I have a 2003 GMC Envoy and just started.

The generator has failed and your battery will be run down very quickly. It is common to go through four to six replacement generators in the life of a GM vehicle. Due to their design they develop huge voltage spikes that can destroy the internal voltage regulator, internal diodes, and can interfere with computer sensor signals. 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 spikes.