The capacity of a sodium-ion battery is an important parameter for determining its actual energy storage capability and is usually expressed in Ah (ampere-hours). The capacity marked on a battery is its rated or designed capacity. During actual use, the measured capacity may differ from the nominal value due to factors such as temperature, charging and discharging current, battery aging, and testing conditions. Therefore, to accurately determine how much energy a sodium-ion battery can still store, the battery needs to be charged and discharged under standardized testing conditions rather than simply judging its capacity by measuring the battery’s resting voltage. Capacity testing involves fully charging the battery according to specified conditions, then using stable testing equipment to perform constant-current discharge until the manufacturer’s specified discharge cutoff condition is reached, and finally calculating the actual capacity based on the discharge current and discharge time.
The testing method is basically similar for individual sodium-ion cells and battery packs consisting of multiple cells, but the testing equipment and operating conditions may differ. Individual cells can be accurately tested using a battery tester, while battery packs require consideration of the BMS, series-parallel configuration, and consistency between individual cells. If the test is used for product inspection before shipment, the testing temperature, charging and discharging rate, cutoff voltage, and resting time should also comply with the requirements specified by the cell manufacturer or relevant standards. Only when the testing conditions remain consistent can capacity data from different batches or different periods of use have good comparative value.
How Is the Capacity of a Sodium-Ion Battery Measured?
Perform the Discharge Test After Fully Charging the Battery
Capacity testing generally begins with a full charge. Use charging equipment that complies with the battery specifications and charge the sodium-ion battery until it reaches the charging termination conditions specified by the manufacturer. After charging is completed, the battery generally needs to rest for a certain period to allow its internal state to stabilize before subsequent discharge. This step should not be omitted. If the battery is not fully charged before discharge, the measured capacity will be lower and cannot accurately reflect the actual capacity of the battery.
Before testing, also confirm:
- Battery model, nominal voltage, and rated capacity.
- Charging termination voltage specified by the manufacturer.
- Recommended charging current and discharging current.
- Specified discharge cutoff voltage.
- Test environment temperature.
Different sodium-ion cells may use different positive and negative electrode materials, so the specific charging and discharging parameters should be based on the data provided by the manufacturer.
Perform Constant-Current Discharge and Record the Time
After charging is completed, connect the battery to a capacity tester or electronic load and perform constant-current discharge according to the specified discharge current. For example, if a sodium-ion battery has a rated capacity of 100Ah and is discharged at a constant current of 10A, it will theoretically take a relatively long time to reach the specified cutoff condition. The testing equipment records changes in current and time throughout the discharge process.
The basic capacity calculation formula is: Capacity (Ah) = Discharge Current (A) × Discharge Time (h)
For example, if the current remains at 10A during the test and the battery reaches the specified cutoff voltage after 9.5 hours of continuous discharge:
10A × 9.5h = 95Ah
Therefore, the actual capacity obtained from this test is approximately 95Ah.
What Conditions Are Required for Capacity Testing?
The Test Temperature Should Remain Stable
Temperature affects the discharge performance of sodium-ion batteries, so capacity testing generally needs to be performed under specified temperature conditions. At different temperatures, the internal resistance, ion movement, and available capacity of the battery may change. If a battery provides a test result of 100Ah at 25°C, a different result may be obtained when it is tested at a lower temperature. This does not necessarily mean that the battery has suddenly developed a fault, but may instead be caused by changes in the testing environment. Therefore, if the capacity before and after battery use needs to be compared, the same temperature should be maintained as much as possible.
The Discharge Current Should Not Be Changed Arbitrarily
Capacity is related to the discharge rate. When discharged at a lower current, a battery can generally release more usable capacity. When a higher current is used, the terminal voltage may drop more significantly, causing the battery to reach the cutoff voltage earlier. Therefore, capacity testing cannot simply specify “discharge until the battery is empty.” The discharge current must be clearly defined.
For example, for a 100Ah battery: 0.1C = 10A 0.2C = 20A
If the manufacturer specifies that capacity testing should be performed at 0.2C, constant-current discharge should be performed at 20A rather than arbitrarily changing the current to 5A or 50A.
The Discharge Cutoff Voltage Must Be Correct
The discharge cutoff voltage is an important condition in capacity testing. Once the battery reaches the cutoff voltage specified by the manufacturer, discharge should be stopped. If the cutoff voltage is set too low, over-discharge may occur. If it is set too high, the actual usable capacity may not be fully released. For sodium-ion batteries, there is no single fixed cutoff voltage that applies to all products. Different cell systems may have different operating voltage ranges, so the parameters of LiFePO4 batteries or other lithium batteries should not be directly applied.
How Can You Determine Whether the Test Result Is Normal?
There May Be a Difference Between Actual Capacity and Rated Capacity
The rated capacity of a battery does not mean that every test must produce exactly the same number. Testing temperature, discharge rate, equipment accuracy, and the current condition of the battery can all cause certain variations. If a 100Ah battery produces a test result of 97Ah under testing conditions that comply with the manufacturer’s requirements, this number alone cannot be used to determine that the battery has been damaged. The result should be evaluated according to the capacity tolerance specified by the manufacturer. If a battery has been used for a long period and its capacity gradually declines from a level close to its rated capacity to a significantly lower level, while its internal resistance increases and operating time becomes shorter, the battery may have experienced significant aging.
Individual Cell Differences Need to Be Considered in Multi-Series Battery Packs
For a battery pack consisting of multiple sodium-ion cells connected in series, the capacity test result of the entire pack cannot fully indicate the condition of each individual cell. For example, in a 4-series battery pack, if one cell has significantly lower capacity than the others, it may reach the cutoff voltage earlier during discharge, causing the BMS to stop discharging prematurely. Therefore, when the capacity of a battery pack is abnormal, individual cell voltage, internal resistance, and cell consistency should also be checked.
Frequently Asked Questions
Q: Can a multimeter be used to determine the capacity of a sodium-ion battery?
A: No, not accurately. Voltage can only reflect part of the current battery condition. Even at the same voltage, the actual remaining capacity of batteries may differ. Accurate capacity testing requires a complete charge followed by discharge under specified conditions.
Q: To what voltage should a sodium-ion battery be discharged during capacity testing?
A: Testing should be performed according to the discharge cutoff voltage specified by the particular cell manufacturer. Different sodium-ion battery material systems and cell specifications have different requirements, so a single fixed voltage cannot be used as the cutoff condition for all products.
Q: Is a battery tester required for capacity testing?
A: For a simple estimate, a stable electronic load and accurate current and time measurement equipment can be used. If relatively accurate capacity data is required, a professional battery tester is recommended because it can continuously record current, voltage, and time and automatically calculate capacity.
Q: How much capacity loss means that a sodium-ion battery needs to be replaced?
A: This cannot be determined based on one fixed percentage alone. The actual replacement criteria should be considered together with the manufacturer’s requirements, equipment needs, and battery usage conditions. If the capacity has significantly affected the operating time of the equipment and is accompanied by increased internal resistance, heating, or abnormal voltage, further inspection is required.
The capacity testing process for a sodium-ion battery consists of fully charging the battery under specified conditions, allowing an appropriate resting period, performing constant-current discharge until the specified cutoff condition is reached, and finally calculating the actual capacity based on the discharge current and time. The most basic calculation is discharge current multiplied by discharge time. However, during actual testing, temperature, charging voltage, charging current, discharge rate, and cutoff voltage must also be controlled. If an ordinary user simply wants to know how long the battery can continue operating, the actual operating time of the equipment can be used for a simple assessment. If it is necessary to determine whether the battery meets its rated capacity or compare performance before and after use, a more standardized capacity testing method should be adopted. The discharge current and cutoff voltage should not be changed arbitrarily during testing, otherwise the results from different tests will not have good comparability.
For sodium-ion battery packs, it is also necessary to consider that the BMS may stop charging or discharging prematurely. If the voltage of one cell becomes too low, the entire battery pack may stop discharging even though other cells still have remaining energy. Therefore, a low measured capacity does not necessarily fully represent the actual capacity of all the cells in the battery pack. In this situation, individual cell voltage, internal resistance, and consistency should be further checked. Through standardized testing conditions and continuous data recording, the actual capacity, aging condition, and ability of a sodium-ion battery to continue meeting the power requirements of the equipment can be evaluated more accurately.
