 In this video, we're going to look at the difference between primary and secondary cells. So in the previous video, we had a look at this diagram here of an electrochemical cell or a battery. So when these two probes were joined together by a wire, electrons would flow from the higher potential zinc to the lower potential at copper. As this occurs, the rod of zinc will start to get smaller as it's turned into zinc ions. And the copper rod will grow larger as copper ions are turned into copper metal. Eventually, either the zinc metal or the copper ions will run out. And this will cause the voltage to decrease and electrons will stop flowing across the cell. We could represent this as a plot of voltage against time. So we start out with an almost constant voltage. So electrons are flowing from the zinc into the copper. So zinc metal is being used up in copper ions as being formed. Then all of a sudden we either run out of zinc or we run out of copper ions. And that means that our battery quickly starts to die and the voltage drops off. As shown in the diagram here. Phenomenal rechargeable batteries. So these are called primary cells. This is the end of the story. They would be discarded. However, there are some batteries that can be recharged and these are called secondary cells. In a secondary cell, energy in the form of an external voltage is applied and it will cause the electrons to flow in the opposite direction back to the high energy environment.