I have a 1995 Oldsmobile Royale 88 LSS that has been diagnosed as having a battery drain. The alternator and battery have been tested as O.K. Yet recently the battery light has been coming on even when the battery is charged. The lights are also go in and out from bright to dim (seems esp. When using headlights). Have cleaned the positive battery cable yet may need to replace this. The car does make a sound also like a swith or silinoid is trying to open or function or someting (intermittant). The car after driving will sometimes not restart. I do charge the battery by using a lead plug on the negative cable so there will not be a drain. Please help as I have had to take off from work in order to fix this car.
The battery warning light surprisingly has nothing to do with the battery, at least not directly. It comes on when the generator is not working. You already observed that you have an intermittent problem which is real common. The generator was tested while it was working. It needs to be tested while it's not working.
These generators are a real miserable design and there is not much that can be tested other than to verify they are working. There are only two wires to test and we know both are working because the generator starts to work sometimes. The only exception would be if there is an intermittent connection in one of them but even that is not likely. If the small wire plugged into the generator has a break in it, the generator will stop charging the battery, but that is the wire that runs the dash warning light so it wouldn't work either. If there was a break in the fat output wire, it's not likely the warning light would turn on. That leaves the generator as the only part that can cause the light to turn on.
One word of warning. This is a terrible design and it is real common to go through four to six replacement generators in the life of the car. What many professionals are finding out is to reduce the repeat failures, you must also replace the perfectly good battery at the same time. It will work fine on older cars from before 1987, but as they age, they lose their ability to absorb the horrendous voltage spikes these generators produce. Those spikes can interfere with computers' sensor readings causing engine running problems, and they can damage the new generator.