This mini-certificate provides a deep yet simple explanation of the operational principles of a rechargeable battery. The certification spotlights the importance and discusses the fundamentals of the ‘electrochemical cell’ which is the heart of any battery system independent of the storage size. A direct link is set forth between the chemical nature of the cell components and the storage properties of interest such as ‘electromotive force’ and gravimetric & volumetric capacities. Specific attention is directed towards lithium-ion batteries by presenting more details about the materials of choice for the anode/cathode together with their synthesis methods.