Quote:
Originally Posted by jim-frank
I'm a little skeptical about any significant savings, here. The alternator's load on the engine is directly proportional to its current output. That means that when it's just producing enough electricity to run the onboard electronics (once the battery is topped off) it's using far less than a horsepower, maybe as little as 250 watts, about a third of a horse. I can see some potential mileage improvements by going with the original idea of using it as a regenerative engine brake, but I really can't see how it could help under any other scenario.  |
I am too, that's why it would be good to physically test this. A battery at saturation will present the highest impedance to the alternator, which will require the least current output from the alternator to maintain at the regulator set point (14.4 volts for this discussion). The battery (and other accessories) are the load, or impedance that the regulator "sees" by it's voltage measurement. (caveat: this applies to typical Delco alternator)
A little of Ohms law: I=V/R (I=current in Amps, V=volts, R=resistance in Ohms). The final voltage at the alternator will always remain constant, based on the regulator setpoint (which varies with temp) but we will assume 14.4 volts. If the battery is low and requires charging, its resistance (actually impedance, but for the simplicity of explaining this) is low. Assuming the regulator attempts to go to 14.4 volts, with a low resistance, that means current output is forced upward. If the battery is saturated, its R is high while the alternator is still shooting for 14.4 volts, that means the current has to drop correspondingly. The current is controlled by the regulator's control of field voltage. In order to test the minimum field voltage, a 1/4 ohm resistor is introduced in the main battery positive lead (which includes the voltage sense as well, and all accessories are turned off) This effectively simulates the impedance of a fully charged battery. The regulator requires less current to maintain 14.4 volts output, as only R has changed. The 1/4 ohm resistor is enough to drive the regulator to minimum field voltage even with a battery not fully charged. This is an actual documented test of the Delco regulator.
Why? It would be good to compare the minimum drag of the alternator while turned on (minimum field voltage) to the drag of the alternator while turned off in order to determine if there will be any fuel economy benefit.