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Aftermarket Alternators
Installation, Capability, Performance

Copyright, 2014, R. Fleischer

Note:  Read 15-A and 15-B, first!!!



Discussion:  (much of this discussion is pertinent to ALL charging systems!!):

Measuring or specifying output in watts is not as simple as it may seem.   The simple part is that watts is equal to voltage multiplied by amperes. 

Unless the output wattage of the alternator system is specified at a practical usable rated voltage, it is possible for the output voltage to sag from a large load (big headlamp, driving lights, heated clothing, whatever), while at the same time the output current could possibly be high.....OR NOT.   If the output voltage was too low to keep the battery fully charged; yet the current availability was high, it is possible for false wattage advertising.  What is really important is the real output with a "reasonable and usable" charging voltage at the battery. 

You want the battery to remain charged or be recharged quickly after starting the bike or perhaps from long periods of waiting at stop lights... and then riding off.   If the battery terminal voltage falls to 12.7 or below whilst on the highway, that is not enough voltage to truly maintain the battery at a full charge.  You need a higher charging voltage to enable the battery, due to inefficiencies in the battery chemistry itself, to be FULLY charged.  It becomes more critical when doing stop and go riding in, for example, city traffic.   At the time you stop where you are going, be it work or home or other, you want the battery to be charged fully.

An aftermarket system should not only have MORE TOTAL REAL WATTS available, but the system should properly maintain the battery under typical operating circumstances.  This includes idling for periods of time in stop and go traffic.  Even if the voltage sags some at a stop light, the alternator desirably should be capable of fast recharging.

The system should be capable of handling all the electrical items likely to be in use at the same time for reasonable periods of time.  Idle rpm output, or slightly above idle rpm output, should be sufficient for normal use, as this is one of the complaints about the stock system, in stop and go commuting use.
NOTE that the idle rpm of Airhead motorcycles should not be below 900 rpm, no matter what you read elsewhere's, and no matter what alternator, stock or otherwise.  I highly recommend a target of 1000 to 1050 rpm for idle rpm, after a full engine warm-up (5 miles or 6 minutes perhaps) at the normal altitude and temperature you ride in.    This rpm was selected by me for a variety of factors, including carburetor synchronization, etc.

I did a careful installation & testing to evaluate the practicality of the ORIGINAL EnDuraLast system.  A known perfect battery was used. The system used light electrical loading & was allowed to fully charge the battery according to the system voltage regulation (14.24 in this conversion instance).  Then the battery was loaded moderately for a while until a known value of battery DEcrease in voltage was noted.  At this point the output was measured by starting the engine (which additionally slightly drained the battery), & then I used, quickly, various rpm during immediate measurements.  That is a simplification of the various tests I did.  My method for testing other alternators, whether aftermarket or stock, is the same, although the voltage noted above might be different, depending on if a fixed setting, or if it was adjustable...and if so, if I adjusted it.

The charging characteristics of batteries of different types varies rather a considerable amount.   For MOST types of lead-acid batteries, if one applies a voltage of about 12.8, constantly, eventually...and this could take a LONG LONG TIME... the battery would reach nearly full charge and then require very little current, from the charger or system, to maintain that stabilized voltage.  For most batteries, 12.8 volts at room temperature is very close to a full charge (but not what is called a higher 'surface charge', which is unimportant here).   

Nerdy:  If the applied voltage is raised, within reason (not exceeding a maximum value, often nearly 15 volts), the battery will reach the new applied voltage & then the charging current needed to maintain that voltage will be higher than at a lower voltage...that is, the charging current needed will be progressively higher for a stabilized higher voltage.  Some battery types are quicker to convert chemically to a full charge.  I deem this characteristic relatively UNimportant.

Batteries vary rather WIDELY as to the current needed from the voltage source for any one specific voltage.   Generally speaking, a typical 'flooded' type of battery of motorcycle size (I will use the larger motorcycle size, about 28AH, as illustration), will need about 2 or 3 amperes to maintain about 14.1 volts AT the battery TERMINALS.   Some Gel and AGM type batteries are quite different. 

A more important thing to know is that the battery REQUIRES higher than 12.8 volts in order for the battery to have a LONG LIFE....even though at 12.8 volts the battery is likely nearly fully charged.  Battery chemistry is such that charging to a higher voltage, and THEN backing off (cutting-back) to a lower stabilizing voltage, is how the best chargers work.   This is for battery chargers you plug-in from your wall socket; and these very specific types are called Smart Chargers. This type of charging can lead to a long lived battery.  NOTE that SIMPLE chargers will always back off on charging current as the charging battery's terminal voltage rises.  This is because the charger has a maximum open circuit voltage.  Unfortunately, there are tradeoffs involved with simple chargers.  I won't get much into that here, suffice it to say that if the simple charger has a high rated output current, it will taper off less, and could injure the battery if left connected and powered, for long periods of time.

UNDERSTAND THAT NONE OF THIS IS HOW THE CHARGING SYSTEM IN THE BIKE operates!!THE ONLY THING THAT TAPERS OFF (or cuts-back) IN THE BIKE'S SYSTEM, IS the maximum voltage, and that there is a small change in charging voltage built into the regulator to deal with the temperature change (ASSUMED to be battery temperature).  A battery requires more voltage at lower temperatures.   For the regulator at room temperature, the bike's system will try to keep the battery at a high floating charge level, perhaps around 14.2 volts (this could be 13.8-14.7, depending on VR and other things), when the rpm is high enough, assuming enough alternator output capability.  As temperature drops, the VR output should increase SLIGHTLY, and vice-versa.  A tenth or two tenths of a volt is typical, with maybe another tenth if the temperature is near freezing or somewhat below.

At the time I am updating this article, on October 22nd, 2014, I have still be unable to arrange the testing of the latest EnDuraLast alternator, nor, the latest Omega, therefore testing results are for the original models.

BMW's original VR settings for the stock bikes is ~13.8 volts.  I have always felt that BMW was WRONG in selecting 13.8 volts but I understand why; which has to do with water use from the flooded batteries, and some minor other things.  The Authorities (Police) VR is set higher than the EnDuraLast, however....nearly 14.5, depending on temperature!  Most voltage regulators have a voltage versus temperature effect, purposely built-in. This is because the voltage needed to maintain any given charge percentage on a lead-acid battery will vary with temperature, AND, the chemical conversion works faster with a slightly higher charging supply voltage, if the battery is COLD.  The COLDER the battery, the more voltage is needed from the system.  The voltage regulator, ideally, is mounted in the same airflow as the battery.  Generally, only a few tenths of a volt is the temperature compensation, from perhaps 120F down to perhaps 20F.....the typical range of temperatures seen by the battery and VR.  Some higher temperatures (and lower) are seen sometimes, and one can add a few tenths of a volt if going lower, subtract a few if going higher.  Motorcyclists rarely ride where battery temperature will exceed 140F (typically from engine heat blown over the battery).  Some ride when the thermometer is below freezing.  As the battery temperature falls towards freezing and below, the battery has a much more difficult time (it is, at its core, a chemical reaction device) producing the current needed to start the engine...which has higher friction due to cold oil and decreased clearances; and the starter motor is going to pull a lot more current.  The chemical conversion of electricity gets more sluggish, and this is not only in battery delivery of electricity, but in re-charging, hence the higher charging voltage.

The STOCK 280 Watt Bosch Alternator:

The following is TRUE ACTUAL USABLE OUTPUT of the stock 280 watt Bosch alternator,
using 3.7 or 2.8 ohm later model rotor; known good connections, wiring, battery, switch, etc.



Output, amperes







Approximately the point of equilibrium with the stock system drain of lights and ignition



see above.  5 amperes is available for charging



MAXIMUM available, battery at 13.5 v, and rising.  This is 270 watts.  Very slightly more will be available as the voltage rises a bit more.
    watts = voltage multiplied by amperes

NOTE that the stock alternator produces what Bosch specified.  Note also that the voltage is a decent value, well above the minimum needed by the battery, to maintain a FULL CHARGE, at maximum output.

The "ORIGINAL" first generation "400" Omega Alternator, as sold by Motorrad Elektrik:
Note that there are two sources for the GENUINE Omega Alternators (mfr by Jeff Lee of Emerald Island...very high quality....);
those sources are Motorrad Elektrik and Beemershop.

The tests on the original Omega were not as extensively done as on the temperature, oscilloscope, stress gauges, hot/cold differences, etc.  Still, what follows for the early Omega is probably what YOU can actually expect of it.

Testing was with a known accurate ammeter with relatively short heavy gauge leads in series with the Omega diode board output.   The battery was drained somewhat to begin with (on purpose); and, a digital voltmeter was placed across the battery terminals.   Readings were taken well before the battery was fully recharged, with heated grips, accessory lights, and headlight all turned on.   Readings were repeated with lights off, and also with the battery fully charged.   The "set point" of the VR appeared to be close to 13.8 volts.
NOTE that the chart below is a summary, and where you see two instances of a higher rpm with lower voltage, that was due to battery charge condition at the instant of measurement, and small deviations from expectations, due to, I think, heating and other effects.



Voltage at battery

Output in amperes


1550 11.70 2 23.4
1700 12.5 5 62.5
1750 11.83 8 95.6
1750 12.5 5 62.5
2200 12.8 12.5 160
4000 12.64 28 354
4000 13.5 20 270
5000 13.2 31.5 415.8


NOTE CAREFULLY, the two 1750 and the two 4000 rpm tests. Note how the wattage varies with the voltage.  I specifically loaded the system in order to get these readings.

You can see that the battery would be MARGINALLY CHARGED at 4000 rpm if 28 amperes of load was used.   This resulted in 354 watts.   If the load was DEcreased, the voltage rose to 13.5 volts.  That is STILL not enough floating voltage...I'd prefer to see 14 volts or even a bit more.    Increasing rpm beyond 5000 did not yield increased output.

If you think about it a bit, you will see how that chart, above, tells you how a manufacturer might 'cheat' on alternator wattage output rating......or, conversely, you would know that you do not want to use excessive loads at times. But, there is another way to think about this.   Even though the voltage might not be high enough at the battery for optimum battery longevity, the battery will still be charged enough!....even at 415.8 watts being put out by the alternator.

NOTE:  output might be better if the VR was set higher, the voltage regulator was NOT the adjustable version on this tested bike... and if a fresh battery had been used.  Note also, that the ammeter and its leads and connections add a very slight series resistance, and thus would have the effect of moving the output higher up in rpm (although slight).   This also applies to the Bosch and EnDuraLast when tested.   While voltage multiplied by amperes is how one converts to watts, it is entirely possible, under some circumstances, for a battery not 100% charged, to have a lesser or greater current input at a particular terminal voltage....and as such, the USABLE system wattage is a bit difficult to interpret to the layperson.   

It is interesting to compare these figures with the stock Bosch system.   While it is possible that there are some anomalies present that I did not go into, especially noted is that the stock Bosch equipped bike I tested had perfect electrical connections, etc.....still, the Omega output seemed low at low rpm, and then rapidly overcame the stock system as rpm into the cruising area was attained.  It is very important for some folks to understand that the output of the Omega is ultimately HIGHER than the EnDuraLast.....but that the EnDuraLast has much higher output at truly low rpm....and, thusly, is PROBABLY better for COMMUTERS.

NOTE:  The weight of the Omega is nearly identical with the stock Bosch....differences are so small as to be of no concern.

The 'new' higher powered '450 watt' Omega:
This unit has a 81 mm rotor (instead of the original Omega's 76 mm) and a larger stator to match.
When tests are run on a 450 system they will be posted here.

SAME, for the EnDuraLast similar to the Omega.

The EnDuraLast Alternator (the original, with the PERMANENT MAGNET ROTOR):

The weight of all removed stock Bosch components:  diode board, stator, rotor, housing, regulator, some wiring, etc., is 87 ounces; all the EnDuraLast items as installed will weigh almost exactly the same.   For the Omega, as noted above, weight is also about the same as the stock Bosch.

Leakage current:  under 1.5 MICROamperes, worst conditions.

Rotor diametrical clearance to stator:  0.006-0.008".  This can vary some with the components, as well as the exact fitment of the inner timing chest to the engine casting, which is adjusted slightly during a timing chain or other similar R/R job.
Rotor runout, axial: negligible.
Rotor lateral runout (side to side):  0.00075" maximum.

Special note on the EnDuraLast Rectifier/Regulator:  In a normal system in a motorcycle, the regulator is USUALLY mounted in an area that allows some engine heat (comparable to battery temperature in a modest way), to influence it.  In most modern cars the the regulator is usually PART OF the alternator ITSELF, and thus as the alternator warms up, the regulating voltage DEcreases slightly, on purpose, to match battery temperature characteristics.   This is not necessarily so on this conversion; depending on where the regulator is mounted, and HOW (it generates heat internally).  However, from my testing results, the EnDuraLast RR unit is OK over a normal temperature range.  

Note that there have been reports of RR unit failures when high levels of alternator output are consistently used.  The RR unit CAN get pretty hot.


The EnDuraLast voltage regulator is supposedly internally fixed at about 14-14.26, and hence a small amount more of the EnDuraLast output is used to keep the battery charged, once charged, than with the stock voltage regulator setup, this is truly a minor thing.

ACTUAL results for the EnDuraLast:

RPM Current, Voltage Watts
900 6 amperes  
1000 7.5 amperes, 13.12 volts 98.3
1000 8 amperes  
1200 12.5 amperes, 13.39 volts 167.4
1500 15 amperes  
1600 16 amperes  
1900 18 amperes, 14.24 volts 256.3
2050 20 amperes, 12.81 volts 256.2


26.5 amperes, 12.8 volts 339.2

I was UNable to obtain more watts, no matter the rpm.   Over 3500 rpm was NOT needed.
Note that for the otherwise stock motorcycle, no additional electrical loads, the EnDuraLast alternator will maintain a reasonable charge at about 1050-1100 rpm.  This means that commuting is no longer an appreciable problem.  Note that the EnDuraLast has more usable wattage output at very low rpm, compared to the stock Bosch, or the 400 Omega.  However, the Omega has higher absolute maximum output wattage. Note also that the voltages at high watts output is not actually all that much different, and sometimes worse, than the stock 280 watt Bosch. Compare very carefully!  The advantage of the EnDuraLast is at low rpm.

***The EnDuraLast is better for in-city stop and go, than the stock or Omega alternators.   The Omega is better for maximum output, and for maintaining the preferred higher battery voltage AT such a higher output....a DUAL benefit.

It takes about 1850 rpm to start reasonable actual charging on the Omega, with normal ignition and lighting loads.  The EnDuraLast is going to do this at around 1150 rpm.

It was noted that the
Regulator set point for the EnDuralast that was test, at 84F, was  14.48-14.50 volts, which is higher than expected, and more to my liking than the Omega.  Since the set-point is slightly affected by where the regulator is mounted, due to LEAD LENGTHS between stator and regulator, this voltage may be a bit different, if you have your regulator mounted next to the battery (which is a good place). MY installation was to have the regulator mounted in the fairing air inlet area...where I could control air flow, etc, for my testing.

1981 and later BMW airhead motorcycles; and some earlier models with aftermarket ignitions, are sensitive to electrical noise in the battery supply. That is ONE reason, of several, that the original MECHANICAL VR was changed to an electronic type in 1981.  BUT, some electronics, including diodes and a RR regulating transistor, can CREATE high-frequency spiking type of noise.    Thus, tests with an dual-trace type of oscilloscope were going to be run on the system; one trace monitoring the battery, and one trace monitoring the ignition pulses. 

 NOTE:   My results did not duplicate the specifications/information on output.   I made some minor changes, eliminating some very TINY voltage drops, that helped some.  I have thought about this at some length.   I have an IDEA of what may be PART of the problem... this is theoretical.   The stator output is via TWO wires, with NO grounding reference.  The Rectifier/Regulator unit MUST have a circuit that involves a type of multiple-diode rectifier called a "Bridge Rectifier" .

While a half-wave rectifier could be used, I am 99% sure THAT is not the design....and would be MUCH worse anyway.   A property of a two-wire source to a bridge rectifier, is that the IMPEDANCE (nearly the same thing as resistance) of the STATOR and connecting wires from that stator, can be MUCH higher, with little deleterious effect, than the same increase in EFFECTIVE output resistance.   THUS, the STATOR wires....the YELLOW wires...can have more resistance in them and still get decent output; but, the RED wires output, and the CASE ground output, will need VERY heavy gauge wires, and negligible voltage drop in connectors and connections.
 I WAS also able to get a modest improvement (about 1.3 amperes, nearly 18 watts) by simply repairing just the bullet connector of the RED wires. This was reported to the manufacturer, so that the KIT could be upgraded.  I think that FURTHER improvement might be mounting the RR unit next to the battery due to the very short output-side wires possible.  As such, my recommendations to the makers of the EnDuraLast was to mount their VR/diode assembly next to the battery, perhaps on the battery box itself....which would be a decent heat sink also.  I have not yet tested such an installation...but I anticipate that it could only be an improvement upon mine.

In June, 2015, I had the opportunity to test a permanent magnet type EnDuraLast alternator in a 1995 R100RT.  Installation was of the 'new method', with the RR unit mounted on the left rear frame member next to the battery.  I made sure all connections, etc., were solid and proper, before I load tested the Odyssey battery, and then ran the battery down a few ampere-hours further, before doing tests.  These were simple tests, with a digital voltmeter at the battery; and an ammeter in series with the RR unit's output (by replacing the fuse with jumpers, which were nearly a foot long overall, and not of heavy enough gauge, to give absolute values but adequate for older installations, with some years on them, etc.).  These are representative numbers, therefore, for such.  If large gauge wires were used, or a clamp-on/over meter, and a more discharge battery, particularly if not an Odyssey, I believe the output would have been higher.
8 amperes output at 900 rpm
10 amperes output at 1000 rpm (and, 12.42 volts)
13 amperes output at 1200 rpm
17 amperes output at 1500 rpm
17.5 amperes output at 1600 rpm
18 amperes output at 1900 rpm
21 amperes output at 2050 rpm
I did a further test as the battery began to recharge, but still was not fully charged.
This test was at 2500 rpm, and I got 19 amperes at 13.95 volts (265 watts)....and, after several minutes, noted the RR unit was getting fairly hot.
I did not try for an absolute maximum wattage output, which would have required discharging the battery again, and this time more considerably.

In the real world, lab tests aside, the OVERALL best performance from all the various alternators and for typical usage and conditions..., is LIKELY going to be with battery terminal voltage of about 14.4 volts.  When one considers lamps life and battery life (most batteries 'prefer' a terminal voltage under VEHICLE charging conditions of close to 14.4 volts or higher), and many other factors, one PROBABLY should have the VR set for 14.3 volts for a reasonable compromise.  The temperature should be around 68F for both VR temperature and BATTERY, for that voltage to be correctly measured.   MOST VR's are deliberately designed to increase voltage output when THEY are colder, the assumption being that the battery is also colder.   If one has an adjustable VR, it should be adjusted on a fully charged battery, with all the accessories turned off, and after only a brief time after engine start, otherwise the temperature of the VR will rise.
It is a waste of time to do voltage measurements, adjustments, etc., unless the battery connections are clean and solid, the diode board push-on connections clean and solid (particularly the large red wire on left side), etc.    Solid diode board mountings are needed for the best performance.  The bikes that CAME with such mountings are the /5, /6, and the 1978-1987 R65 and R80, and some others...just LOOK at yours.   Aftermarket diode boards are available to work with the stock Bosch alternators, but most of my comments still apply.

Reliability is improved, if one has a /5, by using a later, better ventilated, front engine cover.


1.  The stock 280 watt Bosch system is adequate for most Airhead riders.   If driving lights and heated grips and other accessories are contemplated, one may well be marginal or negative on electricity generation.   It depends on how much additional load is being actually used, for how long, and what the average rpm is, peak rpm, and idle time.   Very difficult to make a definitive statement, as riders differ in how they ride and use the engine.  If you want some estimates for the type of loads and your usage, I can do that for you, upon request.

2.  For CITY COMMUTERS with reasonable to high electrical loads who do a lot of very short distance stop and go, you might consider the EnDuraLast over the 400 Omega.   For those with similar electrical loads, or higher, but who do mostly open highway riding, the Omega will produce the additional electricity that might be needed.  There is no question that the EnDuraLast Permanent Magnet unit is better for in-town short distance stop and go riding.

 I may modify recommendations after testing the 450 Omega and other alternators.

 The Omega has the maximum output edge.  This means the Omega would be better for those with very large electrical loads but who are not necessarily city commuters.  I may modify this paragraph after I test the 450 Omega.

rev:  01/27/2008...checked for clarity
02/06/2009:  mention of the Omega 450
09/16/2009:  update article for clarity, numerous places
05/07/2011:  Clarifications for comprehension, entire article
09/17/2012:  Minor updates. QR code added; Google ad layout changed.
2013:            Remove language button, due to problems on some browsers.
10/22/2014:  Minor updates, nothing substantial....but I did include some commentary about
                    real-world usage, at equilibrium and slightly above, that I had not emphasized previously.
06/16/2015:  Addendum


Copyright, 2014, R. Fleischer

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