When the overcharge voltage of lithium battery cell is higher than 4.2V, side effects will start 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 left in the positive material is less than half. At this time, the storage cell often collapses, causing a permanent decline in the battery capacity. If the battery is charged continuously, since the storage cell of the negative electrode is already full of lithium atoms, the subsequent lithium metal will accumulate on the surface of the negative electrode material. These lithium atoms will grow dendritic crystals from the anode surface to the direction of lithium ions. These lithium metal crystals will pass through the separator paper and short-circuit the positive and negative poles. Sometimes the battery will explode before the short circuit occurs, because during the overcharge process, materials such as electrolyte will crack to produce gas, causing the battery shell or pressure valve to swell and crack, allowing oxygen to enter and react with lithium atoms stacked on the negative electrode surface, and then explode. Therefore, when charging the lithium battery, you must set the upper limit of the voltage to take into account the battery life, capacity and safety. The optimal upper limit of charging voltage is 4.2V.

The lithium battery cell should also have a lower voltage limit when discharging. When the cell voltage is lower than 2.4V, some materials will be damaged. Also, since the battery will self discharge, the longer it is discharged, the lower the voltage will be. Therefore, it is better not to put it at 2.4V before discharging. During the period from 3.0V to 2.4V, the energy released by lithium battery only accounts for about 3% of the battery capacity. Therefore, 3.0V is an ideal discharge cut-off voltage.

In addition to voltage limitation, current limitation is also necessary during charging and discharging. When the current is too high, lithium ions will gather on the material surface before they can enter the cell. After obtaining electrons, these lithium ions will produce lithium atom crystals on the material surface, which, like overcharge, will cause danger. If the battery case breaks, it will explode.

Therefore, the protection of lithium ion batteries should at least include: upper limit of charging voltage, lower limit of discharge voltage, and upper limit of current. In general, except for the lithium battery cell, there is a protection plate in the lithium battery pack, which mainly provides these three protections. However, the three protections of the protective plate are obviously insufficient, and the lithium battery explosions around the world are still frequent. To ensure the safety of the battery system, the cause of the battery explosion must be analyzed more carefully.