Battery meter drops when driving with radio and or AC on. GMC says this is normal but the GMC service center said other people have complained about the same thing
Go to autobytel.com and view the forum for the GMC Envoy and Chevy Trailblazer. I briefly looked at these models and decided against them when I saw similar complaints about electrical problems in both models.
It turned out to be the alternator, the lights would dim out, fan would quit, and then it would come back like nothing ever happened. I took it to the dealer and they found that for me.