How to built a cheapest 20 v battery pack for dewalt drill machine  

I built this pack using used laptop batteries and casing from eBay , which cost me only 11 pounds. In this built I used soldering iron , soldering wire , spot welder,
Casing eBay : https: //www.ebay.co.uk/itm/224845977433?
circuit diagram : www.google.com...