A grade vs. B grade LFP battery cells
What’s the difference between A-grade, B-grade, and recycled LiFePO4 battery cells?
It is well known that A-grade cells are significantly more expensive than B-grade cells. Therefore, when comparing products, it's crucial to ensure they are of the same grade.
The best way to determine a cell's grade is to verify if it meets the manufacturer's specifications. This article discusses key parameters from the datasheet that determine a cell's performance, lifespan, and long-term value. By comparing test data to the specifications, we can distinguish between Grade A and Grade B cells.
Different grade Cells Dimension & Weight
The dimension and weight of a battery cell will vary slightly depending on its state of charge (SOC). Therefore, it is important to confirm the SOC state with the supplier before measuring the cell. Once you have confirmed the SOC state, you can measure the cell's dimensions and weight. Then, you can compare your measurements to the values listed in the datasheet. If the measurements match the datasheet, then the cell is likely to be of good quality with better stress stability. However, if the measurements do not match the datasheet, then the cell may be defective or a Grade B cell, which can impact safety and performance.
Different grade Cells Appearance
Each battery cell is stamped with a unique QR code that serves as its identification. This makes it easier for manufacturers to identify the cell when providing after-sales service. Grade B cells are considered substandard and do not usually come with a warranty. To distinguish them from A grade cells appearance, manufacturers often scratch off the QR code. If you find a battery cell with a concealed QR code, it is likely a Grade B cell. However, all Grade B cells are also covered with a new insulating sheet. Therefore, if you do not see any obvious signs of a QR code on the outside of the cell, you will need to remove the insulating sheet to check.
Different grade Cells Capacity
The capacity of different grade lithium battery cells is usually tested at a temperature of 25 degrees Celsius, with a charging and discharging rate of 1C. The real tested capacity value should be recorded and compared to the specifications.
Tip: The capacity of a LiFePO4 cell can be affected by temperature. Therefore, some manufacturers may send cells with a capacity that is slightly higher than the claimed capacity. If your test results are slightly higher than the datasheet, that is not a problem.
Consistent capacity is crucial when cells are used in series/parallel connection, as it directly determines the pack capacity and storage performance, thereby influencing the value over time of the entire battery system.
Different grade Cells Internal Resistance
The internal resistance of a lithium battery cell can be measured at different frequencies. Most manufacturers test the AC internal resistance. However, some manufacturers may test the DC internal resistance. The DC internal resistance is generally considered to be more reliable.
First, check the testing conditions on the datasheet, or confirm that with suppliers. This includes the temperature and SOC state. Once you know the testing conditions, you can measure the cell's internal resistance. If your test results meet the datasheet, then the cell is likely to be a Grade A cell. Lower internal resistance is key to high energy capacity release and good stress stability, while high internal resistance leads to rapid capacity loss and reduces the long-term value of the cell.
Different grade Cells Self-Discharging Rate
The self-discharge rate of different grade cells is calculated by measuring the voltage decrease over time. The self-discharge rate can vary depending on the state of charge (SOC) of the cell. The voltage will decrease more quickly at a 100% SOC state than at a 50% SOC state.
To determine the self-discharge rate, you can follow these steps:
Check the datasheet to make sure you know the testing SOC state.
Stock the cells in the specified testing conditions.
Check and write down the voltage decrease of the cell each day.
Calculate the self-discharge rate per day by dividing the total voltage decrease by the number of days.
Calculate the monthly or weekly self-discharge rate by averaging the daily self-discharge rates.
Here are two possible testing methods for you:
Solution 1: Stock the cells at room temperature (25°C) for 28 days.
Solution 2: Stock the cells at a higher temperature (45°C) for 7 days.
If the self-discharge rate is 3%:
For normal temperature, the self-discharge rate per day is 0.03/28 = 0.00107, which is about 0.107%.
For high temperature, the self-discharge rate per day is 0.03/7 = 0.00428, which is about 0.43%.
A low self-discharge rate ensures better storage performance and reduces the recharging frequency during storage, which is vital for maintaining long-term value.
Different grade Cells Capacity Recovery
The capacity recovery rate of different battery cells is the percentage of its capacity that is regained after a full discharge. To test the capacity recovery rate, you can do a 100% depth of discharge (DOD) charging and discharging cycle.
For example, for a 3.2V 100Ah battery cell with a capacity recovery rate of 95%, you would first measure the cell's capacity at the beginning of the test. If the capacity is 100Ah, then after a 100% DOD charging and discharging cycle, the cell's capacity should be more than 95Ah. If yes, then the battery capacity recovery rate meets the datasheet and the cell is a Grade A quality cell. High capacity recovery rate helps delay capacity loss and supports consistent performance throughout the cell's lifespan, ensuring its value over time.
Conclusion
Well, the conclusion is here. The purpose of testing parameters like capacity recovery is to compare the testing data with the specifications of the cells. This allows you to verify the accuracy of the datasheet and to trust the supplier. If all the data meet the specification or datasheet, then the cells are Grade A. If not, then the cells are Grade B.
In summary, opting for Grade A cells guarantees superior performance, safety, and long-term value. This is especially critical in battery packs using series/parallel connections, where the uniformity of each cell's lifespan and energy capacity is fundamental to the overall pack capacity and system reliability.