Submitted by Radford, B Davis
The issue is that cars (and 'wings and a very few other bikes) have a different type of alternator than our Shadows (and most other motorcycles) have.
Basic theory idea, for those who came in late: Generators of all types make electricity by moving a magnetic field relative to a coil of wire. The stronger the magnetic field and the faster it moves, the more electricity is generated. The moving part is usually called an armature or rotor. The stationary part is called a stator.
The typical car alternator is an excited field type. The magnetic field is created by an electromagnet in the rotor. Supply voltage is controlled by the amount of current going through the electromagnet. Most big AC generators are made this way. If an external power source is applied of a higher voltage than the regulator setpoint is applied to the system, the regulator will recognize the system voltage as being above setpoint and turn the field magnet (usually rotor) off. Result: No problem, as long as the applied voltage isn't so high as to damage anything (typically 25 V or higher on a 12V system). So you can (usually) jump start a car with another car, or a GoldWing without any problems.
Now the bad news:
Most motorcycles don't use an electromagnet to create the magnetic field. Instead ,we have a drum-shaped rotor (usually on the crankshaft) with several permanent magnets placed inside (a magneto). These magnets moving past the stator coils create the electricity we need to run the lights, charge the battery, etc. But you can't change the strength of a permanent magnet. So regulating the generator output is not a straightforward issue of turning the field magnet strength up or down.
The motorcycle voltage regulators I've seen all take the approach of shunting excess generated power to ground. This has the advantage of making sure that the voltage is the same everywhere in the system, but the disadvantage of meaning that the stator is always flowing its maximum rated supply current. This, I think, is why many motorcycles have a reputation for frying stators.
So the design of one of these regulators is completely different from a cage regulator. It has a voltage detection part, like the other regulators, but the big resistor/power transistor package has to be strong enough to carry all the possible excess power generation to ground. It handles a lot more power than the car regulator has to. It generates a lot of heat as it does this, which is why the regulator on my '85 VT1100 is finned and out in the open air--to carry off the heat before it cooks something in or around the regulator. A typical bike magneto makes 30-50 amps at max power. So the regulator is designed to dissipate a maximum of about 700 watts for short periods (this would be full power and no loads on the bike--the battery and lights are all missing). In practice, this cooks the regulator pretty fast--they don't like to dissipate more than about 200 watts for any length of time.
Now consider what happens when your moto regulator is doing a good job keeping the system at a nominal 14.1 volts when running, but the battery is weak, so you have to jump-start it on cold mornings. You hook up the bike to your Toyota with a 95 amp alternator (max output about 1400 watts). The Toyota's voltage regulator keeps *its* system at a cozy 14.3 volts when the engine is running. We now have a problem.
The cage's alternator and regulator want to maintain the system at 14.3 volts. Your bike's regulator, the instant the system is turned on, is going to try to bleed off excess voltage from the system to keep it at 14.1 volts. The car's alternator is rated for 1400 watts. The bike's regulator can dissipate a maximum of 700 for (very) short periods before it cooks itself. It's a tug of war, and the bike regulator ALWAYS loses.
Moral of the story--jump-start your bike from a non-running cage. The quiescent voltage of a car (or bike) battery is in the 13.2-13.8 volt range. The only result of this is that the full output of the bike's magneto will go into the cage and moto batteries once the bike starts. This actually reduces the load on the regulator to near zero, so it's quite happy with this state of affairs.
Corrolary: Want to reduce the load on your bike's voltage regulator? Install MORE (or brighter) lights. No kidding. Since the regulator only handles power output beyond the bike's demands, installing more demands means that the regulator does less (and is happier). The stator does the same amount of work in either case, so no problem there. Be forewarned, though, that Honda didn't exactly break the bank on copper for the wire in our bikes. It's sized to work just right with no corroded connectors and the stock loads. You need to run larger/more supply wires from the battery if you intend to use significantly more power than does the stock system.