Issues related to sources of electrical energy are not only currently relevant but are also increasing in number. A high-quality and stable source of electrical energy is an essential part and requirement in many technical systems of today. The power source is expected to meet quality, stability, and safety requirements crucial in systems whose operation is entirely dependent on the electrical energy source. It is even more important if the consequences of its failure have a catastrophic impact on the system, the operator, people, or the environment. This article is concerned with issues not yet addressed. The content of the article observes the properties and reliability indicators in Li-ion batteries that are the source of energy of an electric airplane and which have been stored for several years. The properties of these Li-ion cells are examined in several stages. The first one is based on i) extensive experiments, the next on ii) a study of the results, modelling, and simulations and, subsequently, on iii) a comparison of the results with new, non-stored batteries. The modelling is performed using modern and up to date methods of non-parametric functional analysis that have not yet been applied. In order to obtain useful time of safe operation, primary focus was directed at studying and modelling safe operating time of the battery. The achieved outcomes for long-term stored batteries are compared to new batteries outcomes. The results not only extend the knowledge of Li-ion batteries but are applicable far beyond the aviation industry. The obtained results show that operating aircraft when using batteries stored for several years ought to be safe. The stored batteries did not show a higher rate of quality or reliability degradation.