A battery produces a direct current, unlike the mains electrical supply, which is an alternating current. In order to charge a battery, a direct current must be used, and similarly, an electrolytic etching process requires a direct current, which is why the mains supply cannot be used until it has been 'rectified' and reduced in voltage from 230 or 110 volts to very much less, usually not above 12 volts. The equipment to do this is known as a transformer and rectifier , and the most common household examples are the little power supplies that one can get for battery driven transistor radios and other electronic equipment. If two metal plates, say of copper, are placed parallel but not touching each other in a conducting solution of the same metal, say of copper sulphate, and they are connected to the terminals of a battery or of a source of direct current like a battery charger, then the current flows from one plate to the other through the solution. A complex process takes place in which the separate positive and negative 'ions' that make up the solution are separated and are attracted to the plate of the opposite polarity.
Copper sulphate consists of positive copper ions and negative sulphate ions.
Normally they are in equilibrium and they stick together like the positive and negative ends of two bar magnets. The current is actually able to flow because of the ions which give the solution the ability to conduct electricity. In fact at the point of contact, the process is exactly the same as being etched by an acid. While copper ions are becoming solid copper on the cathode, an equivalent amount of copper is being removed from the anode, and the copper sulphate is being kept in its original concentration, the copper removed from the anode, combining with the sulphate ions to form new copper sulphate at the same rate that it is losing copper ions at the cathode. There is a common fallacy that copper particles flow from one plate to another, but this is a misleading simplification .