How much wattage can the alternator on a 125cc output roughly?

**spencer2004** · 04-21-2012, 08:51 AM

Just out of interest how much wattage can a standard 125cc engine output without blowing the electrics? I'd like to know as I have a lot of electrical accessories on my motorcycle such as LEDs, heated hand grips and a higher wattage bulb than stock and I'd be interested in knowing how much more my bike could take without damaging the electrical system. Does anyone know the answer or any way to calculate / test it?

**CQBert** · 04-21-2012, 10:41 AM

It is not possible to give a good answer, if you idle the engine, the output is almost nothing, it drains the battery, if you are running it on a good rev, then it depend on what generates the electricity, and how efficient it is and what capacity it has. It varies with the bike and has nothing with the engine size to do. A good way of testing this is to use an artificial "load", which is a box with variable resistances inside connected to the battery. You increase the load and measure the voltage, normally, to keep a good battery charged, the voltage across the battery should be 13.6 Volt, if it drops below 12.5 you are draining the battery. A recommendation is to install a volt meter on the bike and an amp meter, you can them monitor the voltage and the charge or de-charge of the battery. You can add another larger external generator, or replace the original, but it is a bit messy. Typically a 125 cc engine produces about 25 HP, which is about 18,000 watts, or 18 1,000 watt light bulbs, but then there is no pover to move the bike. Unfortunately, by my own experience, the smaller 125cc does not have much excess electrical capacity but for the ordinary lights, the big cruisers are better. Another trick is to replace all light bulbs with LED lights. then you have more left over for the extras.

**spencer2004** · 04-21-2012, 12:02 PM

Thanks for the advice, I'll have to get me a multimeter and do some testing. Some how I don't think my engine produces 25hp, a 2 stroke 125cc might but mine is a 4 stroke. I'll probably take your advice and replace most of the bulbs with LED equivalents.

**humanbeing** · 04-21-2012, 01:27 PM

Typical spec: http://item.taobao.com/item.htm?id=10187522054

You can change to Δ from the original Y if u know what u're doing. "Cons" is no /low output at low rev.
http://www.procycle.us/info/articles...ta-stator.html
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A guy fitted a used "CBR250 MC19" stator on GN125

http://www.motorfans.com.cn/bbs/t_16803658.htm

**spencer2004** · 04-21-2012, 04:21 PM

So it can output is roughly 150w? I thought that it would be more than that. So with my 55w headlight ,20w brake and tail lights, 10w indicators, LEDs, heated hand grips, dash lights and what ever the igniton consumes I don't really have a lot of room here.

Will I be able to fit a stator from a CBR250 to my bike without any modifactions to anything then? The translation dosen't really explain much.

**humanbeing** · 04-21-2012, 11:58 PM

Originally Posted by spencer2004

Will I be able to fit a stator from a CBR250 to my bike without any modifactions to anything then? The translation dosen't really explain much...

1st CBR250 stator is expensive in UK. 2nd CBR250 uses M6 bolt which means "mod" to fit existing M5 NEED .
Y to Δ (+40% in theory) & higher efficiency rectifier is more common mod in Chinese bike.
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The GN250 Y to Δ
http://translate.google.com/translat...8694486_d.html

**CQBert** · 04-22-2012, 12:35 AM

Originally Posted by spencer2004

Thanks for the advice, I'll have to get me a multimeter and do some testing. Some how I don't think my engine produces 25hp, a 2 stroke 125cc might but mine is a 4 stroke. I'll probably take your advice and replace most of the bulbs with LED equivalents.

No - sorry, the "25" was taken a bit out of the air, it varies a lot. I have had this problem myself when I was running around with a lot of"gadgets" live on the bike, as a ham radio (100W transmission power ;-)). Another solution I have seen is a couple of solar panels, but it looks funny. Usually the flywheel and the generator are combines, but some bikes have a combined electric starter and generator. I have also seen people connect a generator to the back wheel. All kinds of solutions, but none really "elegant".

**spencer2004** · 04-22-2012, 08:26 AM

I'm my Haynes manual it says that the alternator rotor is attached to the left side case cover and it looks fairly simple to remove and replace, the only annoyance is that I'd have to drain the oil and get a new side gasket to do so. I think I've seen the CRB250 rotors on Taobao for around £16 but I would imagine that they are fairly heavy and the postage would be expensive but If I was to get one I suppose I could just tap the original m5 holes with a m6 tap. I thought that solar panels would be a really good idea at first as I could attach them to the top of my luggage box so they wouldn't look too bad but most of the energy goes into lighting which I only use at night so that's a no go :(

A small generator that is separate from the engine could be a better idea now that I think about it; there is some room inside my bike where the air box used to be which is directly above the drive chain so I could connect it to the chain using a small sprocket. If I was to do that would I be better off wiring it directly to the battery, the rectifier or it's own voltage rectifier?

It would be nice to have enough electrical power for a couple of 50w spot lights for unlit roads at night or to upgrade to a 100w headlight bulb just for the sake of it ;)