I am looking for extremely basic information that anyone could understand, as I know nothing of electricity. My friend wants to use two automotive alternators to charge two 12-volt car batteries. These batteries are not in a car. He just wants to run a few shop lights and a compressor. I told him two car batteries will not run a compressor of any size but I don't know the math to back it up. He is sure he can run a three-stage compressor intermittently with two car batteries. Assuming that his alternators are up to the task of keeping the batteries charged, it seems to me there is not enough horsepower in two plastic boxes full of lead and acid to run a three stage compressor bigger than my hand. He says his compressor is a cube about 13 inches by 13 inches. I know how to size the compressor once I get the info on what a battery can do. I just don't know how to answer this question: what is the power available from a pair of car batteries to run a mechanical device like a compressor, with a 24 volt DC motor, assuming that the compressor is running about 1/10th of the time? All I know is that a hp is 746 watts and watts is voltage x amperage. I just can't visualize electricity doing something, it must be a trick. Respondents: I need numbers. I already told him it can't work but I have to prove it with numbers. Thanks.