Alternator Output on a MK3

I'm opening this topic to archive some brief test results concerning alternator output. Debbie brought the subject up a few days ago on another topic:

old-britts-start-t6940.html

and I was afaid that it would be too off-topic to capture much interest.

Returning from a lovely ride this morning in brilliant Pacific Northwest sunshine, I did a quick test of the battery voltage (dc) as a function of engine speed. I'm running an 18 A-H Yuasa, one year old, and SPARX 3-phase alternator and rectifier/regulator. Standard filament headlight. Pazon Surefire (ignition consumes 20 Watts or so).

Column 1:Speed (rpm)
Column 2:Voltage (vdc)-no lights, ignition only
Column 3:Voltage (vdc)-lo beam
Column 4:Voltage (vdc)-hi beam

0000 12.50 12.30 12.30
1000 12.35 12.15 12.10
1500 12.40 12.25 12.20
2000 12.80 12.60 12.50
2500 13.40 13.00 13.00
3000 14.00 13.20 13.20

So it looks as though I can putter around town with low-beam on forever as long as my average engine speed stays above 1800 rpm or so. Still, like Debbie, I was expecting a bit more out of the SPARX system.

I don't think your bottom end would want you putting about below 1800 RPM anyway....

There was a post some time ago that stated that the Sparx alternators rated their units in a different way than the original Lucas ones were. I think it may have had to do with the conversion to watts? Anyone remember this post? May have been somewhere else I read it. But the way they were rated made them look a little better than Lucas of the same wattage output. And no I have not been drinking, LOL.

BrianK said:

I don't think your bottom end would want you putting about below 1800 RPM anyway....

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As long as you don't wack open the throttle at that rpm you should be ok.

Hortons Norton said:

There was a post some time ago that stated that the Sparx alternators rated their units in a different way than the original Lucas ones were. I think it may have had to do with the conversion to watts? Anyone remember this post? May have been somewhere else I read it. But the way they were rated made them look a little better than Lucas of the same wattage output. And no I have not been drinking, LOL.

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This post by Stuart over at BritBike, maybe?
http://www.britbike.com/forums/ubbthrea ... Post319496

Based on your data, which strikes me as modest, for a Sparx, you are good somewhere bewteen 2000 and 2500, when systems voltage is above 12.6/12.7 your battery is getting a small charge.

What is the age of your wiring harness? If it is original and unmolested you can get an extra .3/.5 vdc by cleaning your connections and grounds, you may find that your lights are brighter and draw the system down less.

I have a one year old battery, a new harness and a Sparx 3 phase, I'll take some measurements and return.

RS

Rode,

The bike is a complete rebuild, completed last year. The harness is a new custom heavy harness with headlight relays, new connectors, and a multi-point ground wire system (the frame acts as a redundant ground). The state of the battery when I began the tests was 12.50 volts, so I figured that achieving that number with the low-beam headlamp on determined the lower limit on rpm for net positive charge, which occurred a bit south of 2000 rpm. The assimilator light turns off at that point as well.

Looking forward to your results.

Rick

Thanks L.A.B. that was the one, I think Rick may have something else going on. It seems with that alternator you would have plenty even if Sparx did fudge the numbers a little bit. RoadScholar may be onto something, Even if new I would just try to make sure all your connections are solid. Hope you find what's happening to all the juice.

Is it a bad thing that the bike is not generating power at idle? For example my bike idles @ 900-1000 rpm with the lo-beam on.The red ignition is on too at this time which goes off when I blip the throttle or accelerate.Is this normal?

speirmoor said:

Is it a bad thing that the bike is not generating power at idle?

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No, as you wouldn't usually run the engine at idle speed for long periods, would you?

speirmoor said:

For example my bike idles @ 900-1000 rpm with the lo-beam on.The red ignition is on too at this time which goes off when I blip the throttle or accelerate.Is this normal?

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Yes, perfectly normal.

Thanks I just needed reassurance.Its just more noticeable at night time while sitting in traffic lights and such.