Today I was cleaning car and was playing music, well later that day I tryed starting it and battery was dead, luckily I had waranty so I went to get a replacement but now once I put new battery I started it and stays iddle at little less than 500 RPM, it feels like its going to die out, my volt gauge reads 11-12 volts, I checked battery twice at autozone and its fully charged, I rev it up to 2000 and when I let go of gas it turns off. What could it be, help me