Explain how parts of the chloroplast work like a battery?

A battery works by separation of charge.  So too does part of the chloroplast.

The thylakoid membrane of chloroplasts have an electron transport chain in them.  High energy electrons are dropped off by electron carriers and then "fall down" the electron transport "chain", releasing a bit of energy at each step.  This released energy is used to pump protons across the membrane, uphill -- against their concentration gradient.  Thus, one side of the membrane has a huge excess of protons, forming an electrochemical gradient across the membrane.  This gradient, resulting from a separation of charge, is a form of potential energy, like that in a battery; it can be harnessed to do work.


The potential energy of the gradient is converted into kinetic energy as the protons flow back down their electrical and chemical gradients, through enzymes in the plasma membrane; these enzymes (ATP synthase) use the energy to combine ADP + Pi to form ATP.