Batch experiments in a 0.6 m diameter mill have shown that improvements of about 13 per cent in the efficiency of energy utilisation can be realised in ball mills by replacing 25 per cent of the ball charge by an equivalent volume of cost-free critical size pebbles from AG/SAG mills. For a mill of a given size, speed and charge filling, the rate of size reduction of ore particles depends on (i) the frequency of grinding media collisions (ii) the probability of capture of ore particles between the colliding grinding media (iii) the probability of fracture of the captured particles. The probability of fracture depends on the magnitude of collision energy. The distribution of collision energy into shear and impact events determines the relative strengths of attrition breakage and impact breakage. The efficiency of energy utilisation depends on which of the two mechanisms of breakage is more desirable in a particular milling stage. In this paper collision data from a series of DEM simulations of a mill charged with steel media and a mixture of steel media and pebbles is presented in energy spectra and spatial distribution maps. The data provides the necessary detail to elucidate the improvements in the efficiency of energy utilisation in ball mills when using a mixture of pebbles and steel media.