To change Volts and amps to random 12V and 500 amps you need to use step down transformer
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I expect you using electricity from your wall socket , like 220V 20 AMP (europe ) , because if you rise up 20amp to 500 amp , it means difference will be 25 times . like 20A*25=500AMP then voltage go lower 220V/25=8,8V and so on , so you need to find your own values for this logic , you may not get exactly 500AMP and 12V straight from transformer , decide what is more important for you if 12V s what you need but amp can be different , 220/12=18,3 times difference , in that case if you step down volt from 220V to 12V AMPS go up 20A*18,3=366AMP . There is possible to make exactly what values you need but it requires also add some CAPACITORS and some other stuff but its too long story to talk about it.