Battery Internal Resistance: Unraveling Its Composition, Impact, and Selection Methods

In modern life, batteries, as an indispensable energy source, have drawn much attention to their performance, service life, and safety. And battery internal resistance, seemingly insignificant, plays a crucial role.

I. Composition and Influencing Factors of Battery Internal Resistance

1. Ohmic internal resistance

- Ion resistance: When lithium ions transfer inside the battery, they encounter resistance influenced by positive and negative electrode materials, separators, and electrolytes. Different materials and structures affect the transfer speed of lithium ions.

- Electronic resistance: It involves the resistance in the process of electron transfer and is affected by factors such as the contact between active materials and current collectors, the characteristics of active materials, plate parameters, and the substrate of current collectors.

- Contact resistance: Formed by the contact between active material particles and between active materials and current collectors, it is influenced by the adhesion of positive and negative electrode slurries. Good adhesion performance can reduce contact resistance and improve battery performance.

2. Polarization internal resistance

- Ohmic polarization: When an external current flows through the battery, due to the internal resistance, local charge accumulation occurs, making the current performance exceed the resistance provided by ohmic resistance.

- Electrochemical polarization: In the reaction process on the electrode surface, when the electron transfer rate exceeds the electrochemical reaction rate, electron accumulation occurs.

- Concentration polarization: When the ion transfer rate is lower than the reaction rate on the electrode surface and in its solid phase, a concentration difference is formed, which is a direct result of the material transfer process.

Battery internal resistance is influenced by various factors such as the physical properties of battery materials, electrolyte composition, and production processes. Understanding these factors helps better understand the generation mechanism of internal resistance.

II. Characteristics of Lithium Iron Phosphate Battery Internal Resistance

Battery internal resistance is the resistance encountered when current passes through, expressed in internal resistance (mΩAh). The normalized internal resistance range of lithium iron phosphate commercial batteries is 35 to 70 mΩAh, and products from different manufacturers vary. As battery capacity increases, the electrode plate area increases correspondingly. Since the electrode plate area is inversely proportional to resistance, the internal resistance generally shows a downward trend. Energy density can be calculated by multiplying battery capacity by mass and dividing by battery voltage (3.2V).

III. Methods for Testing Battery Internal Resistance

1. Testing ohmic internal resistance

- Direct current discharge method: Perform constant current discharge on the battery and measure the voltage change. According to Ohm's law, R = ΔV / I (R is ohmic internal resistance, ΔV is voltage change, and I is discharge current). This method is simple and intuitive but requires attention to current stability and accuracy.

- Alternating current impedance method: Use alternating current signals to test the battery and measure the impedance at different frequencies to determine ohmic internal resistance. It can provide more detailed internal resistance information but requires professional equipment and data analysis capabilities.

2. Testing polarization internal resistance

- Intermittent discharge method: Perform intermittent discharge, observe the voltage change during discharge and rest periods, and analyze the voltage recovery curve to determine the polarization internal resistance. It is suitable for different types of batteries but has a longer test time.

- Constant current charging-discharging method: First perform constant current charging and then constant current discharging. Measure the voltage change during the charging and discharging process to calculate the polarization internal resistance. Precise control of current and time is required to ensure accuracy.

IV. Impact of Internal Resistance on Battery Performance

1. Impact on battery capacity

- Both ohmic internal resistance and polarization internal resistance consume energy, reducing the actual available capacity. High internal resistance generates more heat during charging and discharging, reducing energy conversion efficiency and available capacity. The impact is more significant during high-current charging and discharging. Reducing internal resistance can improve capacity utilization and usage time.

2. Impact on battery output power

- Internal resistance limits the maximum output power. High internal resistance causes the battery voltage at both ends to drop, reducing output power. Low internal resistance batteries can provide a larger current in a short time to meet the needs of high-power devices. For example, when an electric vehicle accelerates or climbs, it performs better.

3. Impact on battery cycle life

- High internal resistance accelerates battery aging and reduces cycle life. The heat generated during the charging and discharging process increases the internal temperature of the battery, accelerating the aging and damage of materials. Polarization internal resistance leads to polarization phenomena, affecting charging and discharging efficiency and capacity recovery ability. Long-term accumulation reduces cycle life.

4. Impact on battery safety

- High internal resistance causes the battery to generate more heat during charging and discharging. If it cannot be dissipated in time, it may lead to overheating, fire, or even explosion. The impact is more serious in abnormal situations such as overcharging, over-discharging, or short circuits. Reducing internal resistance can improve safety.

V. Methods for Improving Lithium Battery Internal Resistance

1. Optimize material selection

- Positive and negative electrode materials: Select positive and negative electrode materials with high conductivity and good lithium ion diffusion performance. For example, using nano-structured materials can increase the specific surface area and improve the transfer rate of ions and electrons, reducing internal resistance.

- Separator: Select a separator material with high porosity and low resistance to reduce the resistance of lithium ions during transmission. At the same time, improve the stability and high temperature resistance of the separator to enhance battery safety.

- Electrolyte: Develop an electrolyte with high ionic conductivity and good chemical stability. Special additives can be added to improve the performance of the electrolyte, reduce internal resistance, and improve battery cycle life.

2. Improve electrode manufacturing process

- Optimization of contact between active materials and current collectors: Increase the contact area and binding force between active materials and current collectors to reduce electronic resistance. Advanced coating technologies and surface treatment methods can be used to ensure the uniform distribution of active materials and firmly attach them to the current collector.

- Control electrode thickness and porosity: Reasonably design the thickness and porosity of the electrode to ensure sufficient active material loading while reducing the transfer distance of ions and electrons, thereby reducing internal resistance. By optimizing the microstructure of the electrode, battery performance can be improved.

- Optimization of positive and negative electrode slurries: Improve the adhesion of positive and negative electrode slurries to reduce contact resistance. Select appropriate binders and additives to improve the stability and fluidity of the slurry and ensure good contact between active material particles and between the current collector.

3. Battery design and assembly optimization

- Electrode plate area design: According to the application requirements of the battery, reasonably design the area of the electrode plate. As mentioned earlier, the electrode plate area is inversely proportional to resistance. Appropriate increase in electrode plate area can reduce internal resistance. But at the same time, factors such as battery volume, weight, and cost should be considered.

- Battery structure optimization: Adopt advanced battery structure designs such as wound and stacked types to reduce internal connection resistance and space waste. Optimize the battery packaging process to ensure good sealing and stability, reduce internal resistance and improve safety.

- Thermal management design: A good thermal management system can effectively reduce the temperature of the battery during charging and discharging and reduce the increase in internal resistance. By reasonably designing the heat dissipation structure and using efficient heat dissipation materials, keep the battery working within a suitable temperature range and improve performance and life.

VI. Comparison of Internal Resistance of Different State Cells

1. Comparison of cells with good internal resistance and cells with poor internal resistance

- Performance aspect

- Cells with good internal resistance: Usually have high capacity and stable output performance. During charging and discharging, the voltage platform is stable and the energy conversion efficiency is high. It can meet the needs of high-power devices such as electric vehicles and high-end electronic products.

- Cells with poor internal resistance: Performance may be slightly inferior. Capacity may fluctuate to some extent, and output performance may not be stable enough. In high-current charging and discharging, voltage may drop rapidly.

- Life aspect

- Cells with good internal resistance: After strict quality inspection and screening, they have a long cycle life. They can still maintain good performance after multiple charge-discharge cycles, reducing the frequency and cost of battery replacement.

- Cells with poor internal resistance: The cycle life is relatively short. During use, problems such as capacity attenuation and increased internal resistance may occur in advance due to internal defects or unstable performance, requiring more frequent battery replacement.

- Safety aspect

- Cells with good internal resistance: Strict quality control is implemented during production, with high safety. High-quality materials and advanced production processes are used to effectively prevent safety issues such as overcharging, over-discharging, and short circuits.

- Cells with poor internal resistance: The safety risk is relatively high. There may be some potential quality problems, such as a greater risk of internal short circuits and thermal runaway. More attention should be paid to safety during use to avoid safety accidents caused by improper use.

2. Comparison of new cells and old cells

- New cells usually have lower internal resistance. At the beginning of use, their performance is excellent, with sufficient capacity, stable output, and high safety.

- As the usage time increases, the internal resistance of old cells will gradually increase. This is due to reasons such as chemical changes inside the battery, material aging, and structural changes caused by multiple charge-discharge cycles. Old cells may experience problems such as capacity decline, reduced output power, and longer charging time, and the safety risk during use will also increase accordingly.

In short, cells with good internal resistance and new cells are superior to cells with poor internal resistance and old cells in terms of performance, life, and safety. When choosing a battery, various factors should be considered comprehensively according to actual needs to select a cell product with appropriate internal resistance and relatively new.

VII. Summary

Battery internal resistance is an important factor affecting battery performance. Understanding its composition, testing methods, impact on battery performance, and improvement methods is of great significance for selecting and using batteries. At the same time, comparing the internal resistance differences of cells in different states can help make a wiser choice according to actual needs to meet the needs of different application scenarios.