AutoMac always did it's job. Came up to voltage, then cut out. Turn back the rheostat a bit for lower charge current, hit the red button, and off it went again. You couldn't boil batteries with one unless it failed or your cutout voltage was too high. Only time I cooked a battery was when the regulator attached to a 35A Motorola failed and kept sending field current to the alternator. That was way back prior to the AutoMac.