After a sodium-ion battery has been used for a period of time, if its internal resistance is found to have increased significantly compared with when it was first put into use, this usually indicates that the internal condition of the cell has changed. Internal resistance is not a completely fixed value. It is affected by factors such as battery temperature, state of charge, number of cycles, charging and discharging current, cell material condition, and testing methods. Therefore, when an increase in internal resistance is observed, it should not immediately be concluded that the battery has been damaged. The battery’s service time, capacity changes, temperature, and consistency of individual cell voltages should also be considered. For sodium-ion cells from the same batch and of the same model, if the internal resistance of one cell is significantly higher than that of the other cells, it deserves more attention, as this may indicate that the cell is aging faster or that there are manufacturing differences, connection abnormalities, or other issues.
After the internal resistance of a sodium-ion battery increases, the most direct effect is usually a reduction in its ability to handle high-current operation. As the load increases, the voltage drop inside the battery becomes more noticeable, and the equipment may experience insufficient output power, shorter operating time, or premature BMS protection. If the increase in internal resistance is mainly caused by low temperature, the measured internal resistance may decrease again after the battery temperature returns to an appropriate range. If the increase is caused by long-term cycling, high-temperature aging, or changes in electrode materials, it usually cannot be completely restored simply by raising the temperature. Therefore, when evaluating changes in internal resistance, the same testing conditions should be used for comparison. Data measured at different temperatures or different states of charge should not be directly compared.
Why Does the Internal Resistance of a Sodium-Ion Battery Increase?
Cell Aging Is One of the Common Causes
During repeated charging and discharging, the electrode materials and electrolyte in a sodium-ion battery undergo long-term changes. As the number of cycles increases, the active materials inside the battery may gradually change, while the interface conditions between the electrodes and electrolyte may also change. These changes can increase the internal resistance of the battery. When a cell has been used for a long period, it may show gradually increasing internal resistance, declining capacity, and reduced high-rate discharge performance. These are relatively common characteristics during battery aging. If a battery originally had low internal resistance, but its resistance gradually increases after a large number of cycles while its capacity also declines significantly, natural cell aging should generally be considered.
High-Temperature Operation Can Accelerate Changes in Internal Resistance
Temperature has a significant effect on the internal resistance of sodium-ion batteries. Operating or charging and discharging for long periods in high-temperature environments can accelerate the aging of internal materials and electrolytes inside the cell. For example, if a battery is installed inside equipment with poor heat dissipation and remains at a relatively high temperature while frequently undergoing high-current charging and discharging, the thermal load on the cell will increase. After a long period of use, its internal resistance may increase faster than that of a battery operated at normal temperatures. Therefore, the battery installation location, heat dissipation conditions, and actual operating temperature should all be included in routine inspections.
Which Operating Conditions Can Easily Cause Internal Resistance to Increase?
Long-Term High-Current Charging and Discharging
The higher the charging and discharging current of a battery, the more heat is generally generated internally. If the recommended charging and discharging current of the cell is exceeded for a long period, changes in the internal materials may accelerate. This is particularly important in energy storage equipment and electric devices that require high-power output. If the battery capacity is too small but it continuously carries a large load, the cells may remain under relatively high stress for extended periods. It should be noted that occasional high-current operation does not necessarily cause a significant increase in internal resistance immediately. The situation that deserves more attention is the combination of long-term high-current operation and high temperature.
Low Temperature Can Also Increase the Measured Value
In low-temperature environments, the conductivity of the electrolyte changes and the movement of ions inside the battery slows down. As a result, the measured internal resistance is usually higher than at room temperature. For example, if a sodium-ion battery has an internal resistance of 10 mΩ when tested at 25°C, the value may increase significantly when the battery is placed in a lower-temperature environment and tested again. This does not necessarily mean that the cell has suffered permanent damage. Therefore, when comparing internal resistance data from different periods, the testing temperature should be kept as consistent as possible. If conditions permit, the battery can be tested again after reaching a stable operating temperature to avoid misjudgment caused by temperature.
Overcharging and Over-Discharging May Also Affect Cell Condition
If a battery remains in an overcharged or over-discharged state for a long period, the cells may experience additional stress. Although the BMS can provide overcharge and over-discharge protection, protective devices cannot completely eliminate the effects caused by long-term abnormal use. Therefore, if an increase in internal resistance is accompanied by capacity loss, poorer voltage consistency, or frequent BMS protection, the battery’s previous operating conditions should be further examined.
Testing Methods and Cell Differences Can Also Affect the Results
Different Testing Equipment May Produce Different Values
Battery internal resistance can be measured in more than one way. Different testing instruments, test frequencies, test currents, and wiring methods may produce different results. Simply using a multimeter to measure battery voltage and current cannot directly provide an accurate AC internal resistance value for a cell. Professional battery internal resistance testing generally uses dedicated equipment and follows specified testing conditions. Therefore, when determining whether internal resistance has genuinely increased, it is best to use the same equipment, the same testing method, and a similar battery temperature for comparison.
Differences Between Individual Cells Require Particular Attention
For a battery pack consisting of multiple sodium-ion cells, if the internal resistance of all cells changes slightly with usage time, this is generally more consistent with overall aging. If only one cell has an internal resistance significantly higher than the others, that particular cell should be inspected more carefully. It may have faster capacity degradation, an internal connection abnormality, manufacturing differences, or localized aging. Such inconsistency may also cause voltage differences between cells in a series-connected battery pack during charging and discharging. As a result, one cell may reach its protection voltage earlier, ultimately affecting the actual performance of the entire battery pack.
Frequently Asked Questions
Q: Is lower internal resistance always better for a sodium-ion battery?
A: Under the same model, temperature, and testing conditions, lower internal resistance generally indicates better high-current performance. However, battery condition cannot be evaluated based on internal resistance alone. Capacity, cycle count, voltage consistency, temperature, and other data should also be considered.
Q: Does an increase in the internal resistance of a sodium-ion battery mean that the battery is damaged?
A: Not necessarily. If the measurement is taken in a low-temperature environment, the increase in internal resistance may only be a temporary temperature-related effect. If the resistance remains significantly higher over repeated measurements under the same conditions, while capacity and discharge performance also decline, it is more likely to be related to cell aging.
Q: Why is the internal resistance different between cells in the same sodium-ion battery pack?
A: There may naturally be some differences between individual cells. Production batches, capacity, internal resistance, service time, and temperature can all cause different measurement results. If the difference is particularly significant, further inspection of cell consistency and the battery pack’s connection condition is required.
Q: Does increased internal resistance affect battery capacity?
A: An increase in internal resistance does not necessarily cause a significant immediate decrease in capacity, but it generally affects high-current output capability. Under a large load, the battery terminal voltage may drop more significantly, and the usable power of the equipment may also be affected. If the increase in internal resistance is caused by long-term aging, capacity will often gradually decline as well.
The internal resistance of a sodium-ion battery may increase due to normal cell aging, high-temperature operation, long-term high-current operation, abnormal charging and discharging, low-temperature environments, and differences in cell consistency. The result may also be affected by testing equipment and measurement conditions. Therefore, when the internal resistance value changes, it is not advisable to determine whether the battery needs to be replaced based on a single measurement. If the internal resistance only increases in a low-temperature environment and decreases significantly after the battery returns to an appropriate temperature, temperature should generally be considered an important factor. If the battery has been used for a long period and its internal resistance continues to increase while its capacity declines and its loaded voltage drops significantly, cell aging is more likely. If only one cell in the battery pack has significantly higher internal resistance, that individual cell should be inspected carefully.
During routine testing, it is best to keep the testing conditions consistent, such as maintaining a similar temperature, state of charge, and testing method, while continuously recording data over a period of time. This makes it easier to determine whether the internal resistance is only fluctuating temporarily or is continuously increasing. For batteries that already show a significant increase in internal resistance accompanied by abnormal heating, substantial capacity loss, excessive voltage differences, or frequent BMS protection, continued high-load operation is not recommended, and professional inspection should be arranged promptly. Through standardized testing and continuous observation, the actual condition of a sodium-ion battery can be understood more accurately, while incorrect conclusions based on a single abnormal reading can be avoided.
