Evaluating the capabilities of GEANT4 code and COMSOL Multiphysics software can be achieved through coupling for an betavoltaic battery design. Spatial distribution of the deposited energy for the beta particles in the semiconductor transducer has been simulated by using the GEANT4 and then, electron-hole pair generation rate has been obtained. Subsequently, output performances of the battery such as the current-voltage (I–V) characteristics and the maximum electrical power have been determined by using the COMSOL Multiphysics in which the electron-hole pair generation rate from the GEANT4 simulation was utilized as input. Validation is done by considering an optimized planar betavoltaic battery. The results of the current study have been compared with other articles. The results are in good agreement and the relative errors are less than 8%. Our simulation model can be extended to the betavoltaic batteries with other semiconductors and radioactive isotopes, and can be provides a powerful tool for predicting the output performance and optimizing the betavoltaic batteries.