How does it work and what are the downsides of running a motorcycle without a battery?

I recently purchased a Honda S90 (second hand, beat up, and somewhat tweaked) and after looking into it I realized it's got no battery. Just rectifier, electronic ignition points, a CLI, and coil. I don't really understand how it is running. From what I've read online it's possible but it can blow up your electronics?

I'd appreciate if someone can shine a light on this. I'd like to improve the bike over time.

3 Answers 3


Most vehicles use the alternator (driven by the engine) to charge the battery, from which all your electrics are driven.

If you don't have a battery, you can use the output from the alternator to drive electronics directly.

The problem is that the output from the alternator varies dramatically - when the engine is running at high revs the voltage will be higher than at idle, for example. So where the battery normally protects you from variations in voltage, a direct draw from the alternator could potentially damage sensitive electronics.

Of course, if you are only driving your lights, this probably isn't too much of an issue. I don't know the S90, but I'm guessing it doesn't have any complex electrical systems on board.

  • That makes sense. How does it affect the start up though? I was under the impression that a battery was needed. Thanks for the detailed answer!
    – cballenar
    May 4, 2017 at 18:57
  • A kickstarter is perfectly usable on small engines, and even some larger bikes cope okay with them. I had an old 750cc Yamaha with a kickstarter (it killed your leg if you got it wrong, however)
    – Rory Alsop
    May 4, 2017 at 18:59

The real problem is that not all alternators "self start" some do due to residual magnetism but some don't. Unlike dynamos which were capable of charging from very low voltages and eventually completely recharging a battery that was flat.

  • Damn, that's cool. I'm not entirely sure what I have in place. Back to research mode I guess... Thanks!
    – cballenar
    May 4, 2017 at 19:11

Driving motorbike with an electronic dashboard and all necessary indicators including coolant temperature, oil level, fuel level, and all other indicating things... MAY BURN THE electronics, due to a high range of voltage variations. The battery acts like a "sponge" or a capacitor to the excess current, soaking it into itself. If there is no battery expect death to the dash-board, and potentially to other electronic devices on the bike. That’s what happened to me, and had to buy a new one which was a third of the price of a new bike.

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