Permanent magnets are widely employed in levitation-based vibration harvesters and sensors, but their operational range does not span ultra-low frequencies unless a mechanical resonator is added. In this work, an algorithm based on analytical equations has been implemented to design magnetic springs in resonant vibration energy harvesters operating at very low frequencies and the results have been comparatively checked and refined using 3D FEA (Finite Element Analysis). Particularly, a prototype based on a fixed ferrite and a mobile rare-earth magnet is designed, where the semi-analytical optimum solution was properly modified via FEA. The experimental induced pick-up voltage is characterized for ultralow vibrations (resonant frequency 1.7 Hz). This work is funded by Spanish Agencia Estatal de Investigación and Ministerio de Ciencia e Innovación MCIN/AEI/ 10.13039/501100011033 and by “European Union NextGenerationEU/PRTR”, project TED2021-130884B-I00. Open access funding provided by Universidad Pública de Navarra.