The need for protection in rechargeable lithium-ion batteries stems from their inherent characteristics. Due to the materials used in lithium-ion batteries, they cannot withstand overcharging, over-discharging, overcurrent, short circuits, or excessively high temperatures during charging and discharging. Therefore, lithium-ion battery packs are always accompanied by a sophisticated protection board and a current fuse. The protection function of a lithium-ion battery is typically achieved through the coordinated operation of a protection circuit board and current-controlled devices such as PTC (Power Transmission Control) devices. The protection board, composed of electronic circuitry, accurately monitors the cell voltage and charge/discharge current in environments ranging from -40°C to +85°C, promptly controlling the current circuit's continuity. The PTC prevents severe damage to the battery in high-temperature environments.
A typical lithium-ion battery protection board usually includes a control IC, MOS switches, resistors, capacitors, and auxiliary components such as FUSE, PTC, NTC, ID, and memory. The control IC, under normal conditions, controls the MOS switches to conduct, connecting the battery cell to the external circuit. When the battery cell voltage or circuit current exceeds a specified value, it immediately controls the MOS switches to turn off, protecting the battery cell.
1. Charging of lithium-ion batteries
The maximum charging termination voltage for a single lithium-ion battery is 4.2V. Overcharging is not allowed, otherwise the battery will be ruined due to excessive loss of lithium ions at the positive electrode. When charging lithium-ion batteries, a dedicated constant current and constant voltage charger should be used. First, charge at a constant current until the voltage across the lithium-ion battery reaches 4.2V, then switch to constant voltage charging mode. Charging should be stopped when the constant voltage charging current drops to 100mA.
The charging current (mA) can be 0.1 to 1.5 times the battery capacity. For example, for a 1350mAh lithium-ion battery, the charging current can be controlled between 135mA and 2025mA. A typical charging current can be selected at around 0.5 times the battery capacity, and the charging time is approximately 2 to 3 hours.
2. Discharge of lithium-ion batteries
Due to the internal structure of lithium-ion batteries, not all lithium ions can move to the positive electrode during discharge; a portion must remain at the negative electrode to ensure they can readily intercalate during the next charge. Otherwise, battery life will be shortened. To ensure that some lithium ions remain in the graphite layer after discharge, the minimum discharge termination voltage must be strictly limited; in other words, lithium-ion batteries cannot be over-discharged.
The discharge termination voltage of a single lithium-ion battery is typically 3.0V, and should not be lower than 2.5V. The discharge time depends on the battery capacity and the discharge current. Battery discharge time (hours) = battery capacity / discharge current. Furthermore, the discharge current (mA) of a lithium-ion battery should not exceed three times its capacity. For example, for a 1000mAh lithium-ion battery, the discharge current should be strictly controlled below 3A; otherwise, the battery will be damaged.
