I'm not a mechanic by any means but I've narrowed the problem I'm having down to either the battery or the alternator. I have a 96 f150 I6 (not that that matters) and it's run perfect everyday since I bought it not a single problem. Today when I started it up it started just fine but the power gauge, or whatever it's called, didn't jump up like it normally does. I notices the gauge faces and the climate control and even my headlights were much dimmer than usual. I unhooked the headunit and cb and anything else that was drawing power. But it stayed where it was at: between normal and red. Later the check engine light came on. I assume it's the alternator because it seems the battery is not getting a charge and the check engine light is on as opposed to the "battery" light. Am I correct in assuming this??

 

Once your engine is running, The alternator runs everything electrical. The battery Is only used for starting And as a reservoir When the alternator cannot carry the load, or the engine is not running.
Have your alternator tested.