Greetings Chris

I have a 1993 R100GSPD. These old airhead BMW's don't have a lot of excess electrical capacity for ancillary equipment I discovered this after trying run two heated vests, heated grips and headlights. After a couple of weeks travel the battery slowly discharged. This was a bad thing.

I looked into getting a higher capacity alternator and eventually bought one from Motorrad Elektrik which has worked very well. I don't know what bike you have but some of points below might apply to you.

Even though I now have more watts than need (400 versus 230) there are a number issues that needed to be considered before upgrading.

1) Existing wiring - A new alternator may need to have some of the existing wiring replaced. This might be in part of the charging circuit wiring or If you are trying to fit higher wattage headlamps I would not trust the existing wiring to handle the higher current requirements new wiring, relays and even replacement globe holders might be needed.

2) Low RPM Vs Alternator output - The power output from an alternator is proportional to the engine RPM (This is not a linear graph, there are other factors) so while you are riding along at 100kph it might comfortably supply all the electrical demands on your bike and still have enough capacity to charge the battery but in heavy traffic, waiting at lights, etc the battery itself will probably be discharging. This is what happened to us. Fitting a larger alternator might improve the the low end power output but it's likely that if you are idling with all the lights, etc running the battery is still likely to be discharging.

I can only see a partial solution for this and that would be to fit a low voltage cutout circuit before the power outlet on the bike. It's only a partial solution as it would only affect anything plugged into an aux power socket.

I'm in the process of designing a small cutout circuit that can be fitted to a bike. I'm specing it to be able to pass 10-15 amps and it should be able to be connected directly to the battery without discharging it over time if the bike is not being used.

I'll let you know how well it works, if anyone is interested in one send me a PM and I'll send you the circuit details.

3) Voltmeter - As other people have mentioned fitting a voltmeter is the best thing you can do to see how well the bike charging system is working at a given engine RPM. I would highly recommend doing that first before doing any other changes so you have a baseline.

4) Alternator load - As I understand how a permanent magnet alternator works the output of the alternator for a given engine RPM is relatively fixed.

There are basically two types of associated voltage regulator, shunt and series regulation. A shunt regulator works by shorting out a proportional amount of the current to regulate the voltage.

In a lot of modern bikes with permanently on headlights I think you could increase the chance of the regulator failing if the headlights are not running as the voltage regulator would now have to dissipate the extra wattage that would normally be done by the headlights. I don't know if this applies to alternators that use series regulation. I'm still researching this.

For alternators that don't have permanent magnets but use field wound rotors instead, the alternator output is affected by the engine RPM and the current that flows through the rotor which is controlled by the voltage regulator.

The weak point on this type of alternator is when it produces a high current output the rotor current is also high and this can cause a failure in the rotor (as airhead BMW owners know)

Owning an airhead, I like the idea of replacing the pilot bulb with a halogen. I have one question for people who have done this. How long does this bulb last?

The reason I ask this is that when I looked at buying one part of the spec was a 200 hour lifetime. Is this the case in reality?

Anyway that is my $0.02 worth. If anyone can tell me where I am blatantly wrong please let me know :-)

Regards

Ian J

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