Hello, the battery of my jetta 91 car is suddenly discharged , can you clear the reasons please

If your battery is discharging, that means you have a short somewhere, within your electrical system.. Or you left your headlights or something on, that should normally be off, when you turn the car off...I hope this helps you...

There may be a surface discharge. this occurs when the vehicle still draws current from the battery even when the car is off.the problem could have been going on for a while now and the battery just cannot sustain in a charge any more.
most times this occur in the morning when you start. a simple test is to disconnect the battery at night an see of it starts in the morning the battery held its charge and the car is the problem..
Other Reasons:-
the lights was left on
a door was left open
battery not holding charge
no battery water
the alternater not working well to charge the battery