This chapter discusses the main technologies used for electrochemical accumulators such as lead, nickel and lithium accumulators. An electrochemical accumulator is often characterized by a number of values or indicators. These values depend on the technology of the battery, but also on the environment and the usage conditions. One might cite, for instance, the operating temperature, the discharge current, etc. The values which define a battery are: its capacity, its open‐circuit voltage, its end‐of‐discharge voltage, the state of charge (SOC), the depth of discharge, etc. The chapter presents two equivalent electrical circuits used to model the electrical behavior of lithium batteries: the model developed by FreedomCar and the Thévenin model.