Nowadays, electrochemical storage systems represent a very interesting research field. In the literature, several models can be found depending on which aspects, of the storage systems, are represented. Electrical, thermal, ageing aspects, or a composition of the previous ones, can be retrieved. Focusing on the electrical aspects, it is possible to recognize two main subgroups of the electrical models. Models composed of voltage sources and passive elements and models in which only passive elements are present. The former are usually used for batteries whilst the latter for supercapacitors. We can call them, respectively, active and passive models. Indeed, batteries are seen as dc voltage sources whose voltage depends on chemical phenomena, while supercapacitors are seen as large capacitors whose voltage depends on electrical phenomena. Although there is this physical difference between the two devices, from a conceptual point of view, also the batteries should be seen, and thought, as big capacitors whose electric charge is accumulated in chemical way instead of in electrical way. This paper proposes a novel simple passive model for lithium ion batteries in which their parameters are estimated using a simple time domain technique. Experimental and simulation tests are carried out to validate the proposed model.