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Chris Scott 3 Feb 2012 18:16

Increasing alternator output
Overall is it a good idea from a reliability point of view or - like increasing the power of an engine - can it lead to stress on other components unless they too are uprated?

I presume with a higher output either more heat is produced or more bhp is drawn from the engine?

Are high-output windings more bulky anyway, so will be limited by space?

My own bike*puts out 235w @ 8000 rpm so I presume at a more normal 3000 rpm cruise it's a lot less and won't be up to running a vest on a freezing night.

As we know, a standard factory-spec machine is usually best, but I don't know if that applies to high-output alternators too.

I know many of the latest adv bikes have massive output to run the OE gadgets. Old bikes not so good when it comes to heated clothing + extras.


Chris S

Dodger 3 Feb 2012 19:51

You might have to fit a higher capacity reg/rect or give it a bigger heatsink and remount it in a place on the bike where it gets lots of airblast ,but I can't see much problem .You'll lose a couple of horse power from the engine but you won't notice that .

A 65 watt heated liner shouldn't overburden the charging system .
You can compensate by using aftermarket low draw lights .

Most modern big bikes bikes are using 400 watt alternators ,the big beemers use 700 watt because their riders feel the cold a lot more .

backofbeyond 4 Feb 2012 09:05

My current overlanding bike has a 180w alternator and is quite happy running lights and my 75w electric jacket. I've run it for days at a time like this on long winter rides and the voltmeter tells me it's keeping the battery charged.

It depends on the design of your alternator whether it's putting out full power all the time or just producing enough to supply what's needed. If it's the latter type then an upgrade isn't going to make much difference to the stresses and strains on the rest of the system. If it's a permanent magnet type, they produce full power all the time and any excess is shunted off as heat. Maybe you could duct it into your gloves or something :rofl:

Chris Scott 4 Feb 2012 09:36

Thanks for the tips. I've actually done the low draw light thing - at least on the indicators and back. Can one get low output headlight bulbs too?

permanent magnet type - is that an old Brit way of doing it - as opposed to less old Jap?


Maybe you could duct it into your gloves or something
You say that as if it's the most ridiculous idea, riding around with hoses stuffed up your sleeves venting lukewarm air...


Joe C90 4 Feb 2012 16:57

1 hp equates to approximately 750w. So the engine wont care how much you turn to electricity. (even if you have 50% efficiency).

Reg/recs are not the most reliable bits of kit, they have a hell of a job to do, 150 watts is over 10 amps. (remember, bike electric voltage varies from 12.5-14 volts).

On my ta650 outfit, I had lots of electrical stuff, so I monitored the voltages closely, it was noticeable after a prolonged loading, the voltage would suddenly drop, this I believe was due to the reg getting to hot. It was mounted on a large slab of ally plate, to help with reliability. If you go for uprated windings, you need to look at uprating the reg/rec.

Most failures I have seen have been diodes going short circuit in the rectifier stage, or the rectifier failing high, this killed my ZZR in belgium, by frying the battery and then the CDI.

backofbeyond 4 Feb 2012 17:51


Originally Posted by Chris Scott (Post 365903)
Thanks for the tips. I've actually done the low draw light thing - at least on the indicators and back. Can one get low output headlight bulbs too?

permanent magnet type - is that an old Brit way of doing it - as opposed to less old Jap?

You say that as if it's the most ridiculous idea, riding around with hoses stuffed up your sleeves venting lukewarm air...


Yes, you'll get permanent magnet alternators on old British stuff but also on a lot of Japanese trail bikes. The bigger stuff will use an electromagnet instead and they regulate the output by varying how magnetic the rotor is. You can rewind the stator poles on a lot of trail bikes to vary the alternator output and move it up and down the rev range but it's a bit of a tricky thing to do unless you have some electrical engineering background. There is quite a bit of info on the web about it, particularly for some of the easier bikes - Honda XR600R for example, and there are some commercial companies offering off the shelf upgrades. The XR is easy to do because as std Honda only used 4 of the 12 poles on the stator. Rewinding on all 12 ups the output considerably.

You can get low output headlight bulbs (although it kind of defeats the point of a headlight), this guy sells all sorts of bulbs, mainly for old Brit stuff but it's probably because they need it most. The alternatives are could be replacing the stock headlamp with a HID setup (but there may be MOT legality problems with it ) or using LED add on lights to supplement a low output bulb. LED driving lights consume very little current for the amount of light they put out so overall you could save a bit, (especially if you turn the headlight out :innocent:)

I have tried venting hot air from the engine before, using hoses and stuff but not very successfully. With air cooled engines it only seems to work when the engine is hot - ie in the summer or when you're stationary (then there's no airflow) which is the opposite of when you need it. Don't think you'd get enough from the reg/rec to make it worth while.

richie farmer 4 Feb 2012 18:24

Assuming that most of your travelling will be in daylight, and you are just using your headlight to be more visible, you can fit a 20w quartz halogen pilot bulb, and just use that for a daylight riding light.

dajg 5 Feb 2012 00:57

what is that bulb called? where do i get me one..?? what specific bulb type is it supposed to replace? like a 55w halogen or something?

richie farmer 5 Feb 2012 10:43

If you have a pilot light or parking light bulb in the headlamp shell you can replace it with a 20w halogen. You can then run with the pilot light on but the headlight off. It looks like your headlight is on, but only uses 20w instead of 55. It is a common modification on airhead BMWs and other older bikes, especially when running heated vests etc. Have a look here

Quartz Halogen bulbs for motorcycles & cars

stickysidedown 22 Apr 2012 21:22

One thing I have been reading a little about lately is how the design of most reg/recs is of a cheaper diode design that means (through the satanic art of electrickery) as the revs go up the output actually drops??

In any event it seems that for a more reliable and efficient power source you should/could be looking for a reg/rec that uses the more efficient, and of course more expensive MOSFET type.

Apparently the one from a Yamaha R1 uses a MOSFET Reg Rec

Apologies its not a direct answer to your question, more of a sideways one like using HID's as they draw less than incandescents (and in the case of my ktm fitting 03 switchgear so I can turn my headlamp off!)

but getting the most from your stator might be better than having one rebuilt and you'll always have your original reg/rec as a spare too!

stickysidedown 22 Apr 2012 21:28

heres a pretty good link to an explanation re the MOSFET thing


apologies if im not allowed to cross post, I dont even own a VTR Honda, just doing it for thje good of the HUBB community bier

Fern 19 May 2012 20:54

Have you got a mosfet reg?

IanJ 15 Aug 2012 17:02

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 :-)


Ian J

oldbmw 16 Aug 2012 00:02

If the extra power is required to heat the rider there is an alternative.
I use a fairing that is designed to provide protection to the rider from the elements. Cutting out windchill and rain off the rider reduces greatly the heating requirements and also stops an incredible number of big hard insects and the odd piece of gravel.

The fairing I use costs about £250-£300 depending where you buy it.

you can see it here ( admittedly not well )


Redboots 16 Aug 2012 20:33


Originally Posted by IanJ (Post 389320)
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?

You can also solve the problem with LED's. I made a bracket and put some 230,000MC 10mm LED's on it. People do notice them!
I also put one of the same LED's in the headlamp shell to replace the sidelight.
LED's have a very long life with next to no draw.


You could also try the ribbon leds like these:thumbup1:


I took these off as one on the right failed and if you dropped the bike they would be toast. More inboard placement would work though.


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