Safe Temperature Control of Lithium Ion Battery Systems for High Performance and Long Life

摘要

Lithium-ion (Li-ion) batteries have diverse applications such as portable electronics, energy storage, hybrid electric vehicles (HEVs), plug-in HEVs, and electric vehicles (EVs). High energy density and longer life are major reasons for the widespread use of lithium-ion batteries. Presently, almost all major automobile manufacturers have hybrid vehicles in the market. HEVs can also help to reduce greenhouse gases and improve powertrain efficiency, but Li-ion battery life and performance significantly depend on the operating temperature and usage. High battery temperature increases battery degradation but this study proposes a counter-intuitive hypothesis that the life of lithium ion power cells can be increased by judiciously increasing the battery temperature in high power applications such as HEVs. End of life (EOL) in power applications often defined as when the battery is no longer able to provide the required charge/discharge power because the battery voltage exceeds the maximum/minimum allowable voltages associated with the battery's chemistry. It is experimentally shown that battery life can be increased by step-wise temperature increases whenever the battery voltage exceeds a voltage limit.;The temperature and capacity of individual cells affect the current distribution in a battery pack. Non uniform current distribution among parallel-connected cells can lead to capacity imbalance and premature aging. This study develops models that calculate the current in parallel-connected cells and predict their capacity fade. The model is validated experimentally for a nonuniform battery pack at different temperatures. The study also proposes and validates the hypothesis that active temperature control can reduce capacity mismatch in parallel-connected cells. Three Lithium Iron Phosphate (LFP) cells, two cells at higher initial capacity than the third cell, are connected in parallel. The pack is cycled for 1500 HEV cycles with the higher capacity cells regulated at 40°C and the lower capacity cell at 20° C. As predicted by the model, the higher capacity and temperature cells age faster, reducing the capacity mismatch by 48% over the 1500 cycles. A case study shows that cooling of low capacity cells can reduce capacity mismatch and extend pack life.;Lithium ion cells are increasingly being used in high power applications. There are four battery characteristics that are interlinked: Battery life, capacity, operating temperature, and usage. The goal of battery pack design is to minimize the battery pack cost or to maximize the battery pack life or both if possible. In this study, a model based process is developed that selects battery operating temperature and capacity to optimize the life and cost of the battery pack under prescribed usage.;At high temperatures, battery degradation increases and reduces battery life, but battery internal resistance reduces and improves battery performance. Batteries have a maximum allowable voltage limit based on degradation minimization, so the battery capacity is selected large enough to stay within the limit over the entire life of the pack. A real-time control algorithm is developed to vary the temperature of cells to improve their charge acceptance and reduce HEV pack size while maintaining battery life. The proposed algorithm has two strategies. First, the battery pack temperature is increased when its state of charge (SOC) is high because the cell is more likely to exceed maximum voltage limit at high SOC. Second, the battery pack temperature is increased if a high current pulse is expected because higher cell temperature reduces the internal resistance and the corresponding voltage swing.;Besides battery performance, battery safety also has a paramount importance. Battery internal short and overcharge are two dangerous abuse conditions which can lead to the catastrophic results such as fire, smoke, or thermal runaway. Any accident related to battery systems pushes back the acceptability of new technology. This study explores the battery nail penetration and overcharge tests under different conditions. Internal short circuit occurs when a direct current path within a battery is established. A nail penetration test is used to simulate the internal short circuit process, which involves penetrating a test cell/pack with an electrically conductive nail. Gathering useful data at the point of penetration during nail penetration tests is very challenging due to the inherently destructive nature of the test. An intelligent nail (iNail) design is developed to conduct battery cell and pack level nail penetration tests. A prototype stainless steel iNail is manufactured and presented. Multiple thermocouples are placed inside the iNail. The iNail successfully recorded the temperature time history around the penetration point during the nail penetration test of a 4Ah pouch cell. Battery overcharge tests of the cylindrical cells are performed for 2 different chemistries NCA (lithium nickel cobalt aluminum oxide) and NCM (lithium nickel manganese cobalt oxide). Fresh cells and aged cells are overcharged at 1C and 10C rate at 45°C and -20°C.