Sounds like the charging system is not recharging the battery. Use a digital voltmeter to test the charging voltage. It should be between 13.5 and 14.8 volts at idle with no accessories on.
December, 13, 2011 AT 9:12 PM
Do you mean the extra load drains the battery while you're driving? That is a sign the generator can't keep up with demand. Beginning with the 1987 model year, GM switched from the second best generator design to by far, the world's worst pile ever. They develop huge voltage spikes that destroy its internal diodes and voltage regulator. It's common to go through four to six of these in the life of the car, but you already did what it takes to reduce the number of those repeat failures. That is to replace the battery. The battery dampens and absorbs those voltage spikes which also interfere with computer sensor signals. As they age, they lose their ability to do that.
When one of the six diodes fails, you lose two thirds of the generator's rated output. That means a 90 amp generator will only be able to supply 30 amps. That's barely enough to run the electric fuel pump, fuel injection and ignition systems, and the high beam headlights that GM insists on shining in other people's rear view mirrors. Once enough things are turned on that exceed the generator's capability, current is drawn from the battery until it runs dead.
If the voltage regulator fails, the generator won't produce any output at all, and the car will run on just the battery for up to about an hour. The place to start is by having the generator tested with a load test and a ripple test. Excessive ripple indicates a bad diode. No output at all indicates a bad regulator or other internal failure.