I guess, we are talking about a sulfuric cell? I would say, you can only change the voltage. If you change voltage, the ampere will change together with the voltage. 20 A sounds wonderfull, but this means, that 20 A is the maximum. that the cell can suck out of the adapter, without overheating it. So, that's just fine. 12 V worked also fine for me, but I could lower the voltage, when I thought the cell went too hot. I would prefere to build a setup with at least 3V, 6V and 12 V. If I remember right, I could use the adjustable voltage to avoid gas production. I didn't use the adjustable A, because it just means to set the max. Ampere down, so the automatic adjusted the voltage to a lower value. You cannot demand the circuit, how much Ampere it has to use without taking effect on the voltage. This is practically only possible by changing the areal of the electrode surface.
