I connected up this battery with a single wire. It's what we'd call a short circuit. The battery got quite hot after about 20 minutes. That's because electrical energy is being used up inside the battery and dissipated as thermal energy. The reason is because the inside of the battery has resistance - called INTERNAL RESISTANCE. When current flows, it dissipates thermal energy. So electrical energy is being wasted - that's energy that should have been going to components in the circuit. I posted previously about EMF, the amount of chemical energy transferred into electrical energy inside the battery. But since some of the electrical energy is wasted heating the battery, the actual output of the battery is less than the EMF. In fact, the higher the current that flows, the lower the output. (The output is called the TERMINAL pd because it is the potential difference between the terminal.) So although it tells you the EMF on the side of the battery, the actual output of the battery depends on what is connected into the circuit because this affects the current that flows.
DrPhysicsA on Youtube has been recommended to me. He covers the topic here:
https://www.youtube.com/watch?v=8GNKgFB2T30&index=14&list=PL5D99A319E129A5B7
