Recently my car is having kinda electrical problem,when i turn on the A/C the idle drops and the car start hummming,sometimes it is more noticeable when i stop for a while and the gers is on (D), i shecked the battery with a voltmeter it seems the voltage goes up when i rev up the engine.I know it is normal to have this kind of voltage drop but this type of drop just developed recently.Anything i am missing here?