I want to buy a Prodigy 620 Charger, it requires 12-18v dc input. My power supply is 24v and is not adjustable. Is there a regulator I can buy to adjust my current from 24 to the 18 I want? This power supply can push upward of 48 amps

By the time you build or buy a regulator it will be cheaper to get a better charger or another powersupply.

What 24v supply do you have?

I'm using server computer power supplies in series, they are rock solid, low cost but just aren't adjustable. It sounds like I'm gonna just use one at a time for 12 volts. I have two in series for the 24 volt 48 amp.