Second test, trying to use hand-cranked dc motor to charge USB devices (3 Watts working!)

Finally some good news and results. With a 12V DC motor, when used as a generator, it is possible to deliver close to 3.5 W for charging an android phone! This is a continuation to the previous blog.

Working setup

The picture below shows the main components for the final setup.

  • The generator is a geared 12V DC motor that also can be used as generator (10 € at a local store).
  • The DC voltage generator is car 12/24V USB charger with max 1000mA output. (2€ via internet)
  • The crank extention is made of a tent peg (diameter 5mm).


With this setup, it was possible to charge an android phone with 670mA/5V (> 3Watts), great!

Note: The linear voltage regulator 7805 did not work in this case because the measured voltage from generator was between 12-20V. I first tried with 7805, but most of the power went to heat instead to phone. In fact, the output from 7805 was only 200mA (when not using any heat sink). (more about efficiency and heat dissipation: here, here and here).

Other attempt: Charging with batteries

It is possible to charge phone by using 1.2 or 1.5V batteries and a DC step-up converter. However, the current delivered by this method is very low if only 1-2 batteries are used. When using 3 batteries in series, then one can get into 200 mA range (see photo).

Note: It was possible to charge the phone without any step-up converter, when using 4*1.2V chargeable batteries directly connected to USB (good to know for emergencies.The voltage was 4.7V and current was 120mA).


Other attempt: Combining dynamo flashlight and USB power bank

Before the final working setup, I did try some other combinations. The photo below shows a very promising looking charging setup. In the pocket flashlight, the internal battery was replaced by USB power bank. The idea was that the dynamo in the flashlight can be used to charge the USB power bank, as the battery inside USB power bank was 3.7V which is the same as inside the flashlight.


As the photo shows it was possible to charge the phone with 660 mA when the USB power bank was fully charged. But when the USB power bank was without a charge, the dynamo was not strong enough to charge the USB power bank in any reasonable time frame. (The output from the dynamo was close to 100 mA).

Charging current depends on the “load”

Please note that the output current also depends on the load, ie. what USB device is being charged. For example, my android phone takes 660-670 mA at maximum, only when the phone battery is low. If the phone battery is higher, then it might only “take” 200-400 mA.

About printer motor

This will be on hold, since I now have a working setup with a proper generator and since I really do not have proper gears to build a gearbox.

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