I have an old motorcycle that runs on 6 volts. There is no electric starter, just a kick starter, and the headlamp is switchable to 'off'. Since this is an older bike that isn't run on the A roads and sits for long periods, I would like to replace the 6V battery with some capacitors. My question is, is it as simple as getting some big capacitors and put them in place of the battery or is there more to this?

I'm aware that using capacitors may result in a pulsating headlamp at low rpm and that it will take a few kicks to get the caps filled up enough to fire the spark plug, but these are all things I'm amenable to.

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    \$\begingroup\$ You might get better answers to this at a motorcycle forum, assuming that what you want is a common thing to do. From the electronics side, all I can say is be careful what you buy because capacitors have failure modes that I wouldn't want to have happen between my legs. \$\endgroup\$ – The Photon Jan 2 '12 at 19:38
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    \$\begingroup\$ @ThePhoton I figured I would look here as much of the motorcycle forums are filled with speculation and little electronics know-how. My assumption was that battery replacement technology would be the same for any project. That said, I'm A complete novice (obviously)... \$\endgroup\$ – Ofeargall Jan 2 '12 at 19:44
  • \$\begingroup\$ I bought a battery eliminator kit off ebay a while ago. Worked fine. It was a cap and some other components that were potted to keep the 'secret'. It was only forty bucks or so. \$\endgroup\$ – Gf hd d it'ts hd hd hd Jan 3 '12 at 4:31


Information I gave below is good enough. See circuit diagram added at end and reference to electrical manual.

Bike uses classic coil ignition with rectified alternator to battery.

Using a LARGE capacitor may work BUT you risk system damage as voltage may be able to go very high with not enough load - and may not run coils if too much load.Also, higher voltage cap is smaller electrically per size but lower voltage one risks being overvoltaged by system. You could try 50V electrolytic as large as you can find surplus. 16V is too likely to be damaged probably.

My idea of an electronic clamp is an OK one BUT a low self discharge battery is a better idea still (or a solar charger or some other trickle charger).

____________________ End of addition ____________________

You need to get a good feel for what the system does with the 6V. What brand and model of motorcycle is it? It is quite possible that you do not need the 6V at all for the ignition - but you may have already found that you do.

If the 6V is only for the headlight and you have eg magneto or fully self powered electronic ignition then the headlight can probably be powered with no battery BUT you MUST clamp the voltage to stop it going too high - too high and bulb will blow (ask me how I know :-) *).. A clamp can be a bridge rectifier and a MOSFET which applies a large Wattage resistor when voltage gets too high - or even a short circuit is OK in many systms. Regulator bang/bang regulates.

The energy requirements of a spark ignition system are liable to make a capacitor solution unattractive.

Thinking laterally, you could consider using.

  • Eneloop NimH batteries (new style) which give you years of self discharge time.

  • 18650 LiIon x 2. - stripped out of old dead laptop packs. Often some cells are still good. Charger can be a voltage clamp that ensures voltage for two cells in series does not get above say 7.5V. A zener would be sharp enough for this in this application.

  • Small PV (solar) panel to charge battery or keep charged when not in use. (depends on where bike is stored.

  • Long ago. TS90 Suzuki. Convert electrics to 12v. Add halogen headlight . Marvellous. DO NOT miss gear changes at night !!!.
    Gravel road, 24 hour road trial, middle of night, fun!. Corner, change down, whoops - miss gear change, bright, brighter, brighter still, Black. Soon a Honda 750 cometh. We dance together over many gravel miles, I riding as best one may on his headlight beam. Quite a journey.

CB125 wiring diagram

You can download a SLIGHTLY better quality version of the file below from my drobox here - the version below when copied an compared has more jpg artefacs.
My version is a sligtly tidied up version of one I found here
Which was from ths utterlu superb collection of Hinda wiring diagrams - 120+ all free to download.

enter image description here

Rather good CB125 electrical manual -from the days when Honda didn't quite do English proper but put in far more technical material than you'd get nowadays.

Version on my dropbox account which I have unzipped
Zip version on a suspect looking website that actually did work OK not a site I'd frequent if possible.

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  • \$\begingroup\$ It's a 1974 Honda cb125 that is going to receive the battery transplant. All the lighting is set up for 6v right now. That's evidently what is coming off the charging system (+\-)... \$\endgroup\$ – Ofeargall Jan 2 '12 at 20:26
  • \$\begingroup\$ And the ignition can't be completely self powered since I can't keep the bike running on a dead battery. I had to drive by moonlight one night because the battery was dead enough that I couldn't get a good spark with the light on. \$\endgroup\$ – Ofeargall Jan 2 '12 at 20:29
  • \$\begingroup\$ By the way, excellent answer. Provides a lot of great DIY alternatives. I like that. Thank you. \$\endgroup\$ – Ofeargall Jan 2 '12 at 20:51
  • \$\begingroup\$ So, you're saying the best case scenario would be the NimH that I can charge separately or a LiIon with a zener to limit the recharge? I was thinking of using a 6v dry cell with a switch or diode to limit back flow might work too but, again, I'm a novice here. I can rebuild the carburetor, the electrical is a different story... This is a super answer that just keeps getting better over time! \$\endgroup\$ – Ofeargall Jan 3 '12 at 1:26

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