Step 2: Cells
The cells are a common format, and therefore widely available and cheap, called 18650s because they are 18mm by 65mm. They use 6800 18650s in the Tesla Roadster! My brother works at a university where he has access to an e-waste dumpster. Old laptops are often powered by these cells. Often you can look up the date of manufacture and capacity by looking up ID numbers on the cells. There is no way to tell how many cycles they have been through but the savings are so great over new ones that it is easy to ignore most lack-of-charge problems.
There are 18650s all over the net which claim capacities all the way up to 4.2 amp hour and they are half the price of the ones on batteryspace. The highest capacity 18650s on batteryspace is 2.6 amp hour. I called a tech guy and asked him about these 4.2 amp hour Ultrafire 18650s. He said he never heard of Ultrafire and that 2.6 amp hour cells have been the industry standard for years. I haven't tried the Ultrafire batteries and I am not sure if they would work for this project. You can read more about them on Candlepower Forums .
New cells are 7.25 dollars each new on batteryspace here so I saved 116 dollars by finding used ones. If you can't find old batteries for free you can sometimes find used 18650s on ebay.