[fast_e_man]

There are lots of design considerations to package Li+ cells into a battery. each cell is pressurized and contains fammable electrolyte. If you over charge a cell, it will overpressurize and bad things happen. If you over discharge a cell a "voltage crash" occurs, and under certain circumstances the cells can self-reverse bias --- again bad things happen. Also, over a set of charge/discharge cycles, the cells in a battery become divergent in their charge state and don't equally charge. Therefore, you need to design cell balance electronics that will "Bypass" stronger cells while the "weak sisters" catch up. If you have a reliability issue with your charge control electronics, bad things will happen.

While all this is in-play you need a thermal management system designed into the battery packaging to maintain safe thermal operating limits.

Li+ battery design may not fully qualify as "rocket science", but it's way harder that just buying a stack of cells and bolting them under the hood of a car. There is tons of $$$ being poured into Li+ battery technology. Technology maturity is middle of the road, especially in applications where the engineers are trying to wring out maximum energy/kg.