The other thing to consider, most automotive alternators are rated to produce ~50A at 14V at a couple of thousand RPM, which is 700W. One Horsepower is 746W, so allowing for some belt friction, the car engine will see an increased load of ~1HP if the alternator is maxed out. At idle RPM, the alternator is delivering only a few A, meaning less than 0.1HP, which the engine will barely notice... A 5HP lawn mower engine will notice a 1HP load, a 200HP car engine will not.