Connecting battery cells in series VS using boost converter
You can buy a ready-made battery, such as the 60 Ah unit from AA Portable Power or the 100Ah Li through Amazon  for ~US$600, or heavier, less environmentally-friendly but cheaper lead-acid batteries for ~US$160.

This would be simpler than designing your own pack and balancing circuit, and avoid some of the liability should something go wrong... which is not to say that commercial products have no issues.