The Maley procedure has been applied to obtain Ueff(j) of a Bi-n = 2 single crystal for B < 4 T//c-axis and T < 20 K from the time dependence of the non-equilibrium magnetization. Analyzing the resulting Ueff (j) within the collective creep model it is found that for large current densities, flux creep of elastic small (<λ) flux bundles consisting of quasi 2-dimensional pancake vortices occurs. However, at large j better agreement is found with the logarithmic potential model. From the logarithmic Ueff(j) dependence jc in the absence of thermal activation has been derived. It has been found that j > 1010 Am-2 for all fields investigated. These high values of jc indicate that the pancake vortices are pinned independently and that oxygen vacancies in the CuO2 planes are not the only pinning centers.