First, the definition and role of internal resistance
(1) Definition of internal resistance
Internal resistance refers to the resistance encountered by the current passing through the inside of the
lithium battery during discharge or charging. It is determined by the conductivity of the material inside the battery, the ion transport rate of the electrolyte, the contact resistance between the electrode and the electrolyte, and many other factors. The size of the internal resistance directly affects the performance of the lithium battery, including the output power, cycle life, temperature characteristics and so on.
(2) The role of internal resistance in lithium batteries
1. Internal resistance is one of the limiting factors for the output power of lithium batteries. When the internal resistance of the battery is large, the current passing through the battery will produce a large voltage drop, resulting in a decrease in the output power of the battery.
2. Internal resistance will produce self-discharge of the battery. Due to the existence of resistance inside the battery, the battery will also have a certain current flow when it is not working, resulting in the self-discharge of the battery.
3. The internal resistance affects the temperature characteristics of the battery. Batteries with large internal resistance will generate more heat during discharge or charging, which will cause the battery temperature to rise, further affecting the performance of the battery.
Second, evaluate the battery characteristics by DC internal resistance
In practical applications, DC internal resistance is often used to evaluate the health of lithium batteries, to predict life, and to estimate the SOC (State of Charge) and SOP (State of Power) of the battery. By measuring the DC internal resistance of the battery, information about the battery status can be obtained, providing a basis for battery management and control, and further improving the efficiency and life of the battery, as follows:
1. Health evaluation:
The health of a lithium battery refers to the degree of degradation of the battery during use, usually assessed by the capacity decay rate and the internal resistance increase rate. The change of DC internal resistance can reflect the physical and chemical changes inside the battery, so it can be used to evaluate the health of the battery. When the health of the battery decreases, its internal resistance tends to increase.
2. Life prediction:
The increase of battery internal resistance is an important manifestation of battery aging. By monitoring the DC internal resistance of the battery, the life of the battery can be predicted. When the DC internal resistance increases to a certain extent, it indicates that the battery life is close to or has exceeded its design life, and it needs to be replaced or repaired.
3. System SOC estimation:
The SOC of a battery refers to the current state of charge of the battery, that is, the ratio of the remaining available energy in the battery to the total energy. There is a certain relationship between the DC internal resistance and the SOC of the battery. The SOC of the battery can be estimated by measuring the DC internal resistance of the battery. According to the existing internal resistance-charge state relationship model, the SOC of the battery can be calculated by measuring the DC internal resistance of the battery.
4. System SOP estimation:
The SOP of the battery refers to the current power state of the battery, that is, the maximum power that the battery can provide. There is also a relationship between the DC internal resistance and the SOP of the battery. The SOP of the battery can be estimated by measuring the DC internal resistance of the battery. According to the existing internal resistance-power state relationship model, the SOP of the battery can be calculated by measuring the DC internal resistance of the battery.
Dc internal resistance plays an important role in health evaluation, life prediction and estimation of SOC and SOP of lithium battery.
Three, Measurement method of internal resistance
The measurement methods of internal resistance are mainly divided into static measurement and dynamic measurement.
1. Static measurement method
The static measurement method is to calculate the internal resistance of the battery by measuring the difference between the open circuit voltage and the short circuit current. The specific steps are as follows:
(1) Place the battery for a period of time so that the electrochemical reaction within it tends to be balanced.
(2) Measure the open circuit voltage of the battery.
(3) Connect a load between the positive and negative terminals of the battery to measure the short-circuit current.
(4) According to Ohm's law, calculate the internal resistance of the battery.
2. Dynamic measurement method The dynamic measurement method is to measure the internal resistance of the battery by applying an AC electrical signal. The specific steps are as follows:
(1) Apply an AC electrical signal between the positive and negative terminals of the battery.
(2) Measure the current and voltage signals of the battery.
(3) Calculate the internal resistance of the battery according to the phase difference and amplitude ratio of the current and voltage signals.
Four, Factors affecting internal resistance
The size of the internal resistance is affected by many factors, mainly including the following aspects:
1. The conductivity of the battery material The conductivity of the battery material determines the rate at which the current is transmitted inside the battery. The higher the conductivity of the material inside the battery, the lower the internal resistance.
2. Ion transport rate of the electrolyte The ion transport rate of the electrolyte directly affects the internal resistance of the battery. The faster the ion transport rate in the electrolyte, the lower the internal resistance.
3. Contact resistance between the electrode and the electrolyte The contact resistance between the electrode and the electrolyte is also an important factor affecting the internal resistance. The smaller the contact resistance, the lower the internal resistance.
4. The structural design of the battery The structural design of the battery will also have a certain impact on the internal resistance. For example, the larger the electrode area of the battery, the lower the internal resistance.
5. Temperature Temperature also has a great effect on the internal resistance. In general, the higher the temperature, the lower the internal resistance.
Five, Methods to reduce internal resistance
In order to improve the performance of lithium batteries, some measures need to be taken to reduce the internal resistance. Here are some common methods:
1. Optimize the battery material By changing the composition and structure of the battery material, improve the conductivity of the battery material, thereby reducing the internal resistance.
2. Optimize the electrolyte to change the composition and concentration of the electrolyte, improve the transmission rate of ions in the electrolyte, thereby reducing the internal resistance.
3. Improve the contact between the electrode and the electrolyte by changing the contact mode between the electrode and the electrolyte, reduce the contact resistance between the electrode and the electrolyte, thereby reducing the internal resistance.
4. Optimize the battery structural design By changing the structural design of the battery, increase the area of the electrode, thereby reducing the internal resistance.
5. Control the temperature Properly control the operating temperature of the battery to avoid excessive temperature and reduce the internal resistance.
Six. Conclusion
Internal resistance is an important index in the performance evaluation of lithium battery, which directly affects the output power, cycle life and temperature characteristics of the battery. Through reasonable measurement methods and optimization measures, the internal resistance of the battery can be reduced and the performance of the lithium battery can be improved. With the continuous progress of science and technology, it is believed that the internal resistance of lithium batteries will be further reduced, providing better support for the development of lithium battery applications.
