Tunable direct beta-radiation harvester at the nanowatt scale

•Direct β-radiation energy-harvesting from a 63Ni is experimentally investigated.•This technique is interesting toward a reliable long-term miniature battery.•The design, development and experimental evaluation of the harvester are presented.•Steady state harvesting at a rate of 22.5 nW from 340 nW source was demonstrated.•The resulting relations are in agreement with theory.