AG11 battery.Protective measures for lithium phosphate batteries

　　After overcharging the lithium phosphate battery cell to a voltage above 4.2V, it will begin to produce side effects. The higher the overcharging voltage, the higher the danger. When the voltage of a lithium battery cell exceeds 4.2V, less than half of the lithium atoms remain in the positive electrode material. At this time, the storage cell often collapses, causing a permanent decrease in battery capacity. If charging continues, as the storage cell of the negative electrode is already filled with lithium atoms, subsequent lithium metal will accumulate on the surface of the negative electrode material. These lithium atoms will grow dendritic crystals from the negative electrode surface towards the direction of lithium ions. These lithium metal crystals will pass through the diaphragm paper, causing a short circuit between the positive and negative poles. Sometimes, before a short circuit occurs, the battery explodes first. This is because during overcharging, materials such as electrolyte will crack and produce gas, causing the battery casing or pressure valve to swell and rupture, allowing oxygen to enter and react with lithium atoms accumulated on the negative electrode surface, leading to an explosion. Therefore, when charging lithium phosphate batteries, it is necessary to set a voltage limit in order to simultaneously consider the battery's lifespan, capacity, and safety. The optimal charging voltage limit is 4.2V. When discharging lithium batteries, there should also be a lower voltage limit. When the voltage of the battery cell is below 2.4V, some materials will begin to be damaged. Also, due to the self discharge of the battery, the voltage will decrease as it is discharged for a long time. Therefore, it is best not to stop discharging at 2.4V. During the discharge period from 3.0V to 2.4V, the energy released by lithium phosphate batteries only accounts for about 3% of the battery capacity. Therefore, 3.0V is an ideal discharge cutoff voltage. During charging and discharging, in addition to voltage limitation, current limitation is also necessary. When the current is too high, lithium ions cannot enter the storage cell in time and will gather on the surface of the material. After these lithium ions acquire electrons, they will generate lithium atom crystals on the surface of the material, which, like overcharging, can cause danger. If the battery casing ruptures, 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 general, in a lithium battery pack, except for the lithium phosphate battery cells, there is a protective plate that mainly provides these three protections. However, these three protections of the protective board are obviously not enough, and global lithium phosphate battery explosions are still frequent. To ensure the safety of the battery system, it is necessary to conduct a more detailed analysis of the causes of battery explosions.

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