What harm does overcharging and overdischarging do to the battery?

Overcharge
After fully charged and continue to charge to a certain extent, the gas generated by the decomposition of the electrolyte in the lithium-ion battery will cause the battery to swell and even smoke. In this way, the battery will not work properly. Just like a balloon, if you continue to inflate it after it is full, the balloon will burst sooner or later.
Some customers, for the sake of the beauty of their own products, or in order to meet the design requirements of product molds, often only purchase batteries and design protection boards by themselves, which is also easy to cause overcharging. On the one hand, because their battery expertise is not high, the experimental equipment is outdated, and the designed overcharge protection parameters are different; They can't judge, and always use the same process to design the overcharge protection parameters, which is also prone to errors and lead to overcharge.

Overdischarge
The so-called over-discharge refers to the fact that the battery continues to discharge after the internal storage of the battery is discharged, and the voltage drops to a certain value, resulting in over-discharge. Usually, the discharge cut-off voltage is determined according to the discharge current, that is, the critical voltage value for judging whether it is over-discharged. For a 3.7V lithium battery, the 0.2C-2C discharge is generally set at 3.0V, and the discharge above 3C, such as 5C or 10C For discharge, set to 2.4V.
Battery overdischarge often brings catastrophic consequences to the battery, especially high current overdischarge, or repeated overdischarge, has a greater impact on the battery's electrical performance. This is like a person who loses three to five hundred milliliters of blood at a time, which will not affect the person's health, but once the blood loss is too much, the person's normal body functions cannot be maintained, and the person will die to a certain extent. As far as batteries are concerned, over-discharge will increase the internal pressure of the battery, and the reversible function of the positive and negative active materials will be damaged. Even if it is remedied by charging, it is difficult to fully recover.