Conventional ignition coils suffer the disadvantage of being designed to operate best at about 12 volts. Unfortunately, a 12 volt battery often produces as little as 7 volts when "run down" because of excess starter operation, especially in extreme cold. To produce optimum coil performance (and hence nice fat sparks at the spark plugs) under such adverse conditions, the "ballast resistor" system was developed. This system uses a coil which is designed to be most efficient at about 8 volts. For starting, full battery voltage is supplied! This makes this system as efficient at low battery voltage as a "conventional" coil is wlth the battery supplying a full 12 volts. (For any battery voltage above the coil's design voltage, it's even better- an "overboost" condition.) However, an eight volt coil cannot be run continuously at 12 volts without overheating and failing. As soon as the starter switch is released, the coil no longer receives full battery voltage. It is then powered through the ballast resistor which reduces the 12 volts (the generating system is now working) to the coil's design voltage.
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