What is the static voltage on your battery? Let me repeat from previous (with more details) When a battery is just charged it has a "surface charge" which can give a false reading for the actual condition of the battery. When a battery is allowed to sit (like overnight) the surface charge disappates and the actual charge condition of the battery is then the voltage you read. So in reality - if you run your car for a couple of hours at revs and the battery is being charged by the alternator (all accessories off) and you turn the car off and read the voltage at the battery - it can be as high as 13.14 volts - which is the surface charge reading. Let the battery sit over night and without trying to start the car - read the voltage on the battery - it will be the true state of charge. This voltage should NOT be UNDER 12.4 volts (12.65 - 12.90 indicates a real healthy battery - at 12.56 battery is still quite usable but getting weak)- if it is you have battery broblems which could be sucking current from your alternator and not building up a reserve for when you need your accesories. CHECK YOUR BATTERY BEFORE DOING ANYTHING ELSE - It could be time well spent.