I. Charging
Currently, the primary charging method for lithium-ion batteries is the voltage-limited and current-limited method, with constant current (CC) charging initially providing the strongest battery acceptance. This process is primarily endothermic, but at excessively low temperatures, material activity decreases, potentially causing the battery to prematurely enter the constant current stage. Therefore, this method is less effective in cold winters in northern regions.
Preheating the battery before charging can improve the charging effect. As the charging process continues, polarization intensifies, temperature rises more rapidly, gas evolution occurs, electrode overpotential increases, and voltage rises.
When the charge reaches approximately 70-80%, the voltage reaches the maximum charging limit voltage, and the system enters the constant voltage (CV) stage. Theoretically, there is no objective overcharge voltage threshold; if gas evolution and temperature rise are interpreted as overcharging, then...
At the end of the constant current stage, overcharging of varying degrees always occurs, with the temperature rising to 40-50 degrees Celsius. Shell deformation is easily detected, and some of the escaped gases can recombine, while others result in irreversible reactions.
The loss of capacity can be seen as the current intensity exceeding the battery's accepting capacity. During the constant voltage stage, also known as trickle charging, approximately 10% of the capacity is charged in about 30% of the time. The current intensity decreases, gas evolution and temperature rise no longer increase, and they change in the opposite direction.
II. Overcharging
The above process considers the control of the total or average voltage of the battery pack. In reality, there are always individual cells with higher voltages, which means that the cells in the pack have already entered the overcharge stage relative to the other cells in the pack.
If overcharging occurs during the constant current phase, the voltage, temperature rise, and internal pressure will continue to increase due to the high current intensity. Taking a 4V lithium battery as an example, when the voltage reaches 4.5V, the temperature rises by 40 degrees Celsius and the plastic casing hardens. At 4.6V, the temperature rise can reach 60 degrees Celsius, and the casing deformation is obvious and irreversible.
If overcharging continues, the gas valve will open, the temperature will continue to rise, and the irreversible reaction will intensify. During the constant voltage stage, the current intensity is relatively low, and the overcharging symptoms are not as obvious as during the constant current stage. As long as the temperature rise and internal pressure are too high, side reactions will occur, and the battery capacity will decrease.
Side reactions have inertia; if they develop to a certain extent, they may cause the internal materials of the battery to burn during charging or shortly after charging is completed, leading to battery failure. Overcharging accelerates battery capacity decay and causes battery failure, doing nothing but harm.
