If you cycle, one of the safety measures is to have a strong front and rear light. That means you must always check the batteries. One easy way might be using a dynamo. It will remove the problem of weak batteries but add on friction to your cycling.
When I was young, I always hope that someone could invent something that do not need to use battery to power up a light. Dynamo is the closest I could get. Over the years, solar energy becomes another source of energy but the efficiency is totally not practical.
Finally, someone has invented a contactless light that makes use of magnet to power up LEDs. Meet this amazing Magnic Light. It was on Kickstarter by Dirk Strothmann and was funded successfully in March 2012. Too bad, I did not see this. If not, I would have supported it. Here is the video about this Magnic Light.
Now, months after the project is funded, the inventor has even better news. He has shrunk the size of the initial design to become become a micro version, Magnic Micro Light. I am hoping that he would be able to commercialize it.
BTW, here is a short explanation of how the Magnic Light works.
Energy is drawn contactless from the rotating bicycle wheels by making use of eddy currents effected by strong magnets. This new technique is the basis of our completely encapsulated bike light with no batteries and no external cables but with an enormous efficiency factor.