No batteries to throw away, infinitely rechargeable from a USB port (we use a usb solar charger). See if you can fit this in your favorite flashlight body. Supercaps charge quickly, are about 95% efficient, and last almost forever.
For those who never heard about supercapacitor, you can check for more info about supercapacitor here. Supercapacitor is basically a capacitor with very high capacity, and the capacity rating is normally around few Farads. With the “super” capacity, it basically can store a lot of charge, and I am going to use the “super” capacity to store charges for a 5mm LED. Let’s see how long will the 5mm LED last.