The working principle of a lithium-ion battery refers to its charging and discharging principle. When the battery is charged, lithium ions are generated at the positive electrode. These lithium ions then move through the electrolyte to the negative electrode. The carbon used as the negative electrode has a layered structure with many micropores. The lithium ions that reach the negative electrode are embedded in these micropores. The more lithium ions embedded, the higher the charging capacity.
Similarly, when a battery is discharged (i.e., when we use the battery), lithium ions embedded in the carbon layer of the negative electrode are released and move back to the positive electrode. The more lithium ions return to the positive electrode, the higher the discharge capacity. The battery capacity we usually refer to is the discharge capacity.
During the charging and discharging process of a lithium-ion battery, lithium ions are in a state of motion, moving from the positive electrode to the negative electrode and back to the positive electrode. If we figuratively compare a lithium-ion battery to a rocking chair, with the two ends of the rocking chair representing the two electrodes of the battery, then the lithium ions are like excellent athletes running back and forth between the two ends of the rocking chair. Therefore, experts have given lithium-ion batteries another endearing name: rocking chair batteries.
It is important to note that overcharging or over-discharging can damage the battery. To address this, lithium-ion batteries are equipped with overcharge protection and over-discharge protection.
The protection circuit of a lithium-ion battery consists of a protection IC and two power MOSFETs. The protection IC monitors the battery voltage and switches to the external power MOSFETs to protect the battery when there is overcharging or over-discharging.
The principle of overcharge protection IC is as follows: When an external charger charges a lithium-ion battery, to prevent the internal pressure from rising due to temperature increase, the charging process needs to be terminated. At this time, the protection IC needs to detect the battery voltage. When it reaches 4.25V (assuming the battery overcharge point is 4.25V), the overcharge protection is activated, switching the power MOSFET from on to off, thereby stopping the charging process.
In addition, it is important to be aware of potential overcharge detection malfunctions caused by noise, which could lead to the device being flagged as overcharge protection. Therefore, a delay time must be set, and this delay time should not be shorter than the duration of the noise.
Under over-discharge conditions, the electrolyte decomposes, leading to a deterioration in the characteristics of lithium-ion batteries and a reduction in the number of charge cycles. Using a lithium-ion battery protection IC can prevent over-discharge and achieve battery protection.
Over-discharge protection IC principle: To prevent the lithium-ion battery from over-discharging, assuming the lithium-ion battery is connected to a load, when the lithium-ion battery voltage is lower than its over-discharge voltage detection point (assumed to be 2.3V), the over-discharge protection will be activated, causing the power MOSFET to switch from on to off and cut off the discharge, thus preventing the lithium-ion battery from over-discharging and keeping the battery in a low quiescent current standby mode, at which time the current is only 0.1μA.
Since the problems of overcharging and over-discharging have been technically solved during the charging and discharging process of lithium-ion batteries, they can be used with confidence.
