LM338 20A power supply

Upgrade question:
I thought I could use the LM317 but I don’t think it can’t handle the power. So I am looking at the LM338. I found your link and wanted to know if I can use this battery instead of the transformer? Or do you have a better way to do it?

My power source:
It must be portable and I found the Makita 18V Compact Lithium-Ion battery pack works the best for my needs. I am looking for a way to adjust the voltage. The Battery is the 18Vdc and has this ratting of Amp/ Hour: 1.5 aH.

Back ground use:
I am using it to power my custom built slot car Pit-Kit test bed. It is used to tune the motors and sand the traction tires. This true’s them to run smooth on the track. This gives me better traction and performance on the track.

Current setup:
It does not work. I have a 25 ohm pot connected to it now. It kind of works but the adjustment is from 16 to max. I need to bring it down to about 3V and in between max.