Radioisotopic batteries are a good alternative to the conventional chemical batteries for applications requiring a long lifetime with minimum maintenance. For example, thermoelectric type batteries fueled with Pu have been used extensively on NASA space missions. The design of a small beta battery using thallium-204 and a vacuum direct collection method that is intended for powering low voltage electronics in space is described here. Tl is chosen as the source emitter because it can be reirradiated and reused. A vacuum insulation is employed to gain a higher efficiency than prior beta batteries with a dielectric insulator. A zirconium getter film is integrated into the design to maintain the vacuum over long periods of operation. Such batteries are inherently high voltage. However, here a unique voltage downconverter incorporated to efficiently reduce the output voltage from 125 kV to ∼125 V. This converter operates like a "reverse" Marx circuit where capacitor charging occurs in series but the discharge is in parallel. The reference beta battery described here is packaged in a cubic container about 3 cm on a side and has a power of ∼3 mW with a conversion efficiency of ∼17%.