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Randy68 Offline OP
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I'm hoping I can get some help interpreting the results of an alternator test I did getting the Trump ready for summer.
The test consisted of connecting a 1-ohm 100W resistor in parallel with the alternator, measuring the voltage across it (and calculating the current through it)
at 2K RPM (6.2V) & 4K RPM (10.4V). Obviously the current is the same. It appears a 10 amp load would require "help" from the battery to maintain 12V across the load. I'm guessing that something around a 5 amp load is probably about all the "real world" load my alternator can handle without draining the battery. Does that sound resonable? On a side note. Do I need to consider converting my readings to their RMS values. I understand the alternator output is not a true sine wave but trig is about all the math I can handle. Thanks

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What sort of meter are you using? The readings given in the Manual assume an analog meter which reads "average" in the AC position. Most cheap DMMs read peak AC, while the top shelf meters read true RMS (.707 x peak.)
"Obviously the current is the same." Huh? 6.2v across 1 Ohm is 6.2A and 10.4v across 1 Ohm is 10.4A.
Most of the stock 12V alternators were rated at 10A @ 5000rpm. No mention was made whether this was peak or RMS, but one can assume that they were measured with analog ammeters.
In any case, I think you're good as long as you keep the revs up where they should be and don't add any extra lights.


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Hi Randy, I have done this test. I'm familiar with it. I can send video of my results if you PM me your email.

You are disconnecting alternator leads from wire harness right?

I have ohmite 100w 1ohm resistor. Use digital meter on AC volts, quality meter $60. I have old Radio Shack analog meter. Reads same.

I have only tested 2 wire alternators. Shop Manual covers 3 wire testing, but I've never done it.

Exactly what alternator? Year of motor?

The specs change for different 2 wire alternators.

My '73 Tiger all original is tested 3k rpm per Manual. I also tested '73 Bonnie with original Triumph Lucas rotor & SPARX 10A stator. Both tested the same 9v AC @ 3000 Higher rpm goes higher. The resistor gets hot fast so I didn't use really high rpm.

Looking at your volts & rpm my readings were very close to yours. I tested mine just to know.

The '73 Bonnie had smoked a Wassell 16A two wire stator & Tympanium regulator failed allowing volts to go to 20 ish. Stator still charged, but was melted...

New SPARX 10 with new Lucas 4 wire 100A regulator & checked output. Was good similar to yours.

My bike works good in real life. The '73 bonnie has covered several hundred miles since repairs & continues to work good.

The great thing about this test is it tests both rotor strength & stator output at same time. So if good both are good. If bad you must substitute know good parts & recheck.

In this test set up volts = amps. Shop Manual gives specs in volts. Go over shop Manual carefully looking at spec sheet for alternator at hand. Shop Manual just used regular analog AC volt meter. I found my digital meter & analog read the same. As you observed output is very rpm sensitive.

Yes alternator is just barely large enough to handle the load. Fine for country riding. Extended city riding or extended stop/go riding the voltage will go low. They can effect spark at a point especially with EI. Telltale your getting low is flash speed of turn signals if equipped. LED bulbs can really help in city riding. At night headlight gets dim. Bright when revving though. 3 phase alternator is only real cure. It charges higher at much lower rpm. A good battery is a must for city riding. Many of us are now using LED BPF headlight bulbs. We haven't upgraded to 3 phase. In real life riding in our conditions, much city with canyon roads, some freeway the bulbs make it workout fine.

Curious to know what bike & alternator you have. Riding conditions? Are you having problems with low battery? It looks like your stator output is good.

Want some fun? Test the zener exactly per shop Manual with amp & volt meters hooked up. Most interesting. It really does work. Test with incandescent head light on & off. What's your finding? That will show you what you want to know. If you have electronic regulator or no amp meter volt meter will show the volts of course which is enough to see where you're at. What fun is that?

Want more fun. Hook DC volt meter & a 21w or so light bulb to same alternator leads. Be sure to insulate the chassis connectors. Bike started on battery power alone. Remember don't let alternator wires touch anything on bike. Start motor & rev throttle. Volts goes through the roof. My 12v bulb burned out about 25v. It will go way over 30v.

Fun stuff! Don
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Hi Randy,
Originally Posted by Randy68
test consisted of connecting a 1-ohm 100W resistor in parallel with the alternator, measuring the voltage across it (and calculating the current through it)
Obviously the current is the same.
Uh-uh. Resistance is essentially the constant; reason for using specifically a 1-Ohm resistance is the AC Voltmeter connected across both resistor and stator also shows the Amps - the relationship is Ohm's Law, Volts = Amps x Ohms; i.e. if "Ohms" is 1, Volts = Amps. thumbsup

Originally Posted by Randy68
2K RPM (6.2V) & 4K RPM (10.4V).
So the generated Amps are increasing with rpm.

Originally Posted by Randy68
guessing that something around a 5 amp load is probably about all the "real world" load my alternator can handle without draining the battery.
confused Depends on the rpm; as your figures show, your bike's alternator appears to be generating ~10A at ~4 Krpm. "NickL" might chime in with an idea of the rectifier's efficiency? E.g. if it's 90% efficient, the alternator's ~10A should be ~9A DC?

Hth.

Regards,

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Thanks everyone for your feed back. I apparently caused some confusion when I said "Obviously the current is the same" I didn't mean over the RPM range.
What I ment was since I=E/R and R=1 obviously the current is the same. (6.2V/1R=6.2A) (10.4V/1R=10.4A) I checked my DMM and it is a True RMS unit so that answers that question.

TR7/RV. I'm a novice poster and don't know anything about PM-ing. Would like to email you though and maybe share some pictures if I can figure out the PM thing.
My bike is a 68 Bonneville with the original 2-wire alternator. I've replaced the rectifier/zener with a Podtronics rec/reg and replaced the points/aau
with a Pazon EI. I also replaced the original incadesent headlamp with a LED H4 bulb (with new reflector) and added incandesent turn signals. The Tail/Brake light is an incandesent 1157. My constant load while riding is: 2.5 amps. H4 (low beam) 1.75A + .75A tail. The brake light adds 2 amps when on. and the turn signals add 1.5 amps 1hz 50% duty cycle. So my worse case max load is around 6 amps. Most my riding is in the country so, like you, I haven't had any electrical problems. This adventure is mostly for the fun of knowing. A short back story. I got into the alternator stuff because I found pieces of a timing chain roller in the oil during an oil change. That "oil change" resulted in a new timing chain, new clutch plates, new clutch basket, and new shoe. Ain't that how it goes?

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Hi Randy,
“My bike is a 68 Bonneville with the original 2-wire alternator. I've replaced the rectifier/zener with a Podtronics rec/reg and replaced the points/aau
with a Pazon EI. I also replaced the original incadesent headlamp with a LED H4 bulb (with new reflector) and added incandesent turn signals. The Tail/Brake light is an incandesent 1157. My constant load while riding is: 2.5 amps. H4 (low beam) 1.75A + .75A tail.” (are you sure that the LED headlamp low beam draws that much?).

You have omitted to factor in the demand of the Pazon, this will be an additional 3-4A depending on the coil(s) used.
So using your figures for the constant lighting load (2.5A), the constant running load with lights on is ~6A.
With an original incandescent headlamp bulb, you could add ~2A to that figure.

The workshop Manual test for alternator output using the 1 ohm resistor is at 3000 rpm, displaying 9V min (=9A min) for satisfactory function (though supply only marginally satisfied demand with standard electrical equipment, unless you were whizzing around at higher rpm most of the time).

Your measurements at 2000 rpm (6.2A) and 4000 rpm (10.4A) suggest by guesswork interpolation that you’re pretty close to the defined requirement (the generating curve initially rises steeply, then almost exponentially tails off to a limit at higher rpm).

The lower demand of the LED h/l bulb you are using, it looks like you’re in “generating credit” from ~2000 rpm, so you should be laughing.

I wouldn’t factor in the demands of occasional consumers like brake and indicator lamps, that’s what a battery is for, and you have plenty of surplus generating capacity to support that battery.

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the current (reading ) is the same as the voltage reading... under a 1 ohm load ... the circuit voltage is a function of resistance
1 ohm is chosen because it simplifies the math ... (watts) P = V²/R ... and where R=1
the divider is eliminated
so the formula is reduced to V² = P (watts )

6.2 volts is 38.4 watts
10.4 V. is 108 watts ...
the average of the two is 73 watts ... which you could expect in stop and go traffic at lower rpms
... and then will be skewed towards 100 watts or more
when ridden on the open road .
the battey may or may not be fully charged at the end of a ride , depending on the road taken and
the headlight load . ( and the health of the battery )

if you are the type to throw a battery maintainer on berween rides ,
You don't have to end a ride with a full battery , an acceptable goal may be only 1/2 a battery charge .

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I found a handy tool for testing Zeners at Parts Express. It's a 1.5 to 15V variable power supply with built in ammeter. It's only rated at 2A, so by the time you get to 14.5V the protection circuit cuts in. But, by that voltage you've learned all you need to know.


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Some interesting contributions of dubious relevance.

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Good catch on those coils Koan58. Totally forgot about those energy hogs, Out of sight out of mind I guess.
And lets not forget about the efficiency of that Podtronics rec/reg hiding under the seat. I doubt they put that big ol heatsink on there for looks.
At this point I think I'm just going to hook up an amp meter and record the readings at various RPMs.
I was surprized by the current draw of that H4 LED on low beam. They advertise it as a 25 Watt bulb- but I expected it to be much less than that on low beam but it's not. Both current and light output are very close on high or low.

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Hi Randy,
Personally I find a voltmeter more informative than an ammeter.
Anytime the system voltage is above normal battery voltage (12.65V) then the power generation is exceeding demand.

I have a digital voltmeter permanently installed in my console, which is fine on a totally non-original bike like mine.

This would not be appropriate on a classic such as yours, but I have often improvised using a multimeter strapped to the top yoke or fuel tank.

Not only does this inform of the battery status before starting the engine, it clearly illustrates generating performance in real riding time.

Of course an ammeter showing charge/discharge of the battery gives some useful information about how the battery is being treated, but it is the system voltage that determines how well the electrical system functions (even if no battery is connected).

For what it’s worth, BMW boxers had a voltmeter as standard.

Out of curiosity, could you give a link to the LED bulb? Thanks.

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The actual ignition load is more like around 2 amps.
The coils are not 'on' all the time.
With an EI you also have the reduced voltage applied to the coil as the output stage
will drop approx 1.5-2v so with a 14.2v supply the coil will be working at around 12.5v
The ratio of dwell will determine coil current. The coil's first few of milliseconds on
is where it draws maximum current and also as it warms up the current will drop.
If you allow 25watts for the twin's ignition system you will be close enough.
The better EI's control dwell and will give a better suited dwell resulting in less overall
load at normal riding speeds. The huge dwell that basic systems and points allow at lower
rpm result in heating the coils up, nothing more.
The rectifier will have a drop of around 1.5v rising as it heats up. A single silicon junction has
a drop of around 0.6v @ 25deg.
Stuart and the others are right about the 3 phase type stator being the way to go if doing
a lot of town riding, the fact that it will develop a more suited output at 2-3k rpm is good
especially these days when you tend to need your lights on all the time.
As for voltmeters and ammeters, the avearge Lucas ammeter is better named as a vibration monitor
and serves little purpose, a voltmeter will show battery/system status which is what you want to know.
Very few automotive firms used ammeters with an alternator charging system.
Just my opinion.

Last edited by NickL; 03/28/21 1:47 am.
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All you need to do.....start bike with charged battery, hook up voltmeter across battery terminals. increase Rpm to 3000 with headlight on and hold it there for a minute ,got 14 .2 volts or so? Increase to 4000 and check again....Ok? Then remove meter and ride..


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Thanks again to all of you for your input! It's been fun and informative reading your replies.
To wrap things up- here are the measurements I took on my bike.

1968 Triumph Bonneville with original 2-wire alternator:
Prontronics Rec/Reg, Pazon Surefire EI,
H4 LED Motorcycle Headlight, original 1157 tail lamp bulb
Note- Purchased Head lamp on Amazon $22.95 (Nilight 20035H NIXMEN H4/HS1 motorcyle LED Headlight.)
It's still snowing here in northern Michigan so no road test but from what I can tell in the garage the new headlight seems like a
big improvement over the original bulb at 1/2 the current.

Fluke 115 DMM Across battery Terminals,
Cheap +/- 15Adc panel mount amp meter in series with battery

MOTOR OFF, LIGHTS OFF
Ignition Switch Off
battery= 12.13V, 0A
Ignition Switch On, Lights Off.
battery= 12.11, Approxmently 0.030A (ie VERY small needle movement)
This surprized me as I was expecting to read current through the primary coil windings.
Maybe due to the Pazon EI?
Ignition Switch On, Lights ON
battery= 11.8V (slowly dropping), -2.5A

IGNITION ON, MOTOR RUNNING, LIGHTS OFF
Tick-Over (800-1,000 rpm) battery= 11.9V (slowly dropping), -1A
1,500 RPM battery= 13.5V, +1A
2,000 RPM battery= 13.9V, +1A
3,000 RPM battery= 13.9V, +1A

IGNITION ON, MOTOR RUNNING, LIGHTS ON (LOW BEAM)
Tick-Over (800-1,000 RPM) battery= 11.8V (slowly dropping), -2A
1,500 RPM battery= 12.2V, 0A
2,000 RPM battery= 13.3V, 1A
2,500 RPM battery= 13.9V, 1A
3,000 RPM battery= 13.9V, 1A

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Hi Randy, Thanks for the follow.

Hillbilly is correct. If voltage reads good running lights on & off it's good.

Several years ago I zip tied foam pad & volt meter to bars. Went on 100 mile ride with an hr. of city riding. Law here is lights on all the time. '73 Tiger, Motobatt MB9U battery. All stock lighting.

I learned a lot. You'd better figure stop lamp & turn bulbs into consumption for urban city. . Lots of long lights, nearly every block. Lots of turn signal use & most often need to hold brakes at stop as road is not flat. City riding after 30 min, the voltage is really going lower at standstill. No chance for recovery between lights. Stop/go freeway even worse. Move 20' & stop for even 2 hrs. Not to mention engine heat.

I didn't know about good LED headlight bulbs at the time. The only BPF LED on the market then were pretty poor. I'll even say awful. Pilot bulb was already disconnected to save juice.

Here's the crazy part. Just replacing the 2 instrument bulbs was a decided improvement on follow up road test. Savings of just under 6w. That's how close the balance is from factory.

The new 16 LED BPF shine at least decently. They use much less current. Between gauge bulbs & LED headlight I can get a good hour which is enough.

LED tail/stop bulb works good on earlier lens light unit. That saves a lot. One friend has LED tail, 16 LED headlamp on '70 Tiger. Another the same on '68 Tiger. Basically like running with no lights on. These are the bayonet base LED bulb. Not circuit board. The light emitted looks good day or night to me. I've followed them hundreds of miles. Easy to tell tail light from brake light. Both look correct brightness. I've not seen circuit board operation in person.

Pretty much all my riding friends are using BPF headlights & some tail lights. All with blinkers still using incandescent bulbs.

The later large square tail lamp like I have the bayonet LED bulb points down, not out. LED doesn't look as good. The side light shines red/pink on license plate also.

So long as your old alternator is working as it should, LED bulbs can be a viable long term band aid to 3 phase. The band aid has worked very acceptably for us for some time now. If you need a stator, a good time to move to 3 phase.
Don


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Hi All, Thinking about the volt drop of .6 or .7v per diode in the old disc rectifier. Indeed that is normal & expected. I've measured that several times. Poor contact with end discs with center bolt/ground lug adds to the drop of course & is very common in old age. Tightening/cleaning cures that drop.

Here's a question I've wondered for some years. What's the actual volt drop inside electronic regulator/rectifier? They are still using something to convert AD to DC. In the few I've tested the actual output seems fairly close to the disc rectifier.

On the road in real life, it doesn't seem to matter. So long as they are working good, old or electronic systems keep battery charged the same. This is wire alternators. I've never tested 3 wire.

Reliability is a whole other subject. I'm meaning if both systems are in perfect condition, including zener, wiring, ground, connections etc. & battery of course.
Don


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Originally Posted by TR7RVMan
The later large square tail lamp like I have the bayonet LED bulb points down, not out. LED doesn't look as good. The side light shines red/pink on license plate also.
One must be careful when selecting an LED tail light lamp. The ones sold at auto parts stores don't shine in all directions. They provide decent light for the tail/brake, but almost nothing to the license plate with the lamp pointed to the rear. Of course, most of those are common negative only.


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Originally Posted by TR7RVMan
H

Here's a question I've wondered for some years. What's the actual volt drop inside electronic regulator/rectifier? They are still using something to convert AD to DC. In the few I've tested the actual output seems fairly close to the disc rectifier.

Don


The rec/reg includes a bridge which effectively is two silicon junctions, so around 1.2-1.5v drop.
With larger devices the drop may be larger but if you allow around 1.5v you'll be close enough.

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Hi NickL, Thanks!! So about the same as the disc one.
Don


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