Norwegian maker Knut Karlsen got some flexible solar material and decide to try his hand at prototyping solar-powered batteries. He wrapped the cells around some NiMH rechargeables he had on hand and wired them up. He writes:

This setup is in fact a (quite weak) trickle charger. It is not ideal, but really convenient. A second version would have some electronics (to check when full) and capacitors to charge the batteries more efficiently, but the battery would then be smaller if it all needs to fit within a C-cell battery.

The SunCat Batteries – DIY prototypes