Knut Karlsen constructed his very own solar rechargeable batteries that can sit out in the light and convert the suns rays into stored power. He has documented this DIY project to create the prototype SunCat battery.
“The batteries should just bask in the sun like a cat and left for a while, in a sunny window, they would slowly recharge.”
Solar charges meant to juice up your battery powered gadgets, like the Solio, have been on the market for years. Turns out Knut has some connections at the Institute for Energy Technology or IFE (very convenient contacts!) where he was able to procure some flexible solar cells. This may be a slight hiccup in materials for a quick DIY home project but never the less flexible solar cells, or Photo Voltaics, are on the market in all sorts of shapes, sizes, and prices.
Knut attached the solar cells (which were already the correct dimensions) to old rechargeable NiMH batteries. The cells are rated at 1.8V and the batteries at 1.5V, which is acceptable.
Next, he used contacts from an old camera to connect the negative pole to the photo voltaic cells with the help of a conductive silver pen to finish the connection.
This type of charging arrangement is called a trickle charger and works by slowly charging the battery. They can be left charging in this way indefinately without over loading the battery. However, this is the slowest form of charging a battery.
Sources: Bare Knut Blog