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2025-09-17
Causes of abnormal internal resistance of lithium-ion batteries
The reasons for abnormal internal resistance of lithium-ion batteries can be analyzed in combination with materials, processes, usage conditions and other aspects:Material and structural factors1. Electrode material issuesPoor conductivity of the positive and negative electrode active materials or excessive or unevenly distributed binder hinders electron conduction. Oxidation or poor contact of the copper or aluminum foil current collector increases ohmic internal resistance.2. Abnormal electrolyteInsufficient or aged electrolyte (excessive moisture, solvent decomposition) can hinder ion transport.Excessive electrolyte increases internal resistance and affects lithium ion concentration stability. 3. Separator DefectsLow porosity or excessive thickness of the separator restricts lithium ion migration.Workmanship and manufacturing defects1. Electrode and Tab IssuesUneven electrode coating thickness, excessive compaction density, or poor tab welding (such as cold welds) can lead to excessive local current density.High internal resistance between the rivet and the pressure plate, or insufficient positive electrode conductive additive.2.Insufficient pre-formation.The SEI film is not stably formed, and the internal resistance continues to increase during cycling.Usage and aging factors1. Overdischarge ImpactOverdischarge can damage the negative electrode graphite layer, dissolve copper ions in the positive electrode, clog the separator, and increase ohmic internal resistance and charge transfer impedance.Severe overdischarge can cause SEI film decomposition, further increasing internal resistance.2. Temperature and Cycle LossHigh temperatures accelerate electrolyte decomposition, while low temperatures reduce ion mobility, both leading to increased internal resistance.After long-term cycling, the positive electrode material structure collapses, the negative electrode SEI film thickens, and the charge transfer impedance increases.3. Poor battery pack consistencyVariations in capacity or self-discharge rate between battery cells lead to inconsistent voltage and abnormal internal resistance.Other factorsMicro-short circuits: Metal impurities or diaphragm damage during the production process can cause localized current increase and abnormal internal resistance.Seal failure: Gas/liquid leakage after long-term use triggers internal chemical reactions, increasing internal resistance.
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09-122025
The difference between pcs and photovoltaic inverters
PV inverters are only suitable for grid-connected applications, while pcs can be used for both on-grid and off-grid applications. PV inverters and pcs share the same topology. Three-phase inverters/converters use a three-level I-type/T-type topology, ANPC or NPC circuits. Single-phase inverters/converters use an H5/H6 topology. The hardware of PV inverters and pcs is nearly identical, with only slight differences in the DC-side wiring interfaces.The DC side of a photovoltaic inverter is connected to photovoltaic modules. The figure below shows the I-V curve of a photovoltaic module. Under certain conditions, such as an irradiance of 1000W/m², the module's current remains stable at over 18A within the voltage range of 0-35V. As the voltage increases, the current decreases. The I-V curve shows that the photovoltaic cell module maintains a stable current when generating electricity, thus exhibiting the characteristics of a current source. However, its voltage varies continuously, influenced by factors such as irradiance intensity, temperature, air quality, and surface cleanliness.The power generated by a photovoltaic module (P) = voltage (U) x current (I). Along the I-V curve, the area of the rectangle formed by the V value on the horizontal axis and the I value on the vertical axis represents the module's power generation value. Within these rectangles, the maximum area is the power value at which the MPPT (Maximum Power Point Tracking) is located. See the P-V curve for the module below.The MPPT of a module is located at the top of a hill, similar to the peak of a small hill. The P-V curve shows that the power generated by a photovoltaic module is constantly changing.Because photovoltaic batteries cannot generate stable voltage and power during power generation, photovoltaic inverters cannot establish AC voltage and frequency during power generation. They can only be used for grid-connected applications, running a phase-locked loop (PLL) control strategy to inject power following the grid's voltage and current sinusoidal waveforms. Therefore, photovoltaic power sources are often referred to as current sources, also known as P/Q sources.The DC side of the pcs is connected to an electrochemical/rechargeable battery. A typical example is LFP battery. The following figure shows a charge and discharge SOC-V curve and table for LFP battery. The voltage of a lithium battery changes only with the SOC. During transient conditions, its voltage is stable and does not experience sudden increases or decreases. Therefore, a lithium battery has the characteristics of a voltage source.pcs charge and discharge lithium batteries through rectification or inversion. Similarly, charge and discharge power (P) = voltage (U) x current (I). Given a fixed voltage, power output can be controlled simply by controlling the magnitude and direction of the current. When performing charging/discharging/inversion rectification for grid-connected (following) operation, the pcs employs a phase-locked loop (PLL) control strategy, injecting or absorbing energy in accordance with the grid's voltage and current sinusoidal waveforms. During off-grid (connecting to the grid) operation, since the voltage and power of the DC power supply are controllable, the pcs can establish the AC voltage and frequency. A DSP chip controls the generation of the grid voltage/current sinusoidal curves and 50/60Hz frequencies. Therefore,energy storage power supplies used in off-grid applications are often referred to as voltage sources, also known as V/F sources.
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09-032025
What are the common charging methods for lithium batteries?
There are many ways to charge lithium-ion batteries. The most commonly used methods are constant current charging, constant voltage charging, constant current-constant voltage charging and pulse charging.(1) Constant current charging Constant current charging refers to charging the battery with a constant current. When constant current charging is performed with a single current value, if the charging current is too small, the charging time at the beginning of the charging phase will be too long. If the charging current is too large, the charging voltage of the battery will be too large in the later stages of charging, causing a greater impact on the electrodes. (2) Constant voltage charging Constant voltage charging refers to charging the battery while keeping the charging voltage constant. Although this method can automatically adjust the charging current according to the change of the battery SOC during the charging process, its disadvantages are also very obvious: the charging speed is slow, and the charging current is too large due to the low battery voltage in the early stage of charging, which will damage the battery, affect the battery life, and even cause the battery to be scrapped. (3) Constant current-constant voltage charging Constant current-constant voltage charging is currently the most commonly used charging method for lithium batteries. This method refers to first charging with a constant current until the battery voltage reaches a certain voltage value, and then charging the battery at a constant voltage. During the constant voltage charging stage, the charging current value of the battery will gradually decrease. When the charging current decreases to near zero or 0.02C, the battery is considered fully charged. This method combines the advantages of constant current charging and constant voltage charging: during constant current charging, it ensures that the current in the early stage of charging does not exceed the rated current of the battery; during constant voltage charging, the charging efficiency is improved.(4) Pulse chargingThe pulse charging method refers to charging the battery with a constant pulse current. In the initial stage of charging, the battery is first charged with a small current at a constant current. When the battery voltage is charged to a certain set voltage value, the battery is charged with a pulse current. This method provides sufficient time for depolarization operation, allowing the battery to store more energy.The figure below illustrates a typical charging process for a potassium-ion battery under constant current/constant voltage charging mode. The charging process is divided into two phases: constant current charging (t0-t1) and constant voltage charging (t1-t2).
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08-282025
Lithium Battery Pack Process: More Than Just “Assembly”
Many people simply equate lithium battery packs with "battery assembly." However, this process is actually a highly integrated system engineering process that combines electrochemistry, mechanical design, electronics, and thermal management. Every step is crucial to the performance, safety, and lifespan of the battery system.1. Cell Selection: The "Foundation" of the Packing Process, Consistency is KeyBuilding a reliable battery system begins with cell selection. Core requirements include performance consistency screening and on-demand selection. Voltage, internal resistance, and capacity are the three core parameters of a cell, and rigorous screening is required to ensure that each cell's parameters are perfectly matched. If a cell's capacity is 10% lower than others, it will charge and discharge first during long-term charge-discharge cycles, accelerating aging and potentially causing uneven charging and discharging across the entire battery pack, potentially posing a safety risk.2. Structural Design: Balancing Safety and Practicality in SpaceBattery packs must adapt to the end product and withstand the rigors of complex environments. Structural design requires finding the optimal balance between space, weight, and strength. For example, battery packs for new energy vehicles must be designed to closely fit the vehicle's spatial layout while also possessing a high-strength structure to withstand vibrations, bumps, and even collisions during driving, protecting the battery cells from crushing. Energy storage battery packs must also consider cabinet installation dimensions and ensure stacking stability. To reduce energy consumption, especially for automotive applications, battery packs utilize lightweight materials such as aluminum alloy and carbon fiber. However, lightweighting does not mean cutting corners. Engineers utilize topological optimization to strengthen the structure at key stress points, reducing weight while increasing rigidity and protecting the battery cells from damage due to vibration and impact.3. Electrical Connection: A Precise Path for Current and Signals. Even a single error is essential. After the battery cells are assembled, reliable electrical connections are crucial for powering the battery pack and are also a high-risk area for safety hazards. The busbar shape has been optimized to further reduce heat generation. The high-voltage wiring harness in the battery pack, responsible for transmitting high currents, must be thickened and kept away from heat sources. Low-voltage signal lines, responsible for transmitting data, must be routed away from the high-voltage harness to prevent EMI from causing erroneous data and misinterpretation by the BMS. All connections are insulated to prevent electrical creepage and breakdown. The entire battery pack must also meet IP ratings to ensure safety in rainy, submerged, and other environments.4.Thermal Management: The Battery's "Thermostat," Temperature Determines LifespanExcessively high lithium battery temperatures accelerate aging and may even cause thermal runaway. Excessively low temperatures lead to a sudden drop in capacity and slower charging. The thermal management system acts as the battery pack's "thermostat," maintaining an optimal temperature range of 25-40°C. Regarding heat dissipation, new energy vehicle battery packs often use liquid cooling. Liquid cooling plates embedded within the battery pack circulate coolant to remove heat, ensuring more uniform temperature control. Air cooling is cost-effective and simple, making it suitable for applications like energy storage batteries, where heat generation is relatively low. In winter, the battery pack activates its heating function, preheating the battery cells using PTC heating plates or electric heating films to prevent reduced battery life in winter.5. BMS: The "Brain" of the Battery Pack, the Core of IntelligenceIf the battery cell is the "heart" of the battery pack, then the BMS is the "brain," responsible for monitoring, protecting, and optimizing battery performance. The BMS uses sensors to collect real-time data on each cell's voltage and temperature, as well as the current flowing through the entire battery pack. It then uses algorithms to estimate the SOC and SOH, providing the user and the vehicle's control system with constant visibility into the battery's condition. Even if the cell parameters initially match, variations can develop over time. The BMS uses passive balancing (using resistors to discharge high-voltage cells to level the voltage) or active balancing (using energy transfer for greater efficiency and power savings) to prevent overcharging and discharging of individual cells, thereby extending the life of the entire battery pack. The BMS also has a series of preset "safety red lines." If any of these parameters, such as voltage, temperature, and current, are exceeded, the circuit will be immediately disconnected to prevent further damage. This serves as the battery pack's "last line of defense."In addition to the BMS, battery packs must also incorporate multiple safety features from other perspectives, such as electrical and mechanical safety. Before leaving the factory, battery packs must undergo three major tests: electrical performance, safety, and environmental adaptability. This ensures they can function properly in diverse environments and regions.