I had a B&M ? brand (german make) rear bike 'standlight', removed the capacitor as I found it annoying when switching to a battery. Used a 6x cell AA 1,2v pack for awhile, with no issue. Upgraded all the wiring to silicon, and put in a 0,8AHr Yuasa gel lead acid.

For some reason I choose a 12v lead acid instead of a 6v. ?

The front headlight, which was a xenon lamp design with a voltage regulator circuit that I replaced with a LED inside the reflector, same headlamp control circuit, seems to like it. Good light.

However, the rear red 'standlight' got dimmer, and dimmer. And now barely lights up at all. I think it was not designed to operate with such a continuous peak voltage around 12v and wondered what you thought.

  • i think the 12v cell was an attempt to overcome corrosion at connectors (deans connectors) and the poor behaviour of typical bike wire over time... although I had already upgraded most if not all the wiring to silicon... so I was probably thinking too much when I got the cell. :P
    – mwk
    Commented Dec 3, 2012 at 9:55
  • I can't imagine that a lighting system designed for 6V would do well on 12V. You're lucky it's lasted as long as it has. Excess voltage is not the way to overcome connector corrosion -- silicone grease is the solution there. Commented Dec 3, 2012 at 13:28
  • Perhaps better handled by electronics.stackexchange.com?
    – Glenn
    Commented Dec 3, 2012 at 18:18
  • it's on a bike, and it's a light system.
    – mwk
    Commented Dec 8, 2012 at 15:02

2 Answers 2


You most probably damaged the rear light and are risking damage the voltage regulator on the front, too, since the huge majority of generator hubs provides 6 Volts.

If the regulator on the front is sophisticated enough, perhaps it is using just enough current and voltage to feed the light, but I think most circuits use a Zener diode, that actually make the excess voltage to "leak". This is not big issue with generators, whose alternating current waveforms just go above 6V during very short fractions of a wave, but with a battery, it's possible that it is working a bit like "short circuited". If the front light is over-heating, that most probably might be happening.

As for the back light, the light-emitting elements of leds are very sensitive to higher-than-normal voltages, and tend to "burn" due to overheating and chemical/physical damage.

I advise you to check the current voltage of your system, and switch back to a lower voltage if that is the case.

Hope that helps!

  • 1
    Thanks. I'm looking into using a LM317T circuit to bring the power down to what the existing LED's use. The rear light assembly works, the LED's are okay. It's the regulator circuit that appears burnt-out.
    – mwk
    Commented Dec 3, 2012 at 12:38
  • Yeah, I think the point here is to find some regulator that doesn't just "shunt" the extra voltage, but actually limits the voltage without increasing current consumption. I don't know if these LM circuits do this. Some of them have heat-sink mount, which might indicate a "non-efficient" way to regulate, but actually I don't know this. My personal preference would be to find lower-voltage bateries, and everything would be much more electrically safer regarding the rest of the system. Commented Dec 3, 2012 at 13:36
  • I used an LM317T with a 50 ohm resistor to limit current. Will post the full comment as an 'answer' only because there is more room to type. I consider your answer the correct one though.
    – mwk
    Commented Dec 6, 2012 at 14:50
  • If you find my answer is correct, you could accept it by clicking in the green "V" :o). Hope you get your system working! Commented Dec 6, 2012 at 17:34


I used an LM317T as in the photo, with no heat-sink and exchanged a 50 ohm resistor, I think 1/4 watt - for the 4ohm resistor pictured. Using the original bright centre with two side indicator LED's instead of a main headlight 1W. The voltage of the LM317T goes up to ~37 volts input, so no need to reduce the circuit to 6v, at least for the specific tail-light. The other 'generic' tail-light system, mounted on a trailer sometimes sharing the power from the battery, seems to tolerate 12v better that the B&M designed circuit. -> http://www.pedal-pedal.co.uk/union-rear-mudguard-mounting-led-dynamo-light.html <- this one.

The entire new circuit, ahead of the LM317T, draws 24,5 ma. I put my finger on the tab of the LM317T and noticed perhaps a barely-perceptible increase of temperature over room temp. Left it running for 10 minutes and cannot notice any change, it's the same as before. While smaller packages exist like the LM317M and can handle up to 500ma from the specs, the LM317M would be more difficult to work with. The LM317T has large leads as in the photo.

1) Open up the B&M and unscrew and de-solder the board from the power leads and from the main LED to remove it. Rip all the surface mount components off the original board. Save the F00x fuse. 2) Install the LM317T on the back of the board. The back is the side that has no electronics and faces towards the black plastic when installed.

The whole circuit is one resistor and one LM317T. It's all that is needed for battery-supply.

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