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gspd wrote: ↑Sun Sep 26, 2021 12:58 pm
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.

Any details on a voltmeter to install or a how-to guide there?

I'm contemplating something like this for Edgar:



I was primarily looking for dual USB and I found out that they can be had with a volt reading. I'd mount it on the dashboard of the Luftmeister.

The fairing has some always-on running lights and I'd probably take the feed from them. There's probably some voltage drop as compared to what the battery sees, but there would be no draw when the ignition and lights are off.

The issue with a voltmeter (any voltmeter) is that it only 'sees' the voltage at it's input terminals. Unless the input terminals are connected direct to the battery, the voltage reading will be affected by the load carried by the wiring (including the ground) and the resistance of all of those wires. In essence, the voltage you're seeing is not what you want to see (i.e. voltage at the battery terminals) but that voltage less any voltage required to push the required current for all of the circuits shared by the voltmeter.

In practice, provided you're not looking at the voltmeter reading as an empiric reading (i.e. exact), it isn't too much of an issue. If you use the meter as a comparative indicator (i.e. the reading is what you would expect to see under the current circumstances), then all is fine... if the reading is significantly different, then it's time to start checking things.

If you want an accurate reading (within the limits of the instrument), then there's really no alternative but to wire the meter back to the battery terminals. This, of course, means that meter will be permanently 'on' unless you arrange a relay or switch to disable it when the bike is off.

Rob

Rob Frankham wrote: ↑Mon Sep 27, 2021 4:06 amIf you want an accurate reading (within the limits of the instrument), then there's really no alternative but to wire the meter back to the battery terminals. This, of course, means that meter will be permanently 'on' unless you arrange a relay or switch to disable it when the bike is off.

This is what I've done with my /7 with Luftmeister voltmeter. I run the power through an aircraft quality switch which goes through a hole in the interior panel of the fairing. Thus, essentially no losses to the meter.

Kurt in S.A.

Melville,

Before you buy one of the USB and volt meter setups. Check to make sure that it shows battery voltage when using the USB. Some switch to showing the USB voltage while charging.

All, I have thought about using a relay on a volt meter. My question is how much of a loss is there with the relay?

Tom

gspd wrote: ↑Sat Sep 25, 2021 7:19 pm
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).

I'm sorry to say that this statement is basically wrong. At no time does the output from the alternator (which I'm assuming is what is meant by 'load') pass through the rotor. In any case, when the alternator output voltage does not exceed the battery voltage. There is no output from the alternatorat all.

What I think may be meant is that all of the output from the three auxiliary field coil diodes is routed through the rotor... however, that voltage is not battery voltage except at very briefly under the conditions of load and engine speed when the alternator is right on the cusp of starting to produce power. This point, however, is at around 1500 to 1750 rpm which is a rev band seldom used at idle or when the bike is moving so any heating effect is probably over so quickly that it has no real effect.

In fact, the 'worst case scenario', as far as the rotor is concerned, is when the alternator is producing it's maximum output at revs in excess of - say - 3000rpm with the battery voltage around full voltage. That is to say when theload is exactly balancing the maximum output of the alternator. In this scenario, the voltage regulator is doing nothing and the rotor is seeing the full voltage at the output of the auxiliary field coil diodes (to all intents and purposes the same as battery voltage).

So... are these 'full output' scenarios going to damage the rotor? let's take a full voltage output as 14 volts and the rotor impedance as 3.5 ohms. The current through the rotor will be 14/3.5 (i.e. 4 Amps). This isn't enough to heat the field coil wire to anywhere near melting point... or indeed anywhere near the temperature it's going to reach anyway due to proximity to the engine mass. In essence, I doubt whether rotor failures can be attributed wholly or mainly to electrical heating.

Why do they fail? In my opinion, they fail due to a combination of events over a long period. Among them, the most important is vibration with continuous changes in rotation, helped along by continuous heating/cooling cycles. The windings in the stock rotor are not encapsulated (they are in some aftermarket replacements) so a small amount of movement is possible in the winding and gthe contiual flexing will lead to fatigue fractures of gthe copper wire. It's also true that, as a fatigue fracture starts to develop, there will be an increase in resistance at that point leading to increased localised heating which will contribute to the eventual failure.

Rob

Rob Frankham wrote: ↑Mon Sep 27, 2021 12:55 pm Why do they fail?

I can't come close to explaining 'why they fail' with your degree of expertise, won't even try.
I CAN however say with certitude: If your battery is often discharging (12.5V or less) expect your airhead charging system to fail prematurely. Looks like the rotor is the weakest link in our charging system chain so it's the first to go when the going gets tough.

Cops always left their engines idling (even during a heat wave ) when dismounted to direct traffic or to write a ticket (or to get a donut ), because it was police protocol to do so . This protocol made sense. Their airheads often wouldn't restart if even just the police radio was inadvertently left 'on' for a half hour. (The Harleys also had similar issues.)
Police airheads, in city usage, with all their extra lights and police radios, had disproportionally higher (way higher! ) incidents of charging system failures than those used in more rural situations and those used by the general civilian population. Most common failure: open circuit rotors, reoccurring at unbelievably low odometer intervals. BMW tried 'fixing' this with a just as useless heavy duty police regulator. Didn't help. An extra rear mounted battery didn't either.

Tom H Ca wrote: ↑Mon Sep 27, 2021 12:07 pm Melville,

Before you buy one of the USB and volt meter setups. Check to make sure that it shows battery voltage when using the USB. Some switch to showing the USB voltage while charging.

All, I have thought about using a relay on a volt meter. My question is how much of a loss is there with the relay?

Tom

That's a 'How long is a piece of string' question. If you use an automotive relay and the contacts are good, the voltage drop will be negligible, especially as the current used by the voltmeter itself is very small. If you want to go better, an industrial relay used in electronic circuitry will have better quality contacts... if you really want to get serious about it, then a mercury wetted relay wouod be the way to go... but... all of this is a bit academic in the real world since even a cheap automotive relay will have a lower contact resistance than a the accumuated connectors in the circuit and certainly less effect on accuracy than the accuracy of the average automotive voltmeter.

Rob

Rob, thank you for the info. I thought it would be so little that it wouldn't matter. Just wanted to make sure.

Tom

You can increase the Voltage output at low RPM by fitting a diffrent Voltage regulator, its available from BMW Motorworks in the UK and the part is located under the fuel tank, part number is ELA44410

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