I have a couple of hand cranked flashlights laying around and I was never satisfied with their performance.
When I got them fresh out the box they worked great. But when their batteries ran out it was too exhausting to recharge the batteries manually. And after a couple of years, it was almost impossible recharge the batteries because they were too old. Now, a hefty 5 minutes of cranking produces only 20 seconds of light.
I decided to mod one of the flashlights by switching the lithium ion rechargeable battery with a super capacitor.
Surprisingly, the mod was easy and the results were very satisfying. Now, with just a mere 5 seconds of cranking I get 5 minutes of light. A major improvement.
In a further mod I added a joule thief which increased brightness (see step 5)
Step 1: The Back Up Super Capacitor
A super capacitor acts like a battery in computers and electronic devices, and therefore it can serve as a back up power supply.
The neat thing about super capacitors is that they last much longer than rechargeable batteries. They are easier to recharge because they have almost 100% efficient while the batteries lose between 50% to 30% of the energy when recharging.
I am using a 1 Farad super capacitor. But you can use bigger values. The bigger the super capacitor the longer the flashlight works.