In today’s world, various types of protection boards are available for lithium-ion batteries. However, choosing the right protection board to meet the specific needs of the load is often a challenge. With the widespread use of mobile power supplies, the demand for higher load capacity is increasing, with discharge currents reaching several amperes, or even hundreds or thousands of amperes. This has led to the development of secondary battery protection boards with increasingly higher technical requirements, as simple overcharge and over-discharge protections are no longer sufficient. So, how do we choose the best protection board based on the load conditions?

1. Overcharge Protection

To begin, let's consider a lithium-ion battery with the following specifications: nominal voltage 3.6-3.7V, overcharge voltage 4.25V, and over-discharge voltage 2.5V. The key question is whether simply setting an overcharge voltage will suffice. As we know, batteries exhibit resistance beyond just ohmic resistance—there is also polarization impedance. When charging with a high current, the voltage drop due to this impedance becomes significant. For example, with a polarization impedance of 30 milliohms and a charge current of 5A, the voltage drop would reduce the voltage from 4.25V to 4.1V, which is not sufficient for a full charge. Therefore, adjusting the charging current or compensating for this impedance is necessary. By reducing the charging current to 2A, the voltage can be better controlled, ensuring the battery is properly charged without exceeding the voltage limit.

2. Over-discharge Protection

Choosing the appropriate over-discharge voltage is crucial. For example, a 3V over-discharge protection might seem acceptable, but this could result in only using 80% of the battery's capacity. If the discharge voltage is set too low, the battery may be used more efficiently but at the risk of over-discharge. For instance, with a 10A discharge current and a battery impedance of 30 milliohms, the actual battery voltage during over-discharge could be significantly higher than the set cut-off voltage, leading to underutilization of the battery. Therefore, careful consideration of the over-discharge voltage and recovery voltage is essential to ensure safe and efficient battery operation.

3. Managing High Discharge Currents

In most battery protection boards, the discharge current remains relatively stable. However, power tools, such as electric drills, require higher instantaneous discharge currents. For example, the starting current of an electric tool can be as high as 50A, while the normal operation current might range from 2A to 15A. Simple overcurrent protection often fails to meet these demands, as the protection may activate during the startup phase, preventing the motor from running properly. When the current exceeds the protection threshold (e.g., 15A), it is crucial to avoid triggering protection during short bursts of high current that are necessary for motor startup. Proper design of the protection circuit is essential to balance safety and functionality.

4. Balancing for Multi-Series Circuits

In multi-cell configurations, discrepancies between battery voltages can occur, especially during discharge. If one cell has a voltage of 3.0V and another has 3.5V, it is clear that the discharge will not be evenly distributed, leading to inefficiencies. To mitigate this, battery balancing is necessary to ensure uniform discharge across all cells. Without proper equalization, some cells will discharge more quickly than others, potentially causing damage or reducing the overall performance of the battery pack.

Conclusion

When selecting a protection board for lithium-ion batteries, it’s important to consider not only overcharge and over-discharge protection but also factors such as discharge current, balancing, and recovery voltages. Each application has its own unique requirements, and the protection board must be chosen accordingly to ensure optimal performance, safety, and longevity of the battery.