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Eight important parameters of lithium batteries - Lithium battery knowledge popularization

Eight important parameters of lithium batteries - Lithium battery knowledge popularization

Main parameter indicators

Lithium-ion batteries have the advantages of high energy density, high conversion efficiency, long cycle life, no memory effect, no charge and discharge delay, low self-discharge rate, wide operating temperature range and environmental friendliness, so they become an ideal carrier of electric energy , are widely used in various fields.
Generally speaking, when we use lithium-ion batteries, we will pay attention to some technical indicators as the main factors to measure the "quality" of its performance. So, which indicators should we pay special attention to when we use them?

1. Capacity
This is a parameter that everyone is more concerned about. Smartphones have been popularized for a long time. When we use smart phones, what we worry most about is the lack of power, which needs to be charged frequently, and sometimes we can't find a place to charge them. Early functional phones, under normal use, a fully charged battery can stand by for 3 to 5 days, and some products can even stand by for more than 7 days. But in the era of smart phones, the standby time is terrible. One of the most important reasons for this is that the power consumption of mobile phones is increasing, but the capacity of batteries has not increased in the same proportion.

The unit of capacity is generally "mAh" (milliampere hour) or "Ah" (ampere hour), and there is a difference between rated capacity and actual capacity when used. The rated capacity refers to the total power that a fully charged lithium-ion battery can provide when it is discharged to the cut-off voltage at a specific discharge rate (C-rate) under laboratory conditions (ideal temperature and humidity environment). The actual capacity is generally not equal to the rated capacity, which is directly related to temperature, humidity, charge and discharge rate, etc. Generally, the actual capacity is smaller than the rated capacity, sometimes even much smaller than the rated capacity. For example, in winter in the north, if the mobile phone is used outdoors, the battery capacity will drop rapidly.

2. Energy density
Energy density refers to the amount of electricity that can be stored and released by a battery per unit volume or unit weight. It has two units: Wh/kg and Wh/L, which represent weight-specific energy and volume-specific energy, respectively. The power here is the integral of the capacity (Ah) mentioned above and the working voltage (V). In application, the indicator of energy density is more instructive than capacity.

Based on the current lithium-ion battery technology, the energy density level that can be achieved is about 100~200Wh/kg, which is still relatively low and has become a bottleneck in the application of lithium-ion batteries in many occasions. This problem also occurs in the field of electric vehicles. Under the condition that the volume and weight are strictly limited, the energy density of the battery determines the single maximum mileage of electric vehicles, so the unique term "range anxiety" appears . If the mileage of a single electric vehicle is to reach 500 kilometers (equivalent to a traditional fuel vehicle), the energy density of a battery cell must reach more than 300Wh/kg.

The improvement of the energy density of lithium-ion batteries is a slow process, which is far lower than the Moore's Law of the integrated circuit industry. expand.

3. Charge and discharge rate
This indicator will affect the continuous current and peak current of the lithium-ion battery when it is working, and its unit is generally C (short for C-rate), such as 1/10C, 1/5C, 1C, 5C, 10C, etc. Take an example to explain the specific meaning of the rate index. The rated capacity of a battery is 10Ah. If the rated charge and discharge rate is 1C, it means that this type of battery can be repeatedly charged and discharged with a current of 10A. to the cut-off voltage for charging or discharging. If the maximum discharge rate is 10C@10s and the maximum charge rate is 5C@10s, then the battery can be discharged with a current of 100A for 10 seconds and charged with a current of 50A for 10 seconds.

The current value corresponding to the charge and discharge rate is multiplied by the operating voltage to obtain the continuous power and peak power indicators of the lithium-ion battery. The more detailed the charge-discharge rate index is defined, the greater the guiding significance for use. Especially for lithium-ion batteries as the power source of electric vehicles, it is necessary to specify the continuous and pulse rate indicators under different temperature conditions to ensure that the lithium-ion batteries are used within a reasonable range.

4. Voltage
The voltage of lithium-ion batteries includes some parameters such as open circuit voltage, operating voltage, charging cut-off voltage, and discharge cut-off voltage. This article will not discuss them separately, but focus on explaining them.

Open-circuit voltage, as the name implies, means that the battery is not connected to any external load or power supply, and the potential difference between the positive and negative electrodes of the battery is measured, which is the open-circuit voltage of the battery.
The working voltage is the measured potential difference between the positive and negative poles when the battery is connected to an external load or power supply and is in working condition with current flowing. Generally speaking, due to the existence of the internal resistance of the battery, the working voltage in the discharge state is lower than the open circuit voltage, and the working voltage in the charging state is higher than the open circuit voltage.
The charge/discharge cut-off voltage refers to the highest and lowest working voltage allowed by the battery. Exceeding this limit will cause some irreversible damage to the battery, resulting in a decrease in battery performance, and even cause safety accidents such as fire and explosion in severe cases.

There is a certain relationship between the open circuit voltage and working voltage of the battery and the capacity of the battery.

5. Lifespan
The life of lithium-ion batteries will gradually decline with use and storage, and there will be more obvious performance. Still take the smartphone as an example. After using the mobile phone for a period of time, you can clearly feel that the battery of the mobile phone is not durable. At the beginning, it may only be charged once a day, and then it may need to be charged twice a day. This is the continuous decline of battery life. embodiment.

The life of lithium-ion batteries is divided into two parameters: cycle life and calendar life. Cycle life is generally measured in units of times, representing the number of times a battery can be charged and discharged. Of course, there are also conditions here. Generally, under ideal temperature and humidity, carry out deep charge and discharge (100% DOD or 80% DOD) at the rated charge and discharge current, and calculate the battery capacity decay to 80% of the rated capacity. The number of cycles experienced.

The definition of calendar life is more complicated. The battery cannot be charged and discharged all the time, there is storage and shelving, and it is impossible to be in ideal environmental conditions all the time. It will experience various temperature and humidity conditions, and the rate of charge and discharge is also changing all the time. The service life needs to be simulated and tested. To put it simply, the calendar life is the time span for the battery to reach the end-of-life condition (such as capacity decay to 80%) under the environmental conditions of use and after specific operating conditions. The calendar life is closely combined with the specific use requirements, usually need to specify specific use conditions, environmental conditions, storage intervals, etc.

The calendar life is more practical than the cycle life, but because the calculation of the calendar life is very complicated and takes too long, generally battery manufacturers only give the data of the cycle life. To obtain calendar life data, there is usually an additional fee and a long wait.

6. Internal resistance
The internal resistance of a lithium-ion battery refers to the resistance encountered by the current flowing through the battery when the battery is working. It includes ohmic internal resistance and polarization internal resistance, and polarization internal resistance includes electrochemical polarization internal resistance and concentration electrode. internal resistance.

Ohmic internal resistance is composed of electrode material, electrolyte, diaphragm resistance and contact resistance of various parts. Polarization internal resistance refers to the resistance caused by polarization during electrochemical reaction, including the resistance caused by electrochemical polarization and concentration polarization.

The unit of internal resistance is generally milliohm (mΩ). For a battery with a large internal resistance, the internal power consumption is large and the heat is severe during charging and discharging, which will cause accelerated aging and life attenuation of the lithium-ion battery, and will also limit the large rate. charging and discharging applications. Therefore, the smaller the internal resistance is, the better the life and rate performance of the lithium-ion battery will be.

7. Self-discharge
When the battery is placed, its capacity is constantly decreasing. The rate of capacity decrease is called the self-discharge rate, usually expressed as a percentage: %/month.
Self-discharge is something we don't want to see. A fully charged battery will lose a lot of power after a few months. Therefore, we hope that the self-discharge rate of lithium-ion batteries should be as low as possible.

Special attention needs to be paid here. Once the self-discharge of the lithium-ion battery causes the battery to be over-discharged, the impact is usually irreversible. Even if it is recharged, the available capacity of the battery will be greatly lost, and the service life will rapidly decay. Therefore, if the lithium-ion battery is left unused for a long time, you must remember to charge it regularly to avoid over-discharging due to self-discharge, which will greatly affect its performance.

8. Operating temperature range
Due to the characteristics of the internal chemical materials of lithium-ion batteries, lithium-ion batteries have a reasonable operating temperature range (common data is between -40°C and 60°C), if used beyond the reasonable range, it will affect the performance of lithium-ion batteries cause a greater impact.

Lithium-ion batteries of different materials have different operating temperature ranges. Some have good high-temperature performance, while others can adapt to low-temperature conditions. The working voltage, capacity, charge and discharge rate and other parameters of lithium-ion batteries will change significantly with the change of temperature. Prolonged high temperature or low temperature use will also accelerate the decay of the life of lithium-ion batteries. Therefore, efforts to create a suitable operating temperature range can maximize the performance of lithium-ion batteries.

In addition to the limited working temperature, the storage temperature of lithium-ion batteries is also strictly limited. Long-term high-temperature or low-temperature storage will have an irreversible impact on battery performance.

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