Optimizing Internal Resistance: Key to Lithium-ion Battery Efficiency
Lithium-ion batteries, as efficient and environmentally friendly energy storage devices, widely used for fields such as electric vehicles, mobile communications, and energy storage systems. In the performance evaluation of lithium-ion cells/batteries, internal resistance is an essential indicator. Bonnen’s engineering team will provide a detailed introduction and analysis of internal resistance, covering its definition, measurement methods, influencing factors, and measures to improve it.
Definition and Role of Internal Resistance
1. Definition of Internal Resistance
Internal resistance refers to the resistance encountered by the electric current inside a lithium-ion battery during discharge or charge. It is determined by multiple factors, including the electrical conductivity of the battery’s internal materials, the rate of ion transport in the electrolyte, and the contact resistance between the electrode and the electrolyte. The magnitude of internal resistance directly affects the performance of the lithium-ion battery, including its output power, cycle life, and temperature characteristics etc.
2. Role of Internal Resistance in Lithium-ion Batteries
a. Internal resistance is one of the limiting factors for the output power of lithium-ion batteries. When the internal resistance of the battery is high, the current passing through the battery will result in a significant voltage drop, leading to a reduction in the battery’s output power.
b. Internal resistance leads to self-discharge in batteries. Due to the presence of internal resistance, there will be a certain amount of current flowing through the battery even when it is not in operation, causing self-discharge.
c. Internal resistance affects the temperature characteristics of the battery. Batteries with high internal resistance generate more heat during discharge or charge, leading to an increase in battery temperature, which further affects the battery’s performance.
Evaluating Battery Characteristics through DC Internal Resistance
In practical applications, DC internal resistance is often used to assess the health of lithium-ion cells/batteries, predict their lifespan, and estimate the State of Charge (SOC) and State of Power (SOP) of the system. Measuring the DC internal resistance of a battery provides information about its state, serving as a basis for battery management and control, thereby enhancing the battery’s efficiency and lifespan. The specific benefits include:
1. Health Assessment
Battery health refers to the degree of degradation that occurs during battery usage, typically assessed using capacity degradation rate and internal resistance increase rate. Changes in DC internal resistance can reflect the physical and chemical changes occurring inside the battery and, therefore, can be used to evaluate battery health. When the battery’s health deteriorates, its internal resistance often increases.
2. Lifespan Prediction
The increase in a battery’s internal resistance is a significant sign of battery aging. By monitoring changes in the DC internal resistance of a battery, it is possible to predict its lifespan. When the DC internal resistance reaches a certain level, it indicates that the battery’s lifespan is approaching or has already exceeded its design life, necessitating replacement or maintenance.
3. System SOC Estimation
The SOC (State of Charge) of a battery refers to its current charge state, which is the ratio of the remaining available energy in the battery to its total energy capacity. There is a relationship between DC internal resistance and the SOC of the battery. By measuring the DC internal resistance of the battery and using established models that relate internal resistance to SOC, it is possible to estimate the battery’s SOC.
4. System SOP Estimation
SOP (State of Power) of a battery indicates its current power state or the maximum power it can provide. There is also a relationship between DC internal resistance and the SOP of the battery. Measuring the DC internal resistance of the battery can be used to estimate the battery’s SOP. Similar to SOC estimation, models relating internal resistance to SOP can be employed for this purpose.
DC internal resistance plays a crucial role in assessing battery health, predicting lifespan, and estimating system SOC and SOP.
Methods for Measuring Internal Resistance
Methods for measuring internal resistance mainly fall into two categories: static measurement and dynamic measurement.
1. Static Measurement Methods
Static measurement involves calculating the internal resistance of a battery by measuring the difference between its open-circuit voltage and short-circuit current. The specific steps are as follows:
a. Allow the battery to rest for a period to stabilize internal electrochemical reactions.
b. Measure the battery’s open-circuit voltage.
c. Connect a load between the positive and negative terminals of the battery and measure the short-circuit current.
d. Calculate the battery’s internal resistance using Ohm’s law.
2. Dynamic Measurement Methods
Dynamic measurement involves measuring a battery’s internal resistance by applying an alternating current (AC) signal. The specific steps are as follows:
a. Apply an AC signal between the positive and negative terminals of the battery.
b. Measure the current and voltage signals of the battery.
c. Calculate the battery’s internal resistance based on the phase difference and amplitude ratio of the current and voltage signals.
Factors Affecting Internal Resistance
The magnitude of internal resistance in lithium-ion batteries is influenced by several factors, primarily including the following:
1. Electrical Conductivity of Battery Materials
The electrical conductivity of the materials within the battery determines the rate at which electric current can flow within the battery—higher electrical conductivity results in lower internal resistance.
2. Ion Transport Rate in the Electrolyte
The speed at which ions can move through the electrolyte directly affects the internal resistance of the battery. A faster ion transport rate in the electrolyte leads to lower internal resistance.
3. Contact Resistance Between Electrodes and Electrolyte
The contact resistance between the battery’s electrodes and the electrolyte is another significant factor affecting internal resistance. Lower contact resistance results in lower internal resistance.
4. Battery Structural Design
The design of the battery’s structure can also have a significant impact on internal resistance. For example, a larger electrode area within the battery leads to lower internal resistance.
5. Temperature
Temperature plays a substantial role in influencing internal resistance. Generally, higher temperatures lead to lower internal resistance.
Methods to Reduce Internal Resistance
To enhance the performance of lithium-ion cells/batteries, various measures can be employed to reduce internal resistance. Here are some common methods:
1. Optimization of Battery Materials
Altering the composition and structure of the battery materials to enhance their electrical conductivity, thereby reducing internal resistance.
2. Optimization of the Electrolyte
Modifying the composition and concentration of the electrolyte to improve the transport rate of ions, leading to lower internal resistance.
3. Improved Electrode-Electrolyte Contact
Enhancing the contact between the battery’s electrodes and the electrolyte reduces contact resistance and, consequently, lower internal resistance.
4. Optimization of Battery Structural Design
Adjusting the battery’s structural design, such as increasing the electrode’s surface area, decreases internal resistance.
5. Temperature Control
Maintaining an optimal operating temperature for the battery to prevent excessive heat, thus reducing internal resistance.
Internal resistance is a critical parameter in assessing the performance of lithium-ion cells/batteries, with direct implications for factors like output power, cycle life, and temperature characteristics. By employing appropriate measurement methods and optimization strategies, it is possible to decrease the internal resistance of batteries, ultimately improving their performance. As technology continues to advance, it is expected that internal resistance in lithium-ion batteries will continue to decrease, providing better support for their applications.
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