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Long-lived and high-energy-density betavoltaics have a great potential as power supplies for remote and hostile environmental conditions, where volume power density and/or power lifetime are very important considerations. In this paper, we provide new results to aid in the design and optimization of betavoltaics made with Si space solar cells and beta sources. The new results were obtained by using a customized low-energy electron accelerator to characterize the radiation-hardened high-efficiency Si space solar cells while varying the electron beam energy and electron beam current density, i.e., electron beam flux. The betavoltaic conversion efficiency of Si space solar cells increases until 60 keV and then decreases with the increasing electron beam energy. The maximum efficiency (6%) obtained at the electron beam energy of 60 keV suggests that Pm-147 would be a good beta source to make high-efficiency nuclear batteries. The radiation ionization energy is ~ 3.90 eV per electron-hole pair for Si space solar cells. Some radiation damage-induced performance degradation was also observed when the Si space solar cells were exposed to the bombardment of 62-keV electrons with fluence up to 4.92 × 1018betas/cm2, which is equivalent to the radiation from a semi-infinite Pm-147 layer for ~ 2.26 years. The results in this paper suggest that beta-particle entrance window, betavoltaic cells' configuration structure, and device properties such as charge carriers' diffusion length are very important factors to be engineered to improve the conversion efficiency for practical betavoltaics.