An ideal portable power source would store energy with high energy density, would be able to release that energy with high power density, and would operate without degradation over a great many cycles. æThese goals are challenging in practice. BatteriesÍ high energy densities are offset by low power density and limited charge/discharge cycles. Supercapacitors and conventional springs offer high power density but suffer from low energy density. Although large fuel burning devices (e.g. power stations) offer both high energy density and high power density, their performance degrades significantly as they are down-scaled for portable applications. ææA promising alternative is to store energy in the elastic deformation of springs comprising large assemblies of carbon nanotubes (CNTs). æCNTsÍ unique material properties (very high stiffness and maximum strain) offer excellent energy storage per unit weight – more than 1000x greater than steel springs, and comparable to that of Li-ion batteries – while retaining springsÍ inherent compatibility with high power operation. æIn practice, disorder and defect generation make it challenging to translate individual CNTsÍ exceptional properties into high-performance, macroscale springs. æThis project combines Raman spectroscopy with load testing to characterize the structural evolution of CNT springs over large numbers of load cycles. æThe experiments highlight the springsÍ energy storage, the distribution of loads among CNTs, and the defect generation and long-term structural evolution that limits springsÍ performance. æBy unraveling the underlying science of CNT materials, we aim to enable a new generation of powerful portable systems for regenerative braking, green lawn care, and more.