Most PEV battery packs are just 18650s in combination of series/parallel, e.g. the most common e-skateboard pack is 3s10p (3 series of 10 parallel batteries, usually Samsung 30q or 25r 18650s). Add a voltage regulation board and you've got a PEV battery.

I know some DIY battery-makers and they're crazy like lacrosse goalies. But it's way cheaper to make a 4s12p battery that can do 35mph with 40 mile range than to buy one.