If you wanted to calculate the energy supplied by a battery in time t you would use E=VIt where I is the current through the battery. If the internal resistance is r we could also use E=V2rt. So it must be that V2r=VI or V=Ir. But this is false in general (as an example, take emf 12V, current 0.5A and load resistance 15Ω) and I am wondering why it doesn't work?