I'm currently building a second version for a friend with his old battery cells from another macbook. The cells are Sony Fukushima US604496 that are selling for 60USD on a site I found them (10 on ebay though). Holy Moly. 2500mAh per cell, and these are 2 of them joined together.

http://worcell.com/product_info.php?products_id=75708?osCsid=33aa070a35375be6a496fb472e3c50be

I also found 4 Coslight CA603696 LiPos with 2150mAh in an old Laptop battery pack.

Something to consider, especially for friends of apple stuff.

https://learn.adafruit.com/minty-boost/icharging