# Temporary, high-power energy storage

I'm currently building a small, portable human-powered charger for supplying a 1W led. I salvaged a dynamo core from a torch, but I'm unable to use other components inside since the torch is only designed to power 3 70mW LEDs; the internal li-ion battery is only capable of outputting a fraction of a watt.

The problem is one of storage. I know the device is capable of outputting the energy I need; by shortening the crank used to power it and attaching the output to a LM2596 voltage regulator, I was able to output ~1A at 5V (measured via multimeter shunt resistor); this is well in excess of the 1W required to drive the LED. While this power output took a lot of effort (since the dynamo was almost shorted), 1W was fairly sustainable. However, I'd prefer some sort of short-term energy storage that I could use to drive it - that is, at least 10 seconds of full power, so a capacity of greater than 10J.

Possibilities are as follows:

-I have a 10F supercapacitor, but it's only 2.2V, which is too low to drive the LED. While I could get a step-up SMPS, I'm worried about energy losses, since I'm already using one to step the voltage down from the dynamo.

-I could use lithium ion batteries, and intend to (charging apparatus is easy enough to get, and I have the power output capabilities), but I would also like the torch to have a long lifetime; battery lifetime of these is around 3-5 years last time I checked, which I feel is too short. Also, I might want to scale up power production (for instance, leg power instead of arm), which would entail higher energy input which these batteries are incapable of - being limited to a maximum of 1C, so about 1A or less.

-Other possibilities include perhaps a flywheel, but I'm not sure about bulkiness and feasibility.

Main design considerations are efficiency and portability, while still being able to meet power requirements.

Energy stored limit = $\dfrac{C\cdot V^2}{2}$ = $\dfrac{2.2\cdot 5^2}{2}$ = 27.5 joules.