There's a number of things ... but the one that got me was a stuck fuel pump relay. It stayed closed and the fuel pump ran all night.
It doesn't actually drain the battery, but without a working alternator to recharge the battery it will only discharge as the car is operated.
A short in the wiring.
something is causing a drain on the battery when switch is turned off
Bad battery, Parasitic drain.
It could be two things. As you know, the alternator charges the battery so it will not drain. If the alternator is not working properly to charge the battery, the battery will continuously drain. Have the alternator tested at your local Auto Parts store (NAPA or Advance Auto) If it is good, have the battery checked.
theres a good chance your alternator is on its way out
If the alternator is good then the battery is bad.
Battery light usually means alternator not charging. Most common solutions would be be a bad alternator, bad connection, or loose alternator belt. If the battery and alt test ok than check the connections to the alt. You may also have a battery drain (something that is staying on and killing the battery)
Note penciled into friend's 95 GC owner manual sez "pull fuses 6 & 7 (security system) to stop battery drain". Don't know if it works.
If I understand the question. It is not good to run the car without the belt on the alternator. It will drain your battery and damage your battery. You usually have 20-30 miles running on the battery without any recharge without any power on.
Probably have a 'parasitic load'. This is something causing an abnormal battery drain while the car is off.
Something is causing a drain on the battery even with the vehicle off and nothing turned on. A good mechanic can do a drain test on the battery to see what may be draining the charge. A dead cell in the battery itself will also cause the battery to go dead. Disconnect the negative battery cable overnight and if the battery is dead the next morning it has a dead cell and must be replaced. If not look for a light that is on or a relay that is stuck.