Why does my 95 Heritage need a battery to run? It is not fuel injected. I used to push start my 91 Sporster sometimes. Based on what happened when my battery cable broke earlier this year on the way back from the meet up, it apparently needs a battery to run, not just start.

Why is that?

 

Electronic ignition... No mechanial ignition like a magneto.

 

Most alternators need some voltage to "excite" the field coils...no voltage no charge. Even "self exciting" alternators have to get to a certain RPM before they start charging. That is why you cannot push start a vehicle with an alternator and a 100% dead battery. An older vehicle with a generator, can be push started with a 100% dead battery...even with points.

 

For one, your alternator is single phase unless you've put in a three-phase aftermarket stator. So it has a lot of DC ripple on the output. Electronics like the ignition module don't like dirty power. The battery acts a capacitor to smooth the ripple from the alternator output.

Secondly, the stock alternator does not put out enough power with the engine at idle to run all the electrical systems in the bike and the battery discharges at a low rate when the bike is idling at a stop light with the brakes on and turn signals operating, as an example. So without the battery it would just quit.

The third thing is that the rectifier/regulator is a shunt type. The stator puts out full power all the time and the regulator shunts the excess to ground to regulate the voltage at ~14.4VDC. It requires the battery to be connected or its internal circuitry shuts it off to prevent the variable shunt resistor from burning out.

 

Bet you folks didn't know this was going on the garage wall tonight.
Thank you, sir!
Tom

 

Wow! Very impressive! And I was just going to say "because it does!".

 

Can somebody translate this for the slow folk, like me?

 

The charging system used in motorcycles is pretty much the same whether it be Japanese or Harley or whatever. They are permanent magnet AC generators with a rectifier/regulator that rectifies the AC power to DC and controls the voltage so it doesn't boil the battery dry and blow all your bulbs. There is no control of the field like there is with automotive alternators so the alternator puts out full power all the time, although it is dependent on rpm. At slow speeds it puts out less amps and voltage, at high speeds it puts out more amps and voltage.

Since there is no way to control the magnetic field in the alternator core the only way the regulator can control the voltage is to shunt excess current to ground. Most regulators have a self-protection circuit in it that will shut it down if the variable shunt resistor gets too hot. And that will happen if you remove the battery load from it. The regulator circuit is designed to have a huge capacitor in it - that capacitor is the battery. At that point, if you have a broken battery cable the bike has no electrical power so it just quits because there's no ignition.

The old Harley's with a DC generator would run without a battery once the generator field was excited. The newer ones with permanent magnet alternators will either blow the regulator or the regulator will shut it down if you disconnect the battery. And the other thing that is possible if you disconnect the battery with the alternator putting out close to full power is that there will be a momentary voltage spike that could blow some of your bulbs or other electronics. That voltage spike is caused by suddenly removing the capacitor (battery) from the circuit and the regulator now attempts to compensate for it by shunting to ground. But the DC power output is very dirty because the sine wave on the AC side is more of a square sine wave without that battery in the circuit, so the self-protection disconnects it to prevent damage.

In theory, a three-phase permanent magnet alternator could power the bike without a battery, assuming the voltage regulator circuit is fast enough, and robust enough, to control it. But Harley's are all single phase except for some of the newer bikes with ~50-52A alternators, which I believe are three-phase.

 

This is the best explanation of a motorcycle's charging system I have seen. I had some knowledge, but that knowledge was more or less based on a car's alternating system, which I knew was not the same but was unclear of the reasons. This clears up a lot of my questions. Thank you.

 

Don't confuse automotive wound-field alternators with permanent magnet type. PM type alternators are always excited because the rotating magnetic field comes from the magnets. Self-exciting automotive type use residual magnetism in the rotor core that creates a small current in the stator windings, which is in turn rectified to DC by a small diode bridge that feeds the regulator and brushes/slip rings to provide power to the field winding.

So the PM alternators in motorcycles "self excite" but they don't put out enough power at the speed you can turn it at by pushing it to do anything.

The AC voltage from the stator is lower than the rectified DC voltage because AC is measured by RMS (Root Mean Square). The actual peak voltage is 1.414x the AC RMS value. This is true for any AC power source, including the single-phase 120V AC power in your house outlets - the actual peak voltage is around 170 if you put a scope on it and measure the amplitude of the sine wave. When rectified to DC you get the peak voltage value and not RMS.

With a single phase power source there is also a 1.4V forward drop in the diode bridge - 0.7V per diode. A PM alternator produces a set number of volts per revolution, usually expressed the other way - rpm's/volt. And this holds fairly true, although there is some reactance loss in the core so the faster it turns, the more rpm's it requires for every volt increase in output.

In the end, the single-phase unit used in Harley's has to be turning at at least 850 rpm to overcome the reverse bias in the diode bridge before it will start putting out any power to the electrical system.

Three-phase alternators, wired wye, produce 1.732x (square root of 3) single phase voltage, measured phase to phase. So they are more efficient than single phase, with lower resistance stator, and less heat for the same size of alternator. There are several aftermarket manufacturers that make three-phase systems for Harley's.