Copper is used for building battery packs because it is both highly electrically conductive and highly thermally conductive. Copper is an effective means of both transferring power from one cell group to another and wicking away heat generated within the core of the cells. Copper has around 5 times less resistance. Nickel is used to build battery packs because it's both low cost and has excellent anti-corrosion properties. Nickel is easy to work with. This is because common spot welders are simply not powerful enough to directly weld copper. So, a little nickel is needed to form a high resistance. No. A copper battery is only better than a nickel battery if the batteries are completely identical and the same amount of material is being used. The thing is, when you build a copper battery, you have to use a lot less material. Not directly. At least not with the commercially available spot welding machine within reach of the average person. The copper-nickel sandwich was invented to get around this.
