Did you know that you can buy 9V rechargeable batteries? You can, but they're rare in stores. I recently bought some off ebay to use on projects and loved them. Only downside is that I don't have a 9V battery charger and all the ones I found on ebay are quite expensive.
Luckily I'm resourceful and decided to build my own. The entire project only cost me $10 and didn't take that long.
Time: 1/2 Hour