Quote:
Originally Posted by franken
Sorry, but that violates Ohm's Law which we can't do. Lower voltage means lower current.
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Actually, Jim was spot-on.
Your interpretation of Ohm's Law is assuming a constant resistance. With an electric motor, resistance is a function of motor speed. As speed decreases, resistance decreases.
If the relationship were perfectly linear, 20% under-voltage would result in 20% lower speed, then everything would balance perfectly. However, that's not the case. 20% lower voltage (for example, driving a 12V motor with only 10V) results in SIGNIFICANTLY lower motor speed. Like 66% less, perhaps more, especially when we add the load of turning the engine against it's compression. It's huge.
If we assume a typical starter motor to have a 0.12 ohms of resistance at full speed (resulting in 100 amps @12V), then cutting that speed by 66% results in cutting the overall resistance by 66%, leaving us with about 0.04 ohms of resistance. By ohms law, I=V/R, 10V / 0.04 = 400 amps.
So that starter motor that's drawing 100amp @12V, becomes 400amps @10V. (assuming the battery can supply said amperage and voltage).