I'd like to use a standard 6v/3w bike bottle dynamo to power a 18650 battery charging module TP4056 which also powers some LED's.

Diode bridge rectifier steals 1.4v from the generator. In addition to reduced output voltage, a large portion of period would be without any current (all the time while absolute value of voltage is under 1.4v).

So, could I reduce this inconvenience by attaching a transformer right after generator, to multiply voltage by 4 and attaching output of the transformer to the diode bridge and on to the battery charger?

Would the transformer losses be smaller than the loss of having to use only diode bridge? Which kind of transformer would fit this purpose?

  • \$\begingroup\$ Your dynamo is AC dynamo? I haven't check dynamos for bikes though. If it is DC dynamo, you do not need a bridge rectifier. \$\endgroup\$ Mar 15, 2021 at 0:41
  • \$\begingroup\$ A bottle style sidewall generator on a bicycle isn't a dynamo but actually is a magneto and doesn't have a commutator.. It generates AC. It's relatively easy to get them where they generate high voltage, though I don't know if that's ever been done for bicycle bottle style units. \$\endgroup\$
    – jonk
    Mar 15, 2021 at 1:00
  • \$\begingroup\$ Hmm. Since you are looking for power, which is not usually the goal of earlier magneto designs because of poor magnet strength, consider building/adapting your own that uses modern rare earth high powered magnets on the rotor and the coils on the stator. More efficient transfer of mechanical to electrical energy when coupled to modern electronics. Could be a product! \$\endgroup\$
    – jonk
    Mar 15, 2021 at 1:14
  • \$\begingroup\$ We're not talking about superhuman power, I'd only like to charge one or two 18650batteries, which will power like 3 pcs of 3w leds. \$\endgroup\$
    – Milos
    Mar 15, 2021 at 9:21

1 Answer 1


The bottle-type dynamo has a circumference of under 2 inches, and at normal cycling speeds (12 mph), assuming it has two pole pairs, will give AC at about 100 Hz. This is compatible with an AC transformer of the wall-wart type, but not ideal (because those transformers typically have 120V-in, 12V out, and you want circa 6V in...).

Power output is in the 3-5W range for the bicycle lights and generators of old (with incandescent bulbs).

Another option is to use a voltage-doubler rectifier which lowers the drop from 1.4V to half that, or less if you use Schottky rectiiers


simulate this circuit – Schematic created using CircuitLab

and use a DC/DC converter (probably a switchmode buck converter) to deliver suitable charge current to the battery. The light bulb is just a dummy load for the simulation...

  • \$\begingroup\$ This seems like a good option, what is the type of those capacitors? They look non-polarized but their millifarad capacity seems large, in the electrolytic range? \$\endgroup\$
    – Milos
    Mar 15, 2021 at 5:46
  • \$\begingroup\$ @Milos - yes, the capacitors are depicted as nonpolarized, but would be electrolytics with polarity (the (-) ends go, C2 to generator, C1 to GND). The losses in them, at these currents, are minimal. More voltage than you need, though, will show up on output (a stepdown buck regulator would have to precede a TP4056, for instance). \$\endgroup\$
    – Whit3rd
    Mar 15, 2021 at 9:08

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