Shake-to-charge flashlights are a cool idea, particularly if you’re like me and have trouble keeping spare batteries in stock. But beware of cheap counterfeits made in China like this one, which is decidedly lacking in performance, as noted by Discover Circuit’s Dave Johnson:
“The maker used some very cheap 1N4001 diodes in the bridge rectifier circuit instead of more efficient Schottky diodes. They also used a small 0.5 Farad cap with a 5.5V rating. I noticed that that this kind of super capacitor was originally designed for maintaining data in memory chips and has a rather high internal equivalent series resistance. This reduces the overall efficiency, since the device can’t be charged or discharge very quickly. Some of the power that should go to the LED will end up being dissipated inside the capacitor. Better super capacitors do exist.Most white LEDs draw about 20ma of current with a voltage of about 3.6 volts. As the voltage drops from 3.6v, the current will also be lower. Without any regulation, the circuit will not have a consistent light output. I measured the LED current in this circuit at only a few milliamps, even after many minutes of vigorous shaking. This suggests that they decided to sacrifice light intensity for light duration.
See Dave’s complete writeup, including a suggested redesign here.