Charge and Discharge Failures

Obviously, charging of any kind on primary cells should never be done as it has the potential to cause serious accidents. Overcharge and overdischarge (voltage reversal) of rechargeable batteries can occur if the control electronics of the charging station or the battery pack control electronics in the Battery Management System (BMS) malfunction or if severe cell imbalance occurs in a battery pack. 

The ability to withstand overcharge depends strongly on the chemistry of the battery. Aqueous electrolyte systems (e.g., lead-acid, Ni-Cd, and Ni-MH) are less sensitive to overcharge because, after 100% state of charge is reached, additional current drives the electrolysis of water (which produces hydrogen and oxygen) and limits the maximum voltage that the cell experiences. CeUs with aqueous electrolyte 

Li-ion and Li-polymer cells have poor response to overcharge abuse compared to aqueous electrolyte cells because they contain more energy and have potentially flammable electrolyte solvents. Instabihty of certain cathode materials (typically containing nickel oxide) at elevated voltages will produce oxygen that exothermically reacts with organic materials within the cell [27]. Newer cathode materials have improved the situation. 

The plot shows the response of cells to short circuit where the external circuit resistance is less than 100 mS2. The cell current peaks within less than a minute and the temperature maximum occurs a few minutes later. The current often exhibits a plateau with a slow rise, which is due to increased conductivity of the cell at elevated temperature. Cells typically can withstand an external short circuit, since thermal output is small and the cell is in contact with the test fixture. Thermal management will dictate whether response of cells will be benign, as in this test, or exhibit thermal mnaway. Large cells (i.e., over 10 Ah), cells that can sustain very large short-circuit currents, cells that have higher internal resistance, and cells with low inherent thermal stability are more prone to exhibit thermal runaway. 

Internal short circuit is a failure mode where a current path develops within the cell. It can be caused by several factors, including a foreign object, poor cell design (e.g., lack of sufficient separation or insulation of electrodes in the cell), or external pressure on the cell walls. 

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