto parts store for a load test.  This ensures that something has not gone wrong with the battery.

If the battery passes, have the alternator tested.  This can be done with it installed or removed from the vehicle and most auto parts stores will test the alternator for free.

If the alternator and battery are fine, you will need to find a mechanic who specializes in electrical systems to track down the source of the short.