When the lithium battery cell is overcharged to a voltage higher than 4.2V, it will begin to produce side effects. The higher the overcharge voltage, the higher the danger. After the lithium battery voltage is higher than 4.2V, less than half of the lithium atoms remaining in the cathode material, and the storage grid will often collapse at this time, causing permanent capacity loss in the battery. If charging continues, since the storage compartment of the negative electrode is already filled with lithium atoms, subsequent lithium metals will accumulate on the surface of the negative electrode material. These lithium atoms will grow dendritic crystals from the surface of the negative electrode in the direction of the lithium ions. These lithium metal crystals pass through the diaphragm, shorting the positive and negative poles. Sometimes the battery explodes before the short circuit occurs, because in the overcharge process, the electrolyte and other materials will decompose to produce gas, making the battery shell or pressure valve bulge and rupture, allowing oxygen to enter and react with the lithium atoms accumulated on the surface of the negative electrode, and then explode.
Therefore, when the lithium battery is charged, it is necessary to set the upper voltage limit, so that the life, capacity, and safety of the battery can be taken into account at the same time. The ideal maximum charge voltage is 4.2V. Lithium batteries should also have a lower voltage limit when discharging. When the cell voltage drops below 2.4V, some of the material will begin to be destroyed. And because the battery will self-discharge, the longer the voltage will be lower, therefore, it is best not to put it to 2.4V before stopping. During the period from 3.0V discharge to 2.4V, the energy released by lithium batteries accounts for only about 3% of the battery's capacity. Therefore, 3.0V is an ideal discharge cutoff voltage. When charging and discharging, in addition to the limitation of voltage, the limitation of current is also necessary. When the current is too large, the lithium ions do not have time to enter the storage grid and will accumulate on the surface of the material.
When these lithium ions acquire electrons, they produce lithium atom crystallization on the surface of the material, which is as dangerous as overcharging. If the battery casing breaks, it will explode. Therefore, the protection of lithium-ion batteries should at least include: the upper limit of charging voltage, the lower limit of discharge voltage, and the upper limit of current. In the general lithium battery pack, in addition to the lithium battery cell, there will be a protection plate, which mainly provides these three protections. However, the three protections of the protective plate are obviously not enough, and the global lithium battery explosion is still frequent. To ensure the safety of the battery system, the cause of the battery explosion must be analyzed more carefully.