There’s something so wonderfully simple about this concept that I wish it would work better than it does. Ni-MH batteries that have a flexible photovoltaic cell wrapped around them, so that they recharge by just being left on a windowsill.
Unfortunately, the solar cells currently only charge the battery with a tiny, weak trickle, but if they were built into a proper unit, combined with capacitors and the electronics to up the efficiency of the energy conversion, then they could do considerably better.
On a related note, how many devices do you have that still use traditional batteries? Very few, I’d wager – perhaps just a TV remote and a set of portable speakers. Everything’s rechargable these days, and a jolly good thing too.