Battery Capacity Tests: Why Do Results Differ Among Chargers?

When testing the same battery with different chargers, the capacity results may be different, sometimes significantly. This makes it difficult for users to determine the actual capacity of the battery, affecting their experience. Understanding why different chargers measure different capacity readings can help us better select and use chargers.

What problems may cause from inconsistent capacity test results?

Difficulty in evaluating battery performance: When the same battery shows different capacity results on different chargers, it becomes hard to accurately evaluate the battery’s actual performance. This may lead to misunderstandings about the battery quality or even cause you to mistakenly believe the battery needs to be replaced.

Confusion in choosing a charger: Since different chargers show  different capacity results, users often don’t know which charger is more reliable. Not knowing what factors affect battery capacity makes you face great uncertainty when purchasing a charger.

Warranty and after-sales issues: Ordinary users find it hard to determine why the battery capacity results are inconsistent. This will cause them to frequently contact the battery or charger manufacturers, adding unnecessary trouble.

What factors affect battery capacity testing of a charger?

1. Discharge Current

One major factor affecting battery capacity is discharge current. The higher the discharge current, the lower the measured battery capacity, and vice versa. This trend is present in almost all disposable batteries and rechargeable batteries.

For example, the data sheet of XTAR 18650 4000mAh battery shows that as the discharge current increases from 0.8A to 8.5A, the measured capacity gradually decreases. This is because when the battery discharges quickly (i.e. the discharge current is high), the energy that can be extracted from the battery is reduced, resulting in a lower measured capacity.

Therefore, if the discharge current set by the charger is different, the measured capacity results will vary. For capacity testing, the trusted XTAR charger typically utilizes a discharge current of 300 mA, whereas the EBL PD4 charger uses 500 mA. Another reputable charger manufacturer, Vapcell, also offers different discharge currents for users to choose from. The benefit of using a lower diacharge current is that it prevents the battery from overheating and helps extend its lifespan.

2. Cut-Off Voltage

According to the international testing standard IEC 61960-3 2017, the cut-off discharge voltage for 3.6/3.7V lithium-ion batteries is 2.5V. Similarly, from the discharge performance curve of the XTAR 18650 4000mAh battery, it can be observed that the higher the cut-off voltage, the lower the measured capacity.

The cut-off voltage of the XTAR VX4 charger is the same as the IEC standard, so the measured capacity is more accurate. In contrast, Canadian user reported that the cut-off voltage of the MiBoxer charger is 3.1V. When testing the capacity of a 3.6V Li-ion battery, the capacity measured by the MiBoxer will be lower.

3. Test Temperature

The temperature during battery testing has a significant impact on capacity, with the ideal range being 20±5°C. Both low and high temperatures can affect the battery’s discharge performance, then directly influencing its discharge capacity. At high temperatures, the chemical reactions inside the battery become more active, increasing the capacity. At low temperatures, the battery capacity decreases. When using different chargers to test battery capacity, it’s important to ensure the testing temperature is consistent to obtain accurate comparison results.

4. Capacity Grading Accuracy

Another often overlooked factor is the capacity test accuracy of the charger, which is closely related to the charger’s technology level. A high-precision charger can significantly reduce measurement errors and provide more accurate results. For example, compared to professional capacity grading machine, the test accuracy of XTAR latest VX4 charger is over 95%. In a comparison test conducted by the XTAR laboratory under the same conditions, a randomly selected 1.5V AA 1200mWh battery showed a real energy of 1161mWh with the VX4 charger and 1129mWh with capacity grading machine. The accuracy error between them is less than 3%.

Conclusion

From the discussion above, we know that the differences in measured capacity between chargers are mainly caused by discharge current, cut-off voltage, test temperature, and measurement accuracy. Before purchasing a charger, you should check the data sheet or consult the charger manufacturer to understand these important parameters.

Here are some suggestions:

• It is best to choose a high-precision charger and test at 20±5℃ to minimize the impact of environmental factors on the results.

• The IEC standard specifies the cut-off voltage of 3.6/3.7V lithium batteries is 2.5V, but some chargers may set it above 3V. The cut-off voltage of 1.5V rechargeable lithium batteries is determined by the built-in step-down chip, not the charger.

• Different batteries are recommended to use different discharge currents to get the most accurate capacity results. In the following post, lets’s talk about this.