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BMW Motorcycle Aftermarket Alternators

Aftermarket Alternators: Installation, Capability, Performance
AftrMrktAlt.htm

15-C

©


Note:  Read 15-A and 15-B first!!!

 



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

   This is not as simple as it may seem.   In simple output current tests, unless the output voltage is fully specified, it is possible for the output voltage to sag, whilst the output current could possibly rise very fast.  OR NOT.    This would be false wattage, as what is really important is what output is available with a "reasonable" voltage at the lead-acid BATTERY.  The main concern of most riders is that the battery will remain charged.   If the battery terminal voltage is 12.7, that is enough for a rider's purpose, even though the battery will not have any 'surface charge' at that point.  Anything above 12.7 would be gravy.  This is probably the most critical part of in-city commuting.  After all, a major purpose of an aftermarket system should be to not only have MORE watts available, but..to  maintain the battery charge under typical operating circumstances, including idling in stop and go traffic (except with much larger than stock electrical loads), as well as cruising down the road...and...extra lights, etc.   Any idle or slightly above idle rpm output should be sufficient for normal use, as this is one of the complaints about the stock system, in commuting use.    I did a careful installation followed by testing to evaluate the TRUE practicality of the EnDuraLast system, in that a known perfect battery was used, the system used at light loading to fully charge the battery according to the system voltage regulation (14.24 in my conversion), and then the battery loaded moderately for awhile until a known value of battery DEcrease in voltage was noted, at which point the output was measured by starting the engine and using various rpm, rather quickly.  That is a simplification of the various tests I did.  
   The charging characteristics of batteries of different types varies widely.   For MOST types of batteries, if one applied a voltage of about 12.8, constantly, eventually the battery would 'pull' very little current to maintain that stabilized voltage.   As the applied voltage is raised in increments, within reason for a maximum value, the battery will eventually reach the new applied voltage, and at that new stabilized point, the charging current will be progressively higher at that stabilized higher voltage.   Batteries vary WIDELY as to the current drain from an applied voltage source.   Generally speaking however, a typical 'flooded' type of battery of motorcycle size (I will use the larger size, about 28AH as illustration), will 'pull' about 2 or 3 amperes to maintain about 14.1 volts.   Some Gel and AGM type batteries are quite different.   The EnDuraLast voltage regulator is internally fixed at about a 14-14.26, and hence a bit more of the EnDuraLast output is used to keep the battery charged, than with the stock voltage regulator setup.  That is because BMW's original VR settings for the stock bikes is closer to 13.8.  I have always felt that BMW was WRONG in selecting 13.8, but understand why.  The Authorities (Police) VR is set higher than the EnDuraLast, however....nearly 14.5, depending on temperature! 

 

The STOCK 280 Watt Bosch Alternator:

The following is the TRUE ACTUAL USABLE OUTPUT of the stock 280 watt Bosch alternator, using the 3.7 or 2.8 ohm later model rotor and known good connections, wiring, battery, switch, etc.
RPM            OUTPUT in Amperes
1050                  2.5
1550                 10.0 (point of equilibrium with stock system drain)
2100                                             15.0
2850                                             20.0, = maximum, battery 13.5v. and rising.  This is 270 watts.

NOTE:  Watts = voltage multiplied by amperes

 

 

The Omega Alternator, as sold by Motorrad Elektrik:
The tests on the Omega were not as extensively done as on the EnDuraLast...no temperature, oscilloscope, stress gauges, hot/cold differences, etc. Still, what follows for the 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 a higher rpm with lower voltage, that was due to battery charge condition, and there are small deviations from expectations, due to, I think, heating effects.
RPM                          VOLTAGE AT BATTERY            OUTPUT IN AMPERES                WATTS
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:  output did not increase above that point in our measurements, but it is possible that it could, if run at higher rpm, and with some very specific attention to the voltage regulator (which was NOT the adjustable version on this tested bike) and if a fresh battery had been used.  Not also, that the ammeter and its leads and connections would have the effect of moving the output higher up in rpm, than the stock system.   This also applies to the Bosch and EnDuraLast, however.   The ultimate output is also somewhat affected by connections and leads resistance.....and the battery terminal voltage has an effect on calculated watts in a peculiar way on all of the charging systems.     That is, while voltage multiplied by amperes is how one converts those 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 wattage calculated is 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 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.

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

 

 

The EnduraLast Alternator, as sold by Euromotorsports, John Rayski:

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

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, purposely, mounted in an area that allows some engine heat (comparable to battery temperature in a modest way), to influence it. That is not universally true with all.   In most modern cars the placement of the regulator is most-often PART OF the alternator ITSELF, and thus as the alternator warms up, the regulating voltage DEcreases some, purposely, 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).  From my testing, the EnDuraLast RR unit is OK over a normal temperature range.  

  ACTUAL results for the EnDuraLast:
 RPM    current output TO BATTERY             Watts  

900          6A 
1000         7.5A, 13.12V                      98.3
1000         8A
1200         12.5A  13.39V                    167.4
1500         15A
1600         16A  

1900         18A and 14.24V                  256.3
2050         20A and 12.81V                  256.2
MAX          26.5A and 12.8V                 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.
It was noted that the Regulator set point at 84°F was  14.48-14.50 volts.

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.  BUT, some electronics, including diodes and a RR regulating transistor, can CREATE hifrequency spiking type of noise!    Thus, tests with an oscilloscope are going to be run on the system eventually; one trace monitoring the battery, and one trace monitoring the ignition pulses.   This has NOT YET been done. NOTE:!!!!   This is a theoretical comment.   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...again, this is theoretical.   The stator output is via TWO wires, with NO grounding reference.  As such, then, the Rectifier/Regulator unit MUST have a circuit that involves a type of multiple-diode rectifier called a "Bridge Rectifier" ((Whilst 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 sane 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 able to get a modest improvement (about 1.3 amperes, nearly 18 watts) by simply repairing just the bullet connector of the RED wires.  I think that FURTHER improvement, I know not how much, might be possible....and that mounting the RR unit next to the battery will offer the BEST performance, in this particular regard....due to the very short wires possible, the elimination of some connections/interfaces, and so on. 

 

In the real world, lab tests aside, the OVERALL best performance from all the various alternators and for typical usage and conditions..., is LIKELY to be with a battery terminal voltage of about 14.4 volts.  When one considers lamps life, 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 a VR set for 14.4 volts (perhaps 14.6 cold, 14.3 hot, or around these values, with perhaps closer to 14 under very hot conditions).  Temperature here means VR temperature.   VR's are deliberately designed to increase voltage output when they are colder.   If one has an adjustable VR, it should be adjusted on a fully charged battery, with all the accessories turned off, and after some engine running time at cruise, so that the VR is as I mention here.  As accessories are turned on, and/or battery not fully charged to the voltages mentioned, the output of the alternator MAY decrease (in a FEW instances, increase).   This is tricky to explain, so I won't try.

Conclusions:

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.

2.  For COMMUTERS with high electrical loads who do a lot of short distance stop and go, I recommend the EnDuraLast over the Omega.   For those with similar electrical loads, but who do mostly open highway riding, the Omega will produce the additional electricity that might be needed....and has higher maximum output too.

 

EITHER of these systems will perform adequately enough for those that need more electricity.  The Omega has the maximum output edge.


rev:  01/27/2008...checked for clarity

 

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