Battery expansion is usually caused by excessive gas generated inside the battery during the charging process,leading to the expansion of the battery casing.This phenomenon is common in lithium batteries and is mainly caused by the following reasons.

　　Firstly,overcharging is one of the main causes of battery expansion.When the battery is charging,if the battery voltage exceeds the threshold(such as the upper limit of 4.2V for lithium batteries),it will cause abnormal chemical reactions inside the battery.During overcharging of lithium batteries,the reaction of lithium salts inside the battery may produce gas that cannot be released,resulting in battery expansion.If the charger does not cut off charging in a timely manner,overcharging will worsen and may even cause battery damage or explosion.

　　Secondly,a mismatch between the charger and the battery may also cause the battery to expand.If the voltage or current output by the charger is not suitable for the requirements of the battery,the battery may overcharge during charging,causing the internal temperature of the battery to rise and producing excessive gas.This situation is particularly common when using unsuitable third-party chargers,as the quality of the charger is not up to standard or there is insufficient protection mechanism,which can easily lead to excessive current during the charging process,causing expansion.

　　In addition,high temperature is also a key factor in battery expansion.Batteries generate heat during the charging process,especially when the charging current is too high,the internal temperature of the battery may rise sharply.High temperature not only accelerates the chemical reactions inside the battery,but may also cause the decomposition of the electrolyte,generate gases,and lead to expansion.To prevent this phenomenon,chargers are usually designed with temperature control functions to avoid battery overheating.

　　Battery quality issues are also a cause of expansion.Poor quality batteries may have defects in the production process,such as uneven internal materials or unstable electrolyte,which can accelerate gas generation.In addition,aging batteries are more prone to expansion.With the increase of battery usage time,the performance of internal chemical substances in the battery decreases,and the risk of expansion also increases.

　　The failure of the battery management system may also lead to expansion.The Battery Management System(BMS)is responsible for monitoring the voltage,temperature,and other status of the battery to ensure stability during the charging process.If the BMS fails,the charger may not be able to correctly determine the charging status of the battery,resulting in overcharging or high temperature,causing expansion.

　　In short,battery expansion is usually caused by overcharging,high temperature,mismatched chargers,battery quality issues,or battery management system failures.To avoid battery expansion,users should ensure the use of appropriate chargers,avoid overcharging,and choose batteries of reliable quality.Meanwhile,maintaining an appropriate charging temperature and regularly checking the battery condition can also help prevent the occurrence of expansion phenomena.