Energetical modelling of lithium-ion battery discharge and relaxation

Available for a decade for portable electronic applications, lithium-ion battery technology encountered a swift rise, and now it represents broadly 60 % of the market on this segment. From a specific energy and power point of view, lithium-ion accumulators offer performances far more superior to other accumulator technologies. This paper deals with investigations on an accurate 6.8 Ah lithium-ion battery energetical modelling. All electrochemical devices are characterised by an electrical behaviour, which depends on temperature, state of charge and current. In the case of lithium-ion accumulators, the energetical behaviour is moreover deeply marked by line effects, due to the porosity of both electrodes. This paper shows in particular that electrode porosity can be taken into account by means of a diffusion impedance represented by a capacitive transmission line. An energetical model, which couples this line with a current independent capacitance, is proposed and characterised at constant temperature (20degC) over different states of charge intervals (5 %) and for different currents. Reactant diffusion within the electrolyte and relaxation period after discharge are investigated as well. Validation tests carried out on a 6.8 Ah lithium-ion element are conclusive.