Explanation of overcurrent protection measures for Lithium Battery packs

During the charging and discharging process of lithium battery packs, if the current exceeds the safety threshold, it may cause battery overheating, performance degradation or even thermal runaway. Therefore, overcurrent protection is a key measure to ensure the safety of battery packs. The following is a detailed analysis of its principle and implementation methods from three aspects: hardware protection, software control, and failure redundancy design.

First, hardware protection measures

Hardware protection directly cuts off the current path through physical circuits or electronic components, featuring fast response speed and high reliability.

Protection IC and MOSFET switch

Function: The protection chip monitors the current of the battery pack in real time. When the current exceeds the set threshold (such as overcurrent or short-circuit current), it controls the MOSFET switch to cut off the circuit.

Action logic

Overcurrent detection: The protection chip determines whether the current exceeds the limit by detecting the voltage drop across the MOSFET (U=I×RDS×2, where RDS is the on-impedance of a single MOSFET).

Delay design: To prevent false operation, overcurrent protection is usually set with a delay (such as 13 milliseconds). If the current continuously exceeds the limit, the protection chip will turn off the discharge MOSFET and cut off the current.

Recovery condition: After the current returns to normal, the protection chip can be automatically or manually reset to restore the circuit conduction.

Fuses and PTC thermistors

Fuse: When the current exceeds the rated value, the fuse blows, permanently cutting off the circuit. However, the fuse is a disposable component and needs to be replaced before it can be used again.

PTC thermistor: When the current is too large, the resistance value of PTC increases sharply, limiting the current. After the temperature drops, the resistance value recovers and it can be reused.

Current detection resistor

High-precision detection: By connecting an external high-precision current detection resistor, precise monitoring of charging and discharging currents is achieved, reducing the impact of voltage and temperature changes on the detection results.

Low impedance design: Reduce the resistance value to minimize power loss while ensuring the sensitivity of overcurrent detection.

Second, software control measures

Software control is achieved through the battery management system (BMS), which dynamically monitors and regulates the current, featuring flexibility and scalability.

Current threshold limit

Charge and discharge current limit: The BMS sets the maximum charge and discharge current according to the battery specifications (such as no more than 2C). When the current exceeds the threshold, the BMS cuts off the circuit by controlling the MOSFET switch.

Dynamic adjustment: Dynamically adjust the current threshold based on the battery status (such as temperature, SOC) to ensure that the battery operates within a safe range.

Multi-level overcurrent protection

Double protection design

The first level of protection: The set value is relatively small and the delay is relatively long, which is used to prevent false actions caused by transient interference.

The second level of protection: The set value is relatively large and the delay is short, which is used to deal with severe overcurrent or short circuit situations.

Fault diagnosis and recording

Fault reporting: The BMS records information such as the time and current value of overcurrent events, facilitating subsequent analysis and maintenance.

Self-check function: Regularly conduct self-checks on the current detection circuit to ensure its normal operation.

Third, failure redundancy design

To ensure the system remains safe when a single protection mechanism fails, the lithium battery pack adopts a multiple redundancy design.

Dual current detection

Main detection circuit: The current is monitored in real time by the protection chip.

From the detection circuit: The BMS independently monitors the current through ADC sampling. When the detection results of the two are inconsistent, a fault alarm is triggered and the charging and discharging are terminated.

Hardware and software are interlocked

Logical relationship: Hardware protection (such as protection chips) and software control (such as BMS) are independent of each other. If either party detects overcurrent, it can cut off the circuit.

Fault isolation: If the hardware protection fails, the BMS can cut off the circuit through software. And vice versa.

Communication and Monitoring

Real-time communication: The BMS communicates with the protection chip via buses such as CAN/I2C to obtain current detection results in real time.

Remote monitoring: Through Internet of Things technology, remote monitoring of the current status of battery packs and fault early warning are achieved.

Fourth, the collaborative working principle of overcurrent protection measures

Normal working state

The "CO" and "DO" pins of the protection chip output high voltage, the MOSFET conducts, and the battery can be freely charged and discharged.

The BMS continuously monitors the current to ensure it is within a safe range.

The response when overcurrent occurs

Hardware priority response: If the current exceeds the threshold, the protection chip immediately turns off the discharge MOSFET to cut off the current.

Software collaborative processing: After the BMS detects abnormal current, it records the fault, stops charging and discharging, and simultaneously reports the fault information.

Redundant protection: If the protection chip fails, the BMS terminates charging and discharging through current threshold limitation or multi-stage protection design.

Fault recovery and maintenance

The user needs to identify the cause of the fault (such as abnormal load or charger failure), and after the repair, manually or automatically reset the protection circuit.

The BMS records the fault history, which is convenient for subsequent maintenance.

Fifth, the limitations and improvement directions of overcurrent protection measures

Limitations

Hardware aging: The current detection accuracy of the protection chip may decline over time, leading to false operation or failure.

Software vulnerability: The BMS algorithm may have uncovered abnormal scenarios, such as current sampling noise interference.

Cost and performance balance: High-precision protection circuits will increase costs, and a trade-off needs to be made between safety and economy.

Improvement direction

Intelligent diagnosis: Introduce machine learning algorithms to predict overcurrent risks through historical data.

Wireless monitoring: Remote monitoring of the current status of battery packs and fault early warning are achieved through Internet of Things technology.

New material application: Develop higher-precision current sensors or self-healing protection circuits to enhance reliability.