After the lithium battery cell is overcharged to a voltage higher than 4.2V, side effects will begin to occur. The higher the overcharge voltage, the higher the risk. When the voltage of the lithium battery cell is higher than 4.2V, the number of lithium atoms remaining in the positive electrode material is less than half. At this time, the cell often collapses, causing permanent capacity loss of the battery. If you continue to charge, since the storage cell of the negative electrode has been filled with lithium atoms, subsequent lithium metal will accumulate on the surface of the negative electrode material. These lithium atoms will grow dendrites from the surface of the negative electrode toward the direction of the lithium ions. These lithium metal crystals will pass through the separator and short-circuit the positive and negative electrodes. Sometimes the battery explodes before the short circuit occurs. This is because during the overcharging process, the electrolyte and other materials will decompose to produce gas, causing the battery shell or pressure valve to swell and rupture, allowing oxygen to enter and react with the lithium atoms accumulated on the surface of the negative electrode. explosion.
Therefore, when charging a lithium battery, the upper voltage limit must be set so that the battery life, capacity, and safety can be taken into account at the same time. The most ideal charging voltage upper limit is 4.2V. There is also a lower voltage limit when discharging lithium batteries. When the cell voltage is lower than 2.4V, some materials will start to be destroyed. Also, since the battery will self-discharge, the longer it is left, the lower the voltage will be. Therefore, it is best not to stop when the battery is discharged to 2.4V. During the period when the lithium battery is discharged from 3.0V to 2.4V, the energy released only accounts for about 3% of the battery capacity. Therefore, 3.0V is an ideal discharge cut-off voltage. When charging and discharging, in addition to voltage limitation, current limitation is also necessary. When the current is too large, lithium ions will not have time to enter the storage cell and will accumulate on the surface of the material.
After these lithium ions obtain electrons, they will produce lithium atom crystals on the surface of the material, which is the same as overcharging, which is dangerous. If the battery casing breaks, it will explode. Therefore, the protection of lithium-ion batteries must include at least three items: upper limit of charging voltage, lower limit of discharge voltage, and upper limit of current. Generally, in the lithium battery pack, in addition to the lithium battery core, there will be a protective board, which mainly provides these three protections. However, these three items of the protection board