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Aftermarket Alternators for BMW Airhead motorcycles.
Installation, Capability, Performance.
© Copyright 2018, R. Fleischer

SUGGESTED: Read 15-A and 15-B, first!!!



There is nothing wrong with the design of the BMW Airhead motorcycle's stock Bosch alternators. Like most anything they will age, and/or they may be abused, they may fail, although not in excessive numbers. They are often never maintained until there is a failure. Wattage output was adequate for most riders when the motorcycles were originally sold by BMW.   The modest electric output may be of concern if you have lots of accessories ....such as heated clothing, heated grips, extra headlights, etc. I am not for or against aftermarket alternators. I am reporting only facts and testing information and conclusions. You may well do fine with the stock system. But, if you have special needs, such as large city stop and go commuting; or need more watts ....perhaps you are approaching the stock alternator limits (which decreases reliability) ....there is plenty of information in this article. The /5 Airheads had a 180 watt Bosch alternator with a stator diameter of 105 mm where it fit into the engine case. It can be upgraded easily by using an EARLY /6 alternator stator (if it is 105 mm, and NOT 107 mm), and any pre-1985 (approx.) rotor (2.8 ohm rotor not recommended) and any /6 or later diode board. Reliability can be improved for ANY Airhead alternator, stock or aftermarket, and this includes a stock /5 model, or a /5 upgraded to the 280 watt 105 mm Bosch; OR EVEN AN AFTERMARKET ALTERNATOR; using the later, better ventilated, front metal engine cover. If you have an RS or RT model, and the front fiber-glass-like cover is NOT louvred, even more reliability will be had by changing to the louvred type, or putting attractive holes into yours.  
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This article includes information that is far beyond what advertisements for various alternators provide. Charging a battery in a vehicle like your Airhead is NOT as simple as you may think; nor, is interpreting the alternator ratings, typically simply shown as wattage. This article presents a huge amount of information; and, also presents information that helps you select an aftermarket alternator. I suggest you read this article completely through to get an over-view; then refer back to the areas you are interested in. If confused, please inquire on Snowbum's favorite forum for inquiries; an E-mailing LIST, the Airheads List, as hosted by Input will help others, rather than my answering individual inquiries, which are discouraged.

DISCUSSION.  Much of this is pertinent to ALL charging systems:

Measuring or specifying generator (alternator) output in watts is not as simple as it may seem. The simple part is that watts is equal to voltage multiplied by amperes. The output of the alternator can be measured in amperes and voltage, multiplying those together would give you watts. BUT, there are MANY complications.  For the purposes of this Discussion, a lead-acid type battery is assumed.

The output of the alternator system should be AT LEAST specified and measured at a practical/usable voltage. It is wrong to use alternator sales brochures in which voltage, perhaps not even stated, is not high enough to keep the battery charged; or, at least close to fully charged because it is possible for the system output voltage to sag from a large load (big headlamp, driving lights, heated clothing, poor battery, etc). If the output voltage is too low to keep the battery fully charged; yet the current availability was high, it is possible for false wattage advertising. What is important is the real output with a "reasonable & usable" charging voltage at the battery, that keeps the battery charged enough to have good battery life & maintain close to battery rated ampere-hours capacity .....and be able to restart the bike many times reliably, etc. You want reasonable fast recharging, after such as starting, or being at a traffic signal for awhile. The "reasonable & usable" battery voltage can be argued about. A battery is considered to be fully charged at a RESTING voltage of 12.5 to 12.7; and, will pass a formal Load Test.  A battery at 12.4 volts or below is NOT fully charged, and is deteriorating, the lower, the faster the deterioration. A battery with such a voltage is also NOT going to deliver its rated performance. The rider needs to know if the alternator will or will not produce a full charge during extended cruising; as well as knowing what the performance will be in stop and go traffic. 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.

Other considerations:
If the voltage is too high, the battery could deteriorate from too much voltage being applied, which also means too much current, which means wattage (heat) being produced by excessive electricity flowing into the battery. The battery could overheat & warp; or use water too fast, or if a sealed type it might have enough internal pressure to open the safety valve (VRLA types and other sealed batteries). Excessive voltage output which can greatly reduce battery life has NOT been any problem in any of the systems tested.

You want the battery to recharge quickly after starting the bike or perhaps from long periods of waiting at stop lights and then riding off.  Another consideration is if the alternator capability is really very high, the excessive recharging current could damage the battery.   Most modern batteries are capable of withstanding quite large charging currents for short periods of time.

You need considerably higher charging voltage than 12.7, due to inefficiencies in battery chemistry, to allow the battery to reach full charge in a reasonable amount of time. This can become critical when doing stop and go riding in city traffic.

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