During battery discharge to an external load, its voltage gradually decreases. When the battery voltage drops to 2.5V, its capacity is completely discharged. Allowing the battery to continue discharging to the load at this point will cause permanent damage. During battery discharge, when the control IC detects that the battery voltage is below 2.3V (this value is determined by the control IC and varies between different ICs), its DO pin will change from high voltage to zero voltage, causing V1 to switch from conducting to turning off, thus cutting off the discharge circuit and preventing the battery from discharging to the load, serving as over-discharge protection. At this time, due to the presence of the body diode VD1 built into V1, the charger can charge the battery through this diode.
Due to the chemical characteristics of lithium-ion batteries, during normal use, positive chemical reactions occur inside the battery, converting electrical energy into chemical energy. However, under certain conditions, such as overcharging, over-discharging, and overcurrent, side chemical reactions can occur inside the battery. If these side reactions intensify, they can severely affect the battery's performance and lifespan, and may produce a large amount of gas, causing the internal pressure of the battery to increase rapidly and eventually explode, leading to safety issues. Therefore, all lithium-ion batteries require a protection circuit to effectively monitor the battery's charging and discharging status and, under certain conditions, shut down the charging and discharging circuits to prevent damage to the battery.
Generally speaking, over-discharge will increase the internal pressure of lithium-ion batteries, damage the reversibility of the positive and negative electrode active materials, and even if charged, it can only be partially restored, and the capacity will also be significantly reduced.
