What does it actually mean for a battery to be fully charged? In the quantum world, the answer is less obvious than for an ordinary battery. Researchers from the University of Gdańsk have shown that in the smallest systems, it is not only the amount of usable energy that matters, but also the way it is internally organised. The study, published in PRX Energy, suggests that two quantum batteries containing the same amount of usable energy can behave differently depending on how that energy is stored. The findings could help improve the design of future quantum computers, sensors, communication networks and other devices operating at the atomic scale. Physicists and engineers are increasingly designing devices built from individual atoms, ions, photons or tiny superconducting circuits. At this scale, energy does not behave in the same way as it does in conventional batteries. Instead, it must be described using quantum mechanics. Future quantum technologies will need not only to process information, but also to absorb, store and transfer energy, while inevitably losing some of it to the environment. Understanding what it means to charge such systems, how long they can retain energy and how much of that energy can later be recovered is therefore becoming increasingly important. This is where the concept of a quantum battery comes in. A quantum battery is a microscopic system capable of storing energy and releasing it in a controlled way. It may consist of an atom, an ion, a system of light or a small superconducting circuit. For physicists, however, a quantum battery is primarily a model for studying energy storage at the smallest scales. Such systems possess properties that ordinary batteries do not. Under certain conditions, they can charge more quickly, exploit the collective behaviour of many quantum components or store energy in the