Maybe you can find the needed battery pack on Amazon or eBay more cheaply then trying to built your own here.
As you can see there’s quite a lot of engineering involved in making a battery pack. You can get into trouble very easily here! Over charging can cause the battery to explode! And what that happens ignite in a very hot chemical fire. The alignment of the batteries (series and parallel) also can create instability! As some cells will charge/discharge at a different rate which is why you need the circuitry to monitor this.