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Life of lithium battery




I. calculation method of lithium battery life in national standard

Test conditions and requirements for cycle life of lithium battery specified in the national standard: charge with 1C at the ambient temperature of 20 ℃± 5 ℃. When the battery terminal voltage reaches the charging limit voltage of 4.2V, change to constant voltage charging until the charging current is less than or equal to 1 / 20c, stop charging, set aside for 0.5h-1h, then discharge with 1C current to the termination voltage of 2.75V, and then set aside for 0.5h-1h after the discharge If the next charging and discharging cycle lasts for less than 36 minutes, the cycle number must be more than 300.
From the definition of the national standard, we have extracted two points:
1. This definition specifies that the test of cycle life is carried out in the way of deep filling and deep drawing
2. According to this mode, the cycle life of lithium battery still has more than 60% usable capacity after more than 300 cycles.

II. Factors affecting the life of lithium battery
1. Factors related to lithium battery life
According to the battery University, the main factors affecting the life of lithium batteries are as follows:

(1) charging times;
(2) discharge depth;

(3) charging cut-off voltage;
(4) charging speed;
(5) service temperature;
2. Influence of charging times on service life
Many consumers and even some engineers think that the life estimation marked in the specifications of many lithium batteries is about 300 times of charging. They think that if the charger is inserted once, even if it is charged once, then if the consumer inserts the charger twice a day, the life of the battery is less than half a year. Because many people know that the battery still stays on NiMH battery and NiCd battery, because this type of battery has memory effect, but lithium battery has no memory effect, the battery university thinks that a complete cycle of lithium battery should be described as follows: the lithium battery is charged to the rated voltage of 4.2V with 1C (maximum charging current), and charged to the rated voltage of 4.2V with 0.05c charging current The battery is saturated and then discharged to 3V at the discharge rate of 1C, which is a complete charge discharge cycle. It is not a complete charge discharge cycle if it is not filled or discharged. If your battery is charged from full state, it uses one tenth of the power, and then it is fully charged. This is one tenth cycle. If you charge it 10 times, it is basically one cycle. The general life cycle of lithium battery is 300-500 times (at this time, the battery is not completely damaged, but the capacity is only 60% - 70% of the initial capacity).

III. Effect of discharge depth DOD on service life
Many consumers and even some engineers think that too many times of lithium battery charging is harmful to the battery, so during the use process, the battery is fully charged and discharged. When the battery capacity is close to 0, the charging cut-off voltage is set to the highest threshold voltage of 4.2V. However, the University of battery thinks that the lower the discharge depth of lithium battery is, the better. As shown in Figure 2 below, for example, for 1Ah lipo4 battery, the number of cycles is about 600 at 100% DOD, and the capacity provided within the service life is j = 1Ah * 600 = 600ah. If 40% DOD is used for deep discharge, it can cycle 3000 times, and the capacity provided within the service life is j = 1Ah * 0.4 * 3000 = 1200ah, which is 50% longer than 100% discharge depth. Toyota's double engine hybrid car, battery provides 8-year 160000 km warranty, publicity is: shallow charge shallow release, longer life.

IV. influence of charging voltage on service life
According to the University of battery, the effect of charging cut-off voltage on battery life is critical. There are two important criteria for lithium battery below 4.2V:
1. When the charging cut-off voltage is below 4.2V, the number of rechargeable cycles will be doubled for each 0.1V reduction.
2. When the full voltage drops 70mV, the usable capacity of the battery will be reduced by 10%.
The lithium-ion batteries in electric vehicles and satellites must have a service life of more than 8 years. To achieve this, the battery is only charged to 3.90v/battery or less. An interesting finding from NASA is that the lithium ion residing above 4.10v/cell tends to decompose due to the electrolyte oxidation on the cathode, while the lithium ion charged to a lower voltage loses its capacity due to the accumulation of SEI on the anode. NASA reports that once lithium ion passes the eight-year mark after about 40000 cycles in the satellite, the deterioration of battery quality caused by this phenomenon will develop quickly. Charging to 3.92v/battery seems to maximize service life, but this only reduces capacity by about 60%. Sweden's Chalmers University of science and technology reported that using a reduced 50% SOC (state of charge) charge could increase the life expectancy of lithium-ion batteries by 44 – 130%.
Most designers of mobile phones, laptops, tablets and digital cameras will charge lithium-ion batteries to 4.20V. This is the limit threshold for operation. Because the renewal cycle of consumer electronics is generally 3 years, users want a longer endurance time. Many fake brands are in pursuit of draining all the capacity and increasing the charging voltage to 4.3v or higher. On the other hand, the industry pays more attention to life, and may choose a lower voltage threshold. The charging threshold is set by each manufacturer, and there is no uniform standard.

V. how charging speed affects service life
Battery University believes that the charging speed (charging current) of lithium battery has a key impact on the life of lithium battery. The faster the charging speed is, the shorter the life of the battery is. Figure 5 is the 900mAh lithium battery as a comparative experiment, C: the capacity value represented by the battery. For example, for 2600mAH lithium-ion batteries, 1c = 2.6a; for 3400mah lithium-ion batteries, 1c = 3.4A. According to the experimental results, the charge speed of 1C is 800 MAH after 500 cycles, 100 MAH is reduced, and the capacity is reduced by about 11%. If the charging speed is 2c, the remaining full capacity after 500 cycles will be 500mah, 400mah will be reduced, and the capacity will be reduced by about 44%. Now mobile phone manufacturers are pursuing charging speed, but at the cost of battery life. Fortunately, the update frequency of consumer electronics is high enough.

VI . The influence of temperature on life?

The University of battery thinks that the working temperature of lithium battery has a great influence on the battery life. Experiments show that the capacity of NCM lithium battery only drops to 97.7 at 20 ℃ after 100 cycles of charging and discharging, while the capacity of NCM lithium battery only drops to about 60% when the temperature rises to 60 ℃. So try to keep the battery at room temperature.

According to the conclusion of battery University, we know that the life assessment and design of lithium battery is a systematic project, involving five key factors: charging times, discharging depth, charging cut-off voltage, charging speed and service temperature, which are closely related to cost, user experience, performance, quality and reliability, which requires product managers to make trade-offs in specific products, For example, the product manager of mobile phone pursues faster charging and longer endurance, and the service life doesn't matter (the mobile phone will be replaced in a year or two). The product manager of industrial product basically pursues more stable product quality and longer service life, and the charging speed and endurance can reduce the priority.

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