What is the most efficient way to get electricity from shaking? I am thinking about something like shown in the picture below (the magnet is going inside the coil), but maybe there is a better design for it. Efficient I mean by power, like what percentage of vibration energy is converted to electricity. If it is a coil, what would be the best coil design (how many turns of wire and wire diameter I need, etc.)? Maybe there are some articles about it and how to fully calculate it as well.
There is this flashlight design on the internet, but is it the most efficient?