I know that measuring the open-circuit voltage gives a good indication of the state of charge of a battery, both for rechargeables and single-use. But why is this so? As far as I can see the voltage is defined by the electron energies during the chemical reactions at the electrodes, which are constants. I think I can see why the internal resistance increases, since the molecules have to travel further before reacting, but the sub-microamp drain of a DVM is surely insufficient to cause a voltage drop of hundreds of millivolts.
So what am I missing?