Electro-Thermal Modeling of Lithium-Ion Battery

Electro-thermal modelling of Li-ion batteries is one of the forefront topics due to the increasing interest of automotive industries in electric and hybrid vehicles. To control and manage the behavior of such batteries, reliable models are needed. In this thesis, Li-ion battery is modelled using an equivalent circuit analogy approach to study its behavior and its temperature variation under several charging/discharging tests. The model will be developed from battery cell to battery pack which is enclosed in an insulated box with air circulation to ensure temperature uniformity among its cells. First, electrical behavior is modelled using datasheet battery block found in Simulink library. This model characterizes the battery as an open circuit voltage connected with resistor in series. The main advantage of this model is that its main parameters can be obtained from the data of the battery datasheets. Open circuit voltage, internal resistance, initial capacity, number of cells in series and parallel were all defined to build up the electrical model based on the datasheet of Panasonic NCR18650B battery cell. The resistance of the battery depends on both its temperature and state of charge while open circuit voltage depends only on state of charge. Then, the thermal model of the battery is created according to thermodynamics principle and based on the assumption that the heat generated inside the battery is due only to joule’s effect. Thermal model stated that the heat stored inside the battery is the difference between the heat generated due to battery resistance (Pth = RIb2) and heat dissipated into the cooling system. To complete the thermal model, a liquid cooling plate system was designed. The plate is installed at the bottom side of the battery pack and is connected to a pump to deliver the required flowrate and to a heat exchanger that will release the heat absorbed from the battery to the ambient air. The thermal resistance between the battery and the coolant is found to be equal to 0.019 K/W and the global heat transfer coefficient of the radiator is found to be equal to 153.6 W/K based on the assumption that the difference between coolant and air temperature is 5oC and the amount of heat to be removed is equal to the amount of heat generated under 1 C discharge rate. The cooling system is activated when the temperature of the battery crosses 15oC. A PTC heater is also added to the cooling system to increase the temperature of the coolant when battery temperature is below 0oC. After designing the cooling system, the complete model is validated by comparing the results of several tests applied to the model with actual experimental results of the same tests done on 94Ah Samsung prismatic cell. The results revealed the ability of the virtual model to characterize the real behavior of a Li-ion battery up to ±2 V voltage difference and ±4oC temperature difference.