How Long do Lithium Batteries Last?

Lithium battery: the “lifespan” we thought we knew

With the rapid development of science and technology, lithium batteries have been deeply integrated into all aspects of our lives, from mobile phones and laptops used in daily life to electric cars that we rely on for travel. However, there are many misunderstandings about the service life of lithium batteries, among which the saying that “lithium batteries have a lifespan of 500 charges and discharges” is widely circulated. Many people believe that lithium batteries can only be charged and discharged 500 times. Once this number is reached, the battery will die and must be replaced. Based on this understanding, some users wait until the battery is almost exhausted before charging in order to extend the use time of lithium batteries.

But this statement is actually wrong. The so-called “500 charges and discharges” does not simply refer to the number of charges, but a complete charge and discharge cycle. A charge and discharge cycle means that all the power of the battery is used up from full to empty, and then charged from empty to full, which is not equivalent to charging once. For example, on the first day, half of the battery power is used up and then fully charged; on the second day, half of the power is used up and fully charged again. In this way, the two charges together are considered a charge and discharge cycle. Therefore, in real life, it may take multiple charges to complete a complete charge and discharge cycle. Moreover, after completing 500 charge and discharge cycles, the lithium battery is not completely unusable, but its capacity will decrease. Generally speaking, high-quality batteries can still retain 80% of their original capacity after multiple charge cycles, and many lithium-powered products can still be used normally after two or three years.

Obviously, our previous understanding of the life of lithium batteries was too simple and one-sided. This misunderstanding not only affects our daily habits of using lithium battery equipment, but also makes us deviate from the true performance of lithium batteries. So, how should the life of lithium batteries be measured correctly? What factors will affect it? Next, let us delve into the mystery of the life of lithium batteries.

What is the “life” of lithium batteries

Before we delve into the life of lithium batteries, we need to clarify a key question: What is the “life” of lithium batteries? It is not simply measured by time like human life. In the world of lithium batteries, its life has a unique measurement dimension.

Cycle life: Consideration of the number of charge and discharge cycles

Cycle life is one of the important indicators for measuring the life of lithium batteries. It refers to the number of charge and discharge cycles that a lithium battery can withstand before its capacity decays to a certain specified value under a certain charge and discharge system. Simply put, a complete process from fully charged to fully discharged and then fully charged again is counted as one cycle. After a lithium battery has undergone multiple such cycles, its capacity will gradually decrease. When the capacity drops to 80% of the initial capacity, it is usually considered to have reached the end of its cycle life.

Different types of lithium batteries have different cycle lives. For example, the common ternary lithium battery has a theoretical cycle number of about 800-1200 times, while the cycle number of lithium iron phosphate batteries is relatively high, reaching about 2000-2500 times. For example, the 18650 ternary lithium battery used by Tesla in some of its models, under laboratory conditions, after about 1000 cycles, the battery capacity will drop to 80% of the initial capacity. According to official data, the lithium iron phosphate battery used by BYD in some models can have a cycle life of more than 2,000 times under normal use, which means that under ideal conditions, its capacity decays to 80% after more charge and discharge cycles.

Calendar life: consideration of time dimension

In addition to cycle life, calendar life is also a key factor in measuring the life of lithium batteries. It refers to the period from the date of production to the end of the battery life, usually measured in years, covering different links such as shelving, aging, high and low temperature, circulation, and working condition simulation. Even if the lithium battery is idle and not frequently charged and discharged, the chemical substances inside it will undergo slow chemical reactions over time, causing the battery performance to gradually decline. This is the calendar life at work.

Generally speaking, the calendar life of lithium batteries used in cars is 5-10 years. Taking a certain electric car as an example, even if the owner drives less mileage each year and the vehicle is parked most of the time, the battery capacity may decay to about 70% after 6-7 years of use. This is just like the shelf life of food. Even if it is unopened, its quality will be affected after a certain period of time. Many car companies tend to emphasize the cycle life of the battery in their publicity, but reserve the calendar life. This is because the announcement of the calendar life may have a certain impact on the sales of new energy vehicles. However, from the warranty period of the battery pack, we can still roughly infer its calendar life. For example, the warranty commitment of 5 years or more than 100,000 kilometers given by some car companies implies that the battery can maintain relatively stable performance within this time range.

Real data on lithium battery life is disclosed

Comparison of the number of cycles of different types of lithium batteries

The lithium battery family is huge, and different types of lithium batteries have obvious differences in the number of cycles. In order to show this difference more intuitively, we use a table for comparison:

As can be seen from the table, lithium iron titanate battery and lithium iron phosphate battery perform better in terms of number of cycles, among which the number of cycles of lithium iron titanate battery can reach more than 10000 times, which makes it have great advantages in some fields with extremely high requirements for battery cycle life, such as smart grid energy storage, rail transportation, etc. The number of cycles of lithium iron phosphate battery can also reach 2000 – 10000 times, which is widely used in electric vehicles, energy storage power stations and other scenarios. The number of cycles of lithium cobalt oxide batteries and lithium manganese oxide batteries is relatively small. Lithium cobalt oxide batteries usually have only 300-500 cycles, which limits its application range to a certain extent. At present, it is mainly used in some small electronic devices with strict requirements on battery volume and weight and relatively low requirements on cycle life.

Life performance in actual use

The theoretical number of cycles is certainly important, but the life performance of lithium batteries in actual use is what we are more concerned about. In actual use, the life of lithium batteries is affected by a combination of factors, and there is often a certain difference from the theoretical number of cycles.

Taking a certain brand of electric vehicles as an example, the number of cycles of the ternary lithium battery carried by it can reach about 1500 times in a laboratory environment according to the standard charge and discharge test process. However, in actual use, when the vehicle is mainly driven on urban roads, frequently starts and stops, and often uses the fast charging function, after about 800-1000 charge and discharge cycles, the battery capacity decays to 80% of the initial capacity. This is because frequent starting and stopping will cause the battery’s discharge depth to change continuously, and the high heat generated during fast charging will accelerate the chemical reaction inside the battery, causing the battery to age and thus shorten the battery life.

On the contrary, when the car is mainly driving on the highway, the driving conditions are relatively stable, and fast charging is used less and slow charging is used more, the battery capacity will decay to 80% after 1200-1300 charge and discharge cycles. It can be seen that factors such as driving habits, charging and discharging methods, and ambient temperature in actual use scenarios will have a significant impact on the actual life of lithium batteries.

In-depth analysis: key factors affecting the life of lithium batteries

Number of charge and discharge cycles

As the number of charge and discharge cycles increases, the capacity of lithium batteries will gradually decay and the internal resistance will gradually increase. This is one of the key factors affecting the life of lithium batteries. Under normal charge and discharge conditions, the capacity of lithium batteries will decay by about 0.1%-0.2% for each cycle. When the number of cycles reaches a certain level, the capacity decay will accelerate, causing the battery to be unable to meet the normal use requirements of the device.

The figure clearly shows that in the initial cycle stage, the battery capacity decays slowly and the curve is relatively flat; as the number of cycles increases, the battery capacity decays faster and the slope of the curve gradually increases. When the number of cycles reaches about 1,000, the battery capacity has decayed to about 80% of the initial capacity, indicating that the battery is approaching the end of its cycle life.

Significant influence of temperature

Temperature has a significant impact on the life of lithium batteries. Whether it is a high temperature or low temperature environment, it will have a negative impact on the performance of lithium batteries. When lithium batteries are used in a high temperature environment (such as 45°C and above), their capacity decay rate will be significantly accelerated and the cycle life will be greatly shortened. Under high temperature conditions, the chemical reaction rate inside the battery is accelerated, which will lead to the dissolution of the positive electrode material, the decomposition of the electrolyte, and the instability of the interface film (SEI), thereby accelerating the aging of the battery. Studies have shown that at 45°C, the cycle life of lithium batteries may be 30% – 50% shorter than at room temperature (25°C).

In low temperature environments (such as 0°C and below), the charging and discharging efficiency of lithium batteries will be significantly reduced, the internal resistance of the battery will increase, and the available capacity will decrease. Low temperature will slow down the migration of lithium ions inside the battery, making it difficult for the chemical reaction of the battery to proceed normally, thereby affecting the performance of the battery. When the temperature drops to -10°C, the available capacity of the lithium battery may be reduced to about 70% at room temperature, which means that in low temperature environments, the endurance of lithium battery equipment will be greatly reduced.

Charge and discharge rate

Fast charging and discharging also has a negative impact on the life of lithium batteries. Generally speaking, the charge and discharge rate of lithium batteries is expressed in C, and 1C means that the battery is fully charged or discharged within 1 hour. When the charge and discharge rate is too fast, such as when a fast charge mode greater than 1C is used, a large amount of heat will be generated inside the battery, accelerating the aging of the battery. When a lithium battery is charged and discharged at a rate of 2C, its cycle life will be shortened by 20% – 30% compared to charging and discharging at a rate of 0.5C. This is because fast charging and discharging will intensify the polarization phenomenon inside the battery, resulting in an increase in the internal resistance of the battery, thereby affecting the performance and life of the battery.

Other factors

In addition to the above main factors, factors such as the quality of the battery itself, charging maintenance methods, and pressure vibration will also affect the life of lithium batteries. High-quality lithium batteries are more secure in terms of material purity and manufacturing process, and their life is often longer than that of batteries of poor quality. Correct charging and maintenance methods, such as avoiding overcharging and overdischarging, and regular deep charging and discharging, can also help extend the life of lithium batteries. In addition, in some special application scenarios, such as bumps during the driving of electric vehicles and collisions of electronic devices, pressure vibrations may cause damage to the internal structure of the battery, thereby affecting the performance and life of the battery.

How to extend the life of lithium batteries: a practical guide based on data

After understanding the key factors that affect the life of lithium batteries, we can take targeted measures to extend their life and make lithium batteries serve us better.

Control the number of cycles

Try to reduce unnecessary charge and discharge cycles and avoid frequently exhausting the battery before charging. You can charge when there is about 20% – 30% remaining to avoid deep discharge. For example, for smartphone users, when the power display shows 25%, start charging instead of waiting until the power is exhausted and automatically shut down before charging. This can effectively reduce the number of charge and discharge cycles and thus extend the battery life. At the same time, avoid overcharging. When the battery is full, the charger should be unplugged in time to prevent long-term overcharging from damaging the battery. Keeping the battery charge in the range of 40% – 80% for a long time can significantly reduce the number of battery cycles, thereby extending the battery life and allowing the battery to maintain good performance after many years of use.

Maintain a suitable temperature

When using and storing lithium battery equipment, try to maintain a suitable temperature and avoid long-term use or storage in high or low temperature environments. If you need to use it in a high temperature environment, such as using an electric car outdoors in the hot summer, you should try to reduce the use time, or take cooling measures, such as parking the vehicle in a cool place when parking, avoiding direct sunlight. In a low temperature environment, such as in a cold winter, try to use lithium battery equipment in a warm environment, or preheat the battery before use. Controlling the operating temperature of lithium batteries at around 25°C can extend the battery’s cycle life by 30% – 50%.

Control the charge and discharge rate

Try to avoid using the fast charge and discharge function, choose the appropriate charging equipment and charging method, and charge at a lower charge and discharge rate. For electric vehicles, if it is not an emergency, try to choose a slow charging method, such as using a home charging station for nighttime charging. When the charge and discharge rate is reduced from 2C to 0.5C, the cycle life of lithium batteries can be extended by 20% – 30%. In addition, during the charging process, pay attention to the temperature changes of the battery. If the temperature is too high, the charging should be suspended and the charging should be continued after the temperature drops.

Regular maintenance

For some large lithium battery equipment, such as electric vehicles and energy storage power stations, regular maintenance is very necessary. You can regularly check the appearance of the battery to see if there are abnormal conditions such as bulging and leakage; use professional testing equipment to detect the battery capacity, internal resistance and other parameters to promptly discover battery problems. At the same time, the battery management system (BMS) should also be regularly inspected and updated to ensure that it can work properly and effectively protect the battery. For example, the user manual of a certain brand of electric vehicles recommends that the battery needs to be fully inspected and maintained every 10,000 to 20,000 kilometers, so that potential battery problems can be discovered and solved in a timely manner and the battery life can be extended.