A nuclear battery for supplying low level electrical energy for a relatively long period of time. The battery includes a low-energy beta emitter and phosphor dispersed sufficient proximate the beta emitter to capture the low energy betas before decay. A photovoltaic receptor is configured to have a peaked response near the wavelength of the photons emitted by the phosphor. In a preferred embodiment, the photovoltaic, phosphor and beta source are formed into flexible layers which are rolled into a cylinder in order to maximize the capture of photons emitted by the beta-excited phosphor