1. Introduction to Lithium-ion Batteries
A lithium-ion battery is a rechargeable battery that primarily functions by the movement of lithium ions between the positive and negative electrodes. During charging and discharging, Li+ ions repeatedly insert and extract between the two electrodes. During charging, Li+ ions extract from the positive electrode, pass through the electrolyte, and insert into the negative electrode, leaving the negative electrode in a lithium-rich state; the reverse occurs during discharging.
Lithium-ion batteries have a voltage range of 2.8V to 4.2V, with a typical voltage of 3.7V. Batteries are at risk of damage if the voltage is below 2.8V or above 4.2V.
2. The concepts of 1C and 0.1C
The unit of battery capacity is mAh. C refers to the charge/discharge rate of the battery. For example, for a 2000mAh battery, 1C discharge means the discharge current is 2000mA, and 0.1C means 200mA. The same principle applies to charging.
3. Advantages and disadvantages of lithium-ion batteries
The main advantages of lithium-ion batteries:
Lithium-ion batteries have high voltage and high energy density;
It has a long cycle life, typically lasting 500 cycles, and sometimes even exceeding 1000 cycles.
The self-discharge rate is low; the self-discharge rate of a fully charged Li-ion battery stored at room temperature for one month is approximately 10%.
It can be fast charged, reaching 80% of its nominal capacity with a 1C charge.
It has a wide operating temperature range, typically -25 to 45°C, with potential to exceed -40 to 70°C in the future.
Unlike Ni-Cd and Ni-Mh, it does not have a memory effect and does not require the remaining power to be used up before charging;
Compared to Ni-Cd and Ni-Mh, it is environmentally friendly and pollution-free (does not contain heavy metals such as cadmium and mercury);
The main disadvantages of lithium-ion batteries:
High cost;
A protection circuit board is needed, including overcharge and over-discharge protection;
High-current discharge is not allowed; the discharge current should generally be below 0.5C, as excessive current will cause the battery to overheat.
It has poor safety and is prone to explosion and fire.
4. The difference between lithium batteries and lithium-ion batteries
Lithium batteries and lithium-ion batteries are two different concepts, mainly with the following differences:
The positive electrode material of a lithium battery is manganese dioxide or thionyl chloride, and the negative electrode is lithium.
Lithium-ion batteries are lithium batteries that use lithium-containing compounds as the positive electrode. During charging and discharging, no metallic lithium is present, only lithium ions.
Lithium batteries, also known as primary lithium batteries, can be continuously or intermittently discharged. Once the energy is depleted, they cannot be used again and cannot be recharged.
Lithium-ion batteries, also known as rechargeable lithium batteries, can be charged and discharged.
5. Lithium-ion battery charging mode
The ideal charging mode for lithium-ion batteries is called CC CV mode, which is constant current and constant voltage mode.
In the following graph, gray represents battery voltage, green represents charging current, and red represents battery capacity.
When the battery voltage is low, the battery is charged with a fixed constant current. When the battery voltage reaches 4.2V, it will switch from constant current mode to constant voltage mode. Because the battery voltage cannot exceed 4.2V, the system will gradually reduce the charging current until it is close to 0. When the battery voltage is 4.2V and the charging current is 0, it means that the battery is fully charged.
Lithium-ion battery CC CV mode
To ensure absolute battery safety, the actual charging mode will be more refined. Charging an uncertain battery with constant current from the start will cause greater damage to the battery.
In test mode, when the battery voltage drops below 2V, the battery will be woken up with a very small current.
Trickle charging, also known as pre-charge mode, involves pre-charging the battery at 1/10 or 1/20 of the constant current charging current when the battery voltage is between 2V and 3V.
Constant current charging: When the battery voltage rises above 3V, the battery will be charged quickly in constant current mode.
Constant voltage charging: When the battery voltage reaches 4.2V, the battery will be charged in a constant voltage mode.
TI's solution, for example, stops charging when the battery voltage is 4.2V and the charging current is low but not zero, approximately 1/10 of the constant current charging current. At this point, the battery voltage will drop to 4.16V or 4.17V.
Three stages of lithium-ion battery charging
6. Why is the charging cutoff voltage of a lithium-ion battery 4.2V?
The graph below shows the relationship between battery cycle life and charging cut-off voltage. In the initial cycle of the battery, charging to a slightly higher voltage will result in a higher single-cycle capacity, but this will only last for a short period. When the battery charging voltage is 50mV or 100mV higher than the recommended maximum voltage of 4.2V, the battery will age much faster due to slight overcharging in each cycle.
In summary: the battery charging cut-off voltage should be higher than 4.2V. The higher the voltage, the shorter the cycle life and the faster the battery capacity decreases.
Relationship between lithium-ion battery cycle life and battery charging cut-off voltage
7. Lithium-ion battery discharge curve
The following are discharge curves of lithium-ion batteries at different discharge currents. It can be seen that the higher the discharge current, the faster the battery capacity decreases, and the lower the capacity, the less fully the nominal capacity of the battery is used.
The lower the battery capacity, the greater the increase in the battery's internal resistance. When a larger current is discharged, the internal resistance increases more rapidly.
Discharge curves of lithium-ion batteries under different discharge currents
The battery discharge curves at different temperatures show that the lower the temperature, the faster the battery capacity decreases and the less fully it is discharged.
When the battery temperature is below 0 degrees Celsius, the internal active components are very weak, and the internal resistance will increase accordingly; excessively high temperatures can also damage the battery.
Discharge curves of lithium-ion batteries at different temperatures
8. Number of lithium-ion battery cycles
In practice, each time the accumulated discharge capacity equals the design capacity, it is counted as one cycle.
Lithium-ion battery cycle life
The national standard specifies the cycle life test conditions and requirements for lithium-ion batteries: Charge at 1C for 150 minutes at 25°C room temperature, then discharge at a constant current of 1C until the voltage drops to 2.75V to complete one cycle. The test ends when any discharge time is less than 36 minutes, and the number of cycles must be greater than 300.
This definition specifies that cycle life testing is performed using a deep charge-deep discharge method;
The specified cycle life is that after operating in this mode, the capacity must still be more than 60% after more than 300 cycles;
9. Operating voltage range of lithium-ion batteries
The operating voltage of lithium-ion batteries has a range, which may vary slightly depending on the cell manufacturer.
Lithium-ion battery voltage range
