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2025-09-26
Lithium battery DCR: the key significance of DC internal resistance
Definition of DCRLithium battery DCR (Direct Current Internal Resistance) is the sum of all internal ohmic resistances of a battery when DC current flows through it, including ohmic internal resistance, charge transfer resistance, and polarization resistance. The DCR test calculates the internal resistance (R = ΔU/I) by applying a constant current and measuring the change in voltage. This reflects the dynamic impedance characteristics of the battery under actual operating conditions. These ohmic resistances include electrode resistance, electrolyte resistance, and separator resistance. When current flows through the battery, this internal resistance causes a voltage drop within the battery, affecting its performance.DCR measures the role of battery performance(1) Evaluating Battery State of Health (SOH)A battery's state of health (SOH) refers to the ratio of its current performance to its performance when brand new. It reflects the battery's aging and remaining service life. DCR is a key indicator for evaluating the SOH of lithium batteries. A new battery has an extremely low DCR. This is because, in the initial stages of battery manufacturing, the electrode material structure is intact, the electrolyte performance is excellent, the ionic conductivity of the separator is optimal, and the internal ohmic resistance is low. As the battery cycles, the electrode material undergoes structural changes, increasing the internal ohmic resistance and gradually increasing the DCR. By monitoring changes in the DCR, the battery's aging can be monitored in real time, providing a basis for replacement and maintenance.(2) Correlation with Other Battery Performance ParametersDCR is also closely related to other battery performance parameters. For example, it is related to the battery's capacity retention rate. When the DCR increases, the battery's energy loss during charge and discharge increases, resulting in a decrease in the battery's actual usable capacity. Furthermore, DCR also affects the battery's self-discharge rate. Batteries with higher internal resistance experience increased energy loss during self-discharge, shortening the battery's shelf life.Effect of DCR on battery discharge capacity(1) Advantages of a Small DCRThe smaller the DCR, the smaller the battery's voltage drop during high-current discharge. In real-world applications, many devices require batteries capable of high-current discharge, such as mobile phone fast charging and electric vehicle acceleration. For example, when a mobile phone supports fast charging, the charging current increases significantly. If the battery's DCR is small, the internal voltage drop during high-current charging is minimal, ensuring the battery receives energy efficiently and enabling rapid charging. Furthermore, a small DCR ensures more stable battery output power, preventing excessive voltage fluctuations that can affect device operation.(2) Harms of a Large DCRWhen the DCR is excessive, the battery will experience a significant voltage drop during discharge. During electric vehicle acceleration, if the battery's DCR is too high, the motor will require high current, causing the battery voltage to drop rapidly. When the voltage drops below a certain level, the battery management system triggers a protection mechanism to limit the battery's output current, resulting in a decrease in the electric vehicle's power performance and even failure to accelerate properly. In addition, excessive DCR will cause the battery to generate excessive heat during discharge, which will not only reduce the efficiency of the battery, but also accelerate battery aging and shorten the battery life.The DCR of lithium batteries plays an essential role in measuring battery performance and influencing its discharge capacity. It is not only a key indicator for assessing battery health but also closely correlates with actual performance. Monitoring DCR changes through appropriate testing methods provides timely insight into battery performance, providing a basis for optimized battery design, rational use, and effective management.
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09-172025
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).