Have the generator and charging system professionally load-tested for maximum current output and "ripple" voltage. GM has had a huge problem with their generators since they redesigned them for the '87 model year. They develop voltage spikes that can destroy the internal voltage regulator and diodes. If one of the six diodes fails, you will lose exactly two thirds of the output capacity. That leaves you with only 30 amps out of the common 90 amp generator. That's not enough to run the entire electrical system under all conditions. The battery will have to make up the difference until it slowly runs down.
It is common to go through four to six replacement generators in the life of a GM car. To reduce the number of repeat failures, replace the battery at the same time unless it is less than about two years old. As they age, they lose their ability to dampen and absorb those voltage spikes.
Given the engine appears to start the rest of the day, and if the charging system passes the load-test, you may just have a weak battery. Your mechanic will load-test that too.
Thursday, October 10th, 2013 AT 12:12 PM