Perovskite solar cells based on CH3NH3PbBr3 with a band gap of 2.3 eV are attracting intense research interests due to their high open-circuit voltage (V-oc) potential, which is specifically relevant for the use in tandem configuration or spectral splitting. Many efforts have been performed to optimize the V-oc of CH3NH3PbBr3 solar cells; however, the limiting V-oc (namely, radiative V-oc:V-oc,V- rad) and the corresponding Delta V-oc (the difference between V-oc,V- rad and V-oc) mechanism are still unknown. Here, the average V-oc of 1.50 V with the maximum value of 1.53 V at room temperature is achieved for a CH3NH3PbBr3 solar cell. External quantum efficiency measurements with electroluminescence spectroscopy determine the V-oc,V- rad of CH3NH3PbBr3 cells with 1.95 V and a Delta V-oc of 0.45 V at 295 K. When the temperature declines from 295 to 200 K, the obtained V-oc remains comparably stable in the vicinity of 1.5 V while the corresponding Delta V-oc values show a more significant increase. Our findings suggest that the V-oc of CH3NH3PbBr3 cells is primarily limited by the interface losses induced by the charge extraction layer rather than by bulk dominated recombination losses. These findings are important for developing strategies how to further enhance the V-oc of CH3NH3PbBr3-based solar cells.