Analysis of Overcharge Protection Mechanism for Lithium Battery Packs

Overcharge protection for lithium battery packs is one of the core technologies to ensure battery safety and lifespan. Overcharging (i.e., charging voltage or current exceeding the design upper limit) can cause irreversible chemical reactions inside the battery, such as electrolyte decomposition, damage to the structure of electrode materials, lithium dendrite growth, and even lead to thermal runaway, fire or explosion. Therefore, lithium battery packs work in coordination through multi-level protection mechanisms to prevent overcharging. The following is an in-depth analysis of its principle and implementation methods from three dimensions: hardware protection, software control, and failure redundancy design.

First, hardware protection mechanism

Hardware protection is the first line of defense against overcharging of lithium battery packs. It directly cuts off the charging path through physical circuits or electronic components and features fast response speed and high reliability.

Voltage monitoring and Protection chip (Protection IC

Function: Real-time monitoring of the voltage of each battery in the battery pack. When the voltage of any single cell exceeds the preset threshold (such as 4.2V±0.05V), the protection chip will output a control signal.

Action logic

Overcharge detection: If the voltage continuously exceeds the threshold (such as above 50ms), the protection chip determines that it is in an overcharge state.

Control signal output: Through the MOSFET drive circuit, the charging path is turned off (the charging MOSFET is disconnected), while the discharging path is kept on (allowing discharge).

Recovery condition: When the battery voltage drops below the overcharge release threshold (such as 4.1V), the protection chip re-conducts the charging MOSFET to restore the charging function.

Fuses and PTC thermistors (auxiliary protection)

Fuse: In extreme cases (such as when the protection chip fails), when the current exceeds the fuse's rated value, the fuse melts and permanently cuts 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 the PTC resistor increases sharply due to heat generation, limiting the current and providing overcurrent protection. After the temperature drops, the resistance value of PTC recovers and it can be reused.

Balancing circuit (indirectly preventing overcharging)

Function: Through passive or active balancing technology, it balances the voltage differences among individual batteries in the battery pack to prevent overcharging of some batteries due to excessively high voltage.

Implementation method

Passive balancing: The current of a high-voltage battery is shunted through a resistor, and the excess energy is dissipated in the form of heat.

Active balancing: By transferring the energy of a high-voltage battery to a low-voltage battery through a DC-DC converter, it is more efficient but more costly.

Second, software control mechanism

Software control is achieved through the battery management system (BMS), which enables precise monitoring and regulation of the charging process, featuring flexibility and scalability.

Charging voltage and current limitations

Constant current - Constant Voltage (CC-CV) charging strategy:

Constant current stage: Charge with a constant current (such as 0.5C), and the voltage gradually rises.

Constant voltage stage: When the voltage reaches the set value (such as 4.2V), it switches to constant voltage mode and the current gradually decreases.

Termination condition: Charging is completed when the current drops to the cut-off current (such as 0.05C).

Dynamic adjustment: BMS can dynamically adjust charging parameters based on battery status (such as temperature and SOC) to avoid the risk of overcharging.

Charging time limit

Timeout protection: If the charging time exceeds the preset threshold (such as 3 hours), the BMS determines it as an abnormal charging situation and forcibly terminates the charging.

Applicable scenario: To prevent overcharging caused by faults in the voltage detection circuit.

Temperature monitoring and compensation

High-temperature protection: When the battery temperature exceeds the threshold (such as 50℃), the BMS reduces the charging current or pauses charging to prevent thermal runaway.

Low-temperature protection: In low-temperature environments (such as <0℃), limit the charging current or prohibit charging to prevent lithium dendrite precipitation.

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 voltage detection

Main detection circuit: The voltage of individual cells is monitored in real time by the protection chip.

From the detection circuit: The BMS independently monitors the voltage through ADC sampling. When the detection results of the two are inconsistent, it triggers a fault alarm and stops charging.

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 overcharging, it can cut off the charging.

Fault diagnosis: If the hardware protection operates but the software does not detect the anomaly, the BMS can record the fault code and report it.

Communication and self-checking mechanism

Communication monitoring: The BMS communicates with the protection chip via buses such as CAN/I2C to obtain the protection status in real time.

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

Fourth, the collaborative working principle of the overcharge protection mechanism

Normal charging process

At the initial stage of charging, the BMS charges at a constant current, and the voltage gradually rises.

When the voltage approaches 4.2V, the BMS switches to constant voltage mode and the current gradually decreases.

The protection chip continuously monitors the voltage. If the overcharge threshold is not triggered, the charging ends normally.

The response when overcharging occurs

Hardware priority response: If the voltage exceeds 4.2V, the protection chip immediately turns off the charging MOSFET and cuts off the charging path.

Software collaborative processing: After the BMS detects an abnormal voltage, it records the fault and stops charging, while simultaneously reporting the fault information.

Redundancy protection: If the protection chip fails, the BMS terminates charging through charging time limit or dual voltage detection.

Fault recovery and maintenance

The user needs to identify the cause of the fault (such as charger failure or battery aging), 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 of overcharge protection mechanisms and their improvement directions

Limitations

Hardware aging: The voltage 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 voltage 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 overcharging risks through historical data.

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

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