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Motorcycle Capacitors

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Glyn

New Member
Hi - first post to the site.

I'm building a motorcycle "special", putting a yamaha motor into a MZ frame. I will be prob making up a new wiring harness and wish to do away with a battery if possible. The bike has an erly type CDI ignition, run off a seperate coil with a pulsor. The second coil is for the lighting circuit and is rectified.
It is a 6 volt bike.

Could I use a large capacitor instead of a battery for running the lights / horn? I'm not too bothered if the lights flicker or are not of a great quality at night. The bike will be used infrequently and so I don't want a battery to keep going flat. I also have limited space on the bike.

Hope someone can help.

Regards
Glyn Pickering
New Zealand
 

samcheetah

New Member
Glyn said:
Hi - first post to the site.

I'm building a motorcycle "special", putting a yamaha motor into a MZ frame. I will be prob making up a new wiring harness and wish to do away with a battery if possible. The bike has an erly type CDI ignition, run off a seperate coil with a pulsor. The second coil is for the lighting circuit and is rectified.
It is a 6 volt bike.

Could I use a large capacitor instead of a battery for running the lights / horn? I'm not too bothered if the lights flicker or are not of a great quality at night. The bike will be used infrequently and so I don't want a battery to keep going flat. I also have limited space on the bike.

Hope someone can help.

Regards
Glyn Pickering
New Zealand
i dont think so. a capacitor is not the same as that of a battery. u cant use the two interchangably. a capacitor is not a continuous supply of energy. a charged capacitor discharges immediately depending upon the RC time constant of the external circuit. u will have to use a battery to operate your lights and horn. a capacitor is simply a frequency dependent resistor which can be used to store energy. dont get the usage of a capacitor wrong
 

john1

Active Member
Hi Glyn

Yes, that should be ok.
So long as you realise that the capacitor won't run the lights for
long without the engine running.
It won't run them for very long at all without the engine running.
In fact the capacitor may run out of charge before the engine has
come to rest after switching off.
The 'lighting coil' gives quite a 'lumpy' output, generally these
run of the same magnet assembly that the ignition system uses,
and for those who don't know, its not evenly spaced around the fly
wheel. Its very much all over one side, the fly wheels are usually
cast with a shape that will even out the weight distribution and also
machined to balance up. The magnet assembly is actually two magnets
positioned near together so that between them there is a fairly high
magnetic field. It is this that is used, in conjunction with the points
to make the ignition spark. Thats why the flywheel has to be positioned
with respect to engine tdc.

I mention this to show that the lighting coil, which uses the same
magnets, gets a very lumpy voltage. What it gets is a sort of spike,
every revolution of the engine. Now when its driving filament lamps
that matters little, it will even out as an RMS load lighting the
lamp. When the engine revs are low, like tick-over the lights can be
noticeably flickering.

If you want to fit a capacitor on this i would mention two things,
firstly with the lights off, or failed lamps, the cap will charge far
too highly. I suggest a ten or twelve volt zener as limiter, so that
it should not charge to an awkwardly high voltage, which could pop
the lamps if switched on when its charged, or blow the capacitor if
not restrained.

Secondly i suggest you use a diode bridge rather than one diode so
that you get the best use of the output from the lighting coil. Using
just a single sided output will give a higher spike to output ratio.
You will probably find one side of the lighting coil is grounded, so
to use a diode bridge you would have to free that end, to make the
coil 'floating' that is with both ends free.

Using a capacitor will give a much better light from the lamps,
i think that this is something that a lot of people do with these
types of bikes.

Best of luck with it, John :)

just seen russlk's post, he has done this too with 3000MFD, i would
have guessed at more like 10,000, but see how it goes - John
:)
 

Nigel Goodwin

Super Moderator
Most Helpful Member
If the bike needs to be street legal it won't work - headlights on trail bikes (and many smaller motor bikes) commonly have the headlamps fed direct from the flywheel alternator - this results in the lights seriously dimming as you slow down (you would need a hell of a big capacitor to try and stop that!). The indicators and brake lights are usually fed from a smallish lead acid battery, which is charged from the alternator - as you slow down (and charging current disappears) the battery takes over feeding your indicators and brake light.

For an indication of capacitor size use the formula C=IT, so assuming a 10amp headlamp bulb, and 10 seconds of capacity - that means a 100 FARAD capacitor - big beast!.

If you wire it as normal, using a standard motorcycle battery, it won't affect starting at all even if the battery is totally flat - it will simply start charging as soon as you start the engine.

Perhaps Russlk would let us know what the capacitor did on his BSA (sorry, but I don't recognise the model) - presumably it was only to power the ignition?. One device I do remember on various British bikes (not sure if it was BSA or Triumph) was a large zener diode on a heatsink just below the headlamp - this was to prevent the battery getting overcharged at high revs.
 

john1

Active Member
This is from:
http://www.allenmuseum.com/bsa441.htm
it says dirt track racer,
but this version looks street legal to me.
They may have also been known as 'Shooting star'
i think.
Maybe that was the name for the street version.

The generators on some of these dirt bikes had
permanent magnet fields, so voltage control
was by excess power sinking (wasting) using
fairly meaty zener units.

Simple but effective, although they were prone
to failures from overheating damaging the
wires and connections close to the unit.
(at least the ones i met were, but then i didn't
usually get to meet them till there was a failure)

John
 

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