12v Batt charger conversion to variable desk power

I have a 12v power supply. the selector switch went in it. My dad bypassed the switch and gave it back to me as a 100amp charger only.

I want to put it in a new box to power equipment on my desk.

I want to install a selector knob to step the amperage back and whats the best way to do that. is there anything made i could just solder in?

i also would like to have an output where i could vary the voltage for testing circuits.

Is it a "charger" or a "power supply"? Chargers are not usually filtered ( or even regulated), and would not be suitable to use as a bench supply without mucho added components...

Thanks, that's what I needed to know.
I'm looking to do this on the cheap while learning something.
I had a couple years of electrical engineering 25 years ago
I was more interested in socializing back then but I learned a little.
I am looking into the parts needed at Home
thanks for you help.