I have just seen a diagram of the 787 Dreamliner's battery system. In it, the negative terminal is copper and the positive is aluminium.
In a DC system I understood that the current at the supply was equal to the current at the 'sink', i.e. the same amount of current must be passing through both terminals. Why, then are they made from different materials? The only reason I can think of is that it means they could make the copper terminal smaller due to it having less resistance, but I'm just guessing.