REMEMBER THE DYNAMO? No? Neither do I. Dynamos, or dc generators, usually driven by an ac motor, were among the earliest methods for recharging a storage battery. Originally, self-excited or shunt-wound dynamos were used because they had an inherent current limit if the dynamos became overloaded. The overload would reduce the output voltage, thus reducing the excitation, and the output current would “fold back” toward zero as the overload increased. The presence of a battery as a load, however, reduced the fold back, and effective current limiting required electromechanical regulators (similar to those in older vehicles with generators, before the switch to alternators).

The current regulators weren’t user-adjustable; they ran at a fixed current, and tolerances were wide. Early dynamos typically limited their output current to about 120% of their rating, but could range from 100% to 125%. As electronic controls became available, the regulation specs improved.

When dynamos were replaced by static converters in the mid-1960s, and then finally by modern electronically controlled chargers, manufacturers perpetuated the 120% current limit, largely because it was in customers’ specifications.