1985 oldsmobile cutlass supreme brougham

Have the alternator load tested if okay-look for a short that's draining the battery overnite

Check the plug on the back / side of the generator to see if it is loose or the terminals are rusty. When the battery light does not turn on when the ignition switch is turned on, which is what I think you were trying to say, (and is a dandy observation), that means there is a break in the wire going from the battery light to the plug in the generator. One wire in that plug will have full battery voltage all the time. The other one should have full battery voltage IF you unplug it and measure with a digital voltmeter. If you leave it plugged in it will only have around 4 - 6 volts. If you find full battery voltage while it's plugged in, there is a bad connection on that plug. That is the turn-on signal for the internal voltage regulator. Sometimes the residual magnetism in the spinning field winding is enough to self-start the system working minutes or hours after starting the engine. If and once that happens, it will continue to work just fine until you stop the engine.