So if you're looking to change 20v to 12v then you need a buck converter capable of 30a https://www.amazon.com/VOLRANTISE-Regulator-Converter-Waterproof-Transformer/dp/B09X1ZGX9H/ref=sr_1_1_sspa?crid=3M7ZCJHVCJQYK&keywords=buck+converter+12v+30a&qid=1654611468&sprefix=buck+converter+12v+30a%2Caps%2C103&sr=8-1-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUEyOEVMRURXRDRWVEoyJmVuY3J5cHRlZElkPUEwMDE2OTcwMUlPNjFZREkxVlRNTyZlbmNyeXB0ZWRBZElkPUEwNjY5ODI0NDkwVjJaTzBOSkYyJndpZGdldE5hbWU9c3BfYXRmJmFjdGlvbj1jbGlja1JlZGlyZWN0JmRvTm90TG9nQ2xpY2s9dHJ1ZQ==

If you just want something to reduce the speed then you need a PWM like this. https://www.amazon.com/Controller-Stepless-Adjustable-Potentiometer-Forward-Brake-Reverse/dp/B089273KVZ/ref=sr_1_4?crid=RPL3713EFASE&keywords=PWM%2Bdc%2Bmotor&qid=1654611400&sprefix=pwm%2Bdc%2Bmotor%2Caps%2C115&sr=8-4&th=1

The difference is, A Buck converter actually reduces the voltage down. A PWM doesn't reduce the voltage it pulses the voltage to create a specific speed setting.