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altcapability.htm
15-A

All the alternators after the /5 (180 watts) had 238, 240, 250, or 280 watts. There were some variations in the original stock rotors, in the R90S model it was slightly smaller in diameter; and, in last of the airheads, in particular those that had the solenoid valves for the fuel and vapors. There are various combinations of things, not very pertinent here. The Authorities (Police) models had 238 watts similar to the R90S, but was NOT done with the same parts, and did NOT perform the same as the R90S alternator. After about 1990, rotors for the civilian models were 2.8 ohms and produced a bit more at lower rpm, but same at upper rpm.   These drew more current from the system, and probably should not be used with mechanical regulators, and iffy with the earliest electronic (unproven idea of mine).

Except for the /5 180 watt stator assembly which is smaller at 105 mm, and the /5 diode board not having the alternator stator center-tap function (one CAN use the /6 and later board with the /5; but the /5 board used with the /6 and later alternator will REDUCE the output some), all the other parts are interchangeable physically are compatible electrically (maybe that VR ...see above paragraph), and.... with the various rotor rebuilding going on there is no telling, necessarily, what rotor might be in an airhead, without measurements of diameter and resistance.  I have never bothered to measure the physical diameter of all the various rotors. After the /5, the stators are all 107 mm; EXCEPT that some 1974-1975 were also 105 mm size, but were 280 watts. THUS certain 1974-5 STATORS can be installed in a /5, to increase the /5 output by 100 watts...if one uses the /6 and later diode board.

The earliest rotors (this means /5 and /6) had a much higher resistance, close to 7 ohms. These were designed to be used with the MECHANICAL voltage regulator, although an electronic type will work fine with them.   That higher resistance rotor reduced the CURRENT (amperes) that went through the regulator, reducing the wear on the regulator contact points....the regulator being a Bosch mechanical back then, as noted.    YES, that DOES mean that if one uses a later low ohms rotor, that the old mechanical regulator points will probably wear faster.   I have NOT done any testing in this regards. It is possible, due to how the mechanical regulator actually works, that this is a wrong idea, and the mechanical regulator might well work OK on the later, lower resistance rotors. Note that any of the later regulators, electronic, should work fine, and so should most any regulator from a car, that fits the plug and mounting area. Adjustable voltage output regulators are available, and the Bosch metal can electronic regulator that came with some 1981+ Airheads can be made adjustable.
Adjustable regulators at a quite reasonable price are available from Stan Smith at RockyPointCycle.

The alternators with the 3.4 and 2.8 ohm rotors produce output beginning at slightly lower rpm.  The maximum output is UNaffected.

The Authorities models (if you have an ex-Authority, or Police model) produce at even lower rpm....but, unfortunately, will produce about 42 LESS watts TOTAL, than a regular 280 watt alternator. Since I have never had the opportunity to fully test an Authorities alternator, I am not treating it, below, but my guess is that no one will want one, pricey too, unless you are driving only in the city....and then maybe an aftermarket alternator like the EnDuraLast or Omega is the better way to go.....not a bad way if you need more watts and charging at lesser rpm anyway, if $$$.

If anyone has a REAL authorities alternator they want to swap for a higher power standard BMW one, let me know...I have a few rotors and stators on the shelf.    Authorities models had the "high output voltage regulator"....a misnomer really, as it simply had the voltage set slightly higher.. this is easy to do internally on some of the other regulators...it helps a little bit with city driving.

There are some other variables, including dealing with the mentioned setting of the voltage regulator; but, in general, you can expect what follows from the system.   Keep in mind that just because the system is rated at some particular wattage, does NOT mean you will get ALL those watts....typically you will get maybe 13% less; AND, even more losses are possible, and to get even near the maximum usually requires some goodly rpm....upwards of 5000 typically....because YOUR system is likely not as clean, shiny, and perfect, as was the system I specially prepared for the testing information below. Still, if your system is reasonably in decent condition for connections, etc., you can expect the full-rated output at 3500-4000 rpm.

/5:   stock 180 watt unit is adequate for the original lighting, and adequate if the headlight is upgraded to the larger 55/60 watt type.   It will probably handle a heated vest, and maybe a conversion to rear running lights, but that is the practical absolute limit. You will need 4000+ rpm to obtain full output. The 280 watt unit can be installed, if you have the correct 105 mm stator parts from 1974 (approx. 1974, maybe into some of 1975).

/6 and later:   You can expect the system to be adequate for the stock lighting, and can probably add a heated vest and usually heated grips and rear running lights conversions, and probably 80 watt headlight conversion or some other 20-40 watt accessory. That is the absolute limit, however.   I don't recommend that headlight conversion (needs relays if you do it) and heated grips and a vest are roughly the practical limits.

If you have a headlight modulator, that frees up some watts in itself, as the AVERAGE drain is lower. Using a headlight modulator on the original 40/45 watt NON-halogen /5 lamp is a BAD idea.

The problem that usually comes up is that the battery does not recharge fully.   The voltage regulator setting for all regulators is fully described on this website elsewhere's, and somewhat on the Club's .org site, so I won't tackle them here.    Those that ride in town, stop and go, MIGHT have to charge the batteries at the end of the day, overnight on a Smart Charger of some sort. When you sit at a signal light waiting for it to change, you have a fair amount of battery drain at idle rpm, powering the ignition, lights, ETC.   It takes time after rpm's rise enough, to replenish the drain from the battery....it can take a fair amount of rpm too. Those that are cruising down the road for longer distances, above 3500 rpm, likely will have no problems, unless they are pushing the alternator limits, then it will just take longer. In those instances where 5000+ rpm are needed, it is, contrary to information you may have heard elsewhere's, a problem with small additive corrosion and insufficiently clean and shiny connections, and so on.

Here is ROUGHLY what the /6 and later bike can do, with the STOCK lighting and NO 'extras'....remember, I say here "stock"....This information assumes your system is in good condition, with good brushes, shiny tight connections, good diode board grounding, etc.:::
     Depending on system maintenance, and the rotor, you probably will be able to keep the battery adequately charged if you can be at 2800-3200 rpm. That is fairly low for the motor, and it will take longer, even at 4000, to replenish the battery if it is drained from a stop light for a couple or three minutes.   That is correct...it will take LONGER to replenish, even at a goodly rpm, than it took to drain.    If your stock fairing voltmeter INDICATES about 12.5 volts or higher, at any time, chances are pretty good that the battery is reasonably charged.   It would be nicer if it indicated about 13.7 or 13.8 whilst at speed (actual battery voltage is likely 0.3 higher than indication of the fairing voltmeter).

    If you try to use the maximum output of the alternator for long periods of time, you will cause additional heat stress on the diodes.....and the rotor and stator also will increase in temperature.  The diode board is in a very hot area to begin with. The stators tend to be mostly unaffected, but the rotors, being OFTEN accelerated and suddenly decelerated, sometimes violently with poor clutch and throttle technique, tend to not fair as well, and sometimes the windings move and shorts (or opens especially), will occur.   .....later rotors are potted in epoxy, which helps.   The alternator and diode board cooling on the faired models is not as good.   Cooling on faired models leaves something to be desired, and just the later louvered front piece probably helps. The cooling air, heated well, has to go someplace.....and how it gets out, taking some heat with it, is different on the early machines with the scoop vent on top of the starter cover.

Aftermarket alternator conversions such as the EnDuraLast and the Omega will give INcreased total wattage output, do it at a lower rpm; and also produce usable watts at near idle rpm.

REAL WORLD: Whilst the above article gives some generalities, here are some specifics. I want to explain first, what was done:
A 1984 R100RT with Bosch metal can electronic regulator was used. The alternator and diode board were all stock items. The diode board; with known perfect diodes soldering joints, was mounted on aftermarket solid metal mounts, not rubber mounts as came on this model bike. 100% of ALL electrical connections were gone through prior to tests. In particular, the red wires at the starter relay plug were gone over. The starter motor (alternator output goes there) power input terminal was clean, shiny, tightened. ALL connections, everyplace I could get to, were in top condition. The ignition switch contacts resistance was excellent. The battery was a flooded type, in excellent condition. Everything was done to allow measurements to show the very best that a perfect charging system would do. The Bosch voltage regulator was set for 14.3 AT the battery, when the system was running maximum output, light load. This motorcycle has a "3.7 ohm" rotor; the brushes were near new, and the resistance of the rotor through the brushes was 4.2 ohms.

The diode board output (big red wire) was disconnected, and a special, very short length heavy gauge pair of wires was used, connected to an accurate ammeter, between diode board output and that heavy red BMW lead that plugs into that diode board large spade connection (right side, facing from front).

Because of this method, the ACTUAL output of the alternator, NOT including any effects from the diodes that energize the rotor, was measured. Battery voltage was monitored by a known-accurate digital voltmeter.

The motorcycle had all lights, etc., disconnected that were not stock as factory shipped, except that an extra 20 watts was added to simulate small additional loads often used by many riders, see below. All tests were run with normal lighting, headlight on low beam. The battery was drained some prior to each series of tests, so that the maximum possible output of the alternator could be measured withOUT using special resistance loads. Every attempt was made to eliminate errors. The tachometer was calibrated. A method of measuring input and output from the BATTERY was made up, it was loss-less in design. The purpose of this measurement was to absolutely know where equilibrium was.

Results:
at 1050 rpm, the true output was 2.5 amperes.
at 1550 rpm, the true output was 10.0 amperes, and there was NO drain from the battery; this was the rpm for equilibrium...that is, the battery was neither charging, nor discharging. For this test, I used an extra 20 watts of load to simulate the very common addition of two rear running lights (#1157 lamps, one section for RUN function, converting the turn signals to Turn-Run function); or, small additional accessories often added by riders.
at 2100 rpm, the true output was 15.0 amperes, and the battery would be charging at that rate.
at 2850 rpm, MAXIMUM output was reached of 20.0 amperes at a voltage at the battery of 13.5....where the battery was fully charged....and if kept at that rpm, the charging current would slowly decrease, as the voltage regulator slowly dropped the charging.....and the battery would reach its normal 14+-, shortly thereafter.

NOTE carefully what these figures mean!....they mean that a stock 280 watt alternator is fully capable of recharging the battery after starting, and maintaining a fully charged system, at 2100 rpm or more, assuming a stock electrical system; and if all was perfect, and there were no additional loads.

Other tests were run, simulating the drain of a heated vest, small auxiliary lights, etc. There is no reason to believe that in excess of 3300 rpm is needed, under any conditions, including fully heated engine, etc.

www.euromotoelectrics.com has updated Valeo parts; Bosch parts, does rebuilding of electrics, and offers the EnDurLast permanent magnet alternator upgrade, for ALL airheads from the /5 onwards. That kit is somewhat involved in installation. The kit works well for those needing more watts, and especially for commuters whose batteries, due to much stop-and-go riding in towns, never gets fully charged, unless plugged into a charger.

Another higher output alternator, basically an oversized version of the stock Bosch, is the Omega kit, from Motorrad Elektrik: www.motoelekt.com
Motorrad Elektrik has many more electrical items for airheads.

12/21/2005: final editing and release
03/12/2006: Add more information on EnDuraLast and Omega
10/17/2006: Edited for clarity and typos