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The cold weather is coming.

One of the most detrimental things for our antiquated anemic marginal airhead charging systems is a high load at low (engine) speeds. Voltmeter readings at (or below) nominal battery voltage (12.5 volts) mean you're charging system is maxxing out; the demand is exceeding the supply. Basically, you little rotor is working like a dog. (I get a bit anxious when my voltmeter goes below 13V)

My OE heated grips draw approximately 30W, my BMW heated vest ditto.
Over the decades I've developed the habit of switching 'the heat' off when idling (like stopped at a red light) and at low speeds (like in stop 'n' go traffic). This is to minimize the load on my charging system.

I was mulling over the idea of adding a separate relay to power my heated vest (via auxiliary socket) and OE heated grips.
This new relay would be triggered by my neutral switch, easy hookup.
In neutral, the grips and vest would turn off.
If and when I need to pump up a tire on the side of the road with my mini-compressor* (via auxiliary socket), I'd just shift it out of neutral.
Does anyone have any positive or negative thoughts concerning this setup?

*I also carry a mini bicycle tire pump.
Belt AND suspenders.

Although I have not evidence, it is my opinion that load on the alternator at idle does not negatively affect the alternator. Since less current is being generated at idle, it is probably running cooler. Yes, the voltage regulator is sending more current for the electro-magnetic rotor, but the slower spinning generates less current. It does impact the battery some from being discharged and recharged, but assuming you don’t idle at a light for long periods of time, it shouldn’t be a problem. While idling, the system voltage will be lower so your heaters won’t generate as much heat.

Bottom line, I don’t believe the relays you mentioned would provide much benefit. I think the use of both grips and and vest causing the alternator to generate near its power limit at higher RPM will generate the most heat and cause the system to fail sooner.

The link below is the closest I’ve found as relevant information

https://www.elreg.com/blog/factors-that ... lternator/

The link you posted states "Alternators do not work efficiently at idle until they reach 2,400 RPM"..."At a 3:1 pulley ratio, those speeds correspond to 800rpm on most vehicles" (paraphrased)

On our airheads, and many most other bikes, the alternator spins at crankshaft speed.
Some bikes have a belt drive alternator providing a 'car type' ratio advantage.

When my bike was new, and on full warranty (3 years, unlimited mileage), I didn't monitor my voltage, didn't give a $#!^. I didn't even have a voltmeter on my bike back then. I would leave my vest and grips and high beam on anytime I felt like it. As long as the bike started, I was content. Every so often, the 'battery light' would come on while riding, or I would turn on the ignition and not see a 'battery light'. That meant it was rotor replacement time again. I (c/o BMW) was constantly replacing rotors (on warranty). Like maybe 4 or 5 in those first 3 years (130,000km).
All the police airheads (that idle a very high percentage of their lives) had the same issue at ridiculously low mileages. BMW replaced hundreds of rotors on warranty worldwide.

The way it was explained to me in my youth, by no other than Phillip Funnell, BMW Guru, is that when the alternator is not spinning fast enough to generate more than battery voltage (12.5V), all the load is being routed through the rotor, which overheats it. This does not cause an immediate failure per se but it will shorten the rotors' looong haul lifespan. (Rob F's opinion on this would be interesting).

When my warranty ended and I was on my dollar, as per Phil's advice, I installed a voltmeter and I started conscientiously monitoring my voltage, and reducing the draw at low rpm, so the voltage would very rarely be forced to drop below 12.5V. Since then I have rarely had to replace any of my charging system parts, except brushes regularly every 100,000km or so. My present rotor has roughly 350,000km on it. In all fairness, all my charging system wiring and grounds have been beefed up, which I believe also helps immensely.

Being the cynic that I am, I still carry a spare rotor and diode board on long trips. I also have a frame mounted spare regulator, just in case.

First, you can't argue with success.

That got me thinking more about it. I think we'll all agree that heat is the enemy.
The rotor gets charged by power coming from the voltage regulator, but the output of the stator goes to the diode board. So when the alternator is pumping out a lot of current, it's the stator that's taking a beating, along with the diode board. When the alternator isn't producing enough power to keep the voltage at the correct level, the voltage regulator increases the current through the brushes into the rotor, increasing the electro-magnetic field. Although I'm not totally sure about this, I believe the actual voltage sent through the rotor is whatever is available in the system, and the voltage regulator turn off the current when the system voltage gets to it's predetermined level. When the system voltage gets up to the appropriate level, the voltage regulator with pulse the current to maintain that level.
Therefore, if the motor isn't spinning fast enough, the voltage regulator will maintain solid current through the rotor, creating more heat with no "pulses" to cool down. And that would explain heat in the wiring of the rotor at low engine speed in conjunction with a heavy electrical load.

Is the above fundamentally correct?

I know you would rather have a simple rather than a complicated solution.
Given all that, I accept your initial premise that a relay to reduce electrical load at idle to save the rotor from overheating.

But this device would reduce the load when undercharging for any reason, not just the neutral light being on.
A more complex device (more electronics on a simple machine?), but it's still cool.

https://www.amazon.com/Undervoltage-Aut ... 07RS49D6B/

How about an amp meter?

Maybe one of these:

Seth wrote: ↑Sat Sep 25, 2021 10:36 pm
Therefore, if the motor isn't spinning fast enough, the voltage regulator will maintain solid current through the rotor, creating more heat with no "pulses" to cool down. And that would explain heat in the wiring of the rotor at low engine speed in conjunction with a heavy electrical load.

Is the above fundamentally correct?

That sounds right to me. The voltage regulator will be trying to to get full output from the alternator by passing maximum average current to the rotor. I'm not sure if the regulator gives up switching all together to produce a solid current or just shifts to something like 99% on and 1% off but either way it will produce maximum heating in the rotor.

I'm reminded that VW used something similar on their Type 4 powered vehicles. Those used an auxiliary fan for the heater, but it was wired through a relay and would only run if the charge light was out.

So a worthy concept and one with 50+ years of application behind it.

Zombie Master wrote: ↑Sun Sep 26, 2021 2:19 am 800WattAlternator copy.jpgMaybe one of these:

BMW figured it out on their other bikes by the time airhead production was on death row.
The pic you posted looks cool at first glance, somebody put a lot of thought and work into that.
But look closely; That top pulley is way too big, that thing will still spin at about crankshaft speed.

Zombie Master wrote: ↑Sun Sep 26, 2021 2:12 am How about an amp meter?

Ammeters need heavy duty wiring.
Old british bikes and cars mostly had ammeters rather than voltmeters.
If the needle was left of center, it meant "turn something off!".

Voltmeters can use pretty much any wire gauge, they draw almost no current.
Below 12.5? Turn something off!

If interpreted properly, they both provide useful info.