I've had this problem with my charging system in my car and it has been ongoing for a couple of weeks now. I've got a new battery with new cables and everything is well. When things work well (alternator battery) the amps gauge is a little past half (13 amps). Then out of nowhere, the gauge will start settling around 9 and even 8 amps. The lights will all dim and i eventually set a SES light. I go to check for the codes and nothing. It wont even go into diagnostic mode. I check under the hood and low and behold the wire coming off of the positive post on the battery to the little computer wire (the one you disconnect to reset the computer for the codes) fried itself where they are connected (with one of those yellow 12? guage female connectors). The wire was melted and the connector was hardly existant. Once i reinstall a new connector, all will be well. Sure enough two weeks later it happens again. This time i get a loud whining sound (much like a blower) that is consistent with engine rpms. Im going to have my alternator checked tomorrow but im wondering what could cause that same wire to melt itself twice in two weeks. I cant think of any feasible reason as to why this is happening.

 

Your alternator is bad. Replace it, and make sure you replace that wire with the correct guage one.