Lithium batteries start to have side effects when they are overcharged to a voltage above 4.2V. The higher the overcharge voltage, the higher the risk. When the voltage of a lithium cell is above 4.2V, less than half of the lithium atoms remain in the positive electrode material, and the storage compartment will collapse, resulting in a lasting loss of capacity. If charging continues, subsequent lithium metal builds up on the surface of the negative electrode material as the storage compartment is already full of lithium atoms. These lithium atoms will grow dendritic crystals from the surface of the negative electrode in the direction from which the lithium ions came. These lithium metal crystals can pass through the diaphragm and short-circuit the positive and negative electrodes. Sometimes the battery explodes before the short circuit occurs because during the overcharge process, the electrolyte and other materials will decompose and produce gas, causing the battery casing or pressure valve to bulge and rupture, allowing oxygen to enter and react with the lithium atoms that have accumulated on the surface of the negative electrode, which then explodes.

Therefore, when charging Li-ion batteries, it is important to set an upper voltage limit in order to take into account the battery's life, capacity and safety. The ideal upper voltage limit for charging is 4.2V. There is also a lower voltage limit for discharging lithium cells. When the cell voltage falls below 2.4V, some of the materials begin to be damaged. As the battery will self-discharge, the longer it is discharged, the lower the voltage will be, so it is recommended not to stop at 2.4V. The energy released during the period from 3.0V to 2.4V is only about 3% of the capacity of the battery. Therefore, 3.0V is the ideal cut-off voltage for discharging. In addition to voltage limits, current limits are also necessary when charging and discharging. If the current is too high, the lithium ions will not be able to enter the storage compartment in time and will collect on the surface of the material.

When these lithium ions gain electrons, they produce crystals of lithium atoms on the surface of the material, which can be just as dangerous as overcharging. In the event of a rupture of the battery casing, it can explode. Therefore, the protection of lithium-ion batteries must include at least three items: upper charge voltage limit, lower discharge voltage limit and upper current limit. Generally, in addition to the lithium cell, there is a protection plate in the lithium battery pack, which mainly provides these three protections. However, these three types of protection are obviously not enough, and lithium battery explosions are still common worldwide. To ensure the safety of battery systems, the causes of battery explosions must be analysed more carefully.