Just went through this..
6/2 romex can handle 44 Amps Continuous, 55 Amp max. This is ok if you charge at 40 Amp with a 50 Amp Breaker. Wire should have more capacity than breaker. If you use 6/2 for 48 Amp charging, the wire can overheat. No Bueno.
If you want to charge at 48 Amp, you will...