On September 1, 2025, the new national standard for electric bicycles came into effect, setting higher requirements for the safety and performance of electric bicycles. In the design of electric bicycles, the battery management system (BMS) is of vital importance, and the problem of static surge poses a serious threat to the reliability and stability of the BMS. As an FAE engineer of Shanghai Lebao Electronics, I would like to share with the engineers designing the BMS for electric bicycles the risks of static surge and the corresponding strategies.

The risk of electrostatic surges to the BMS of electric bicycles

1. Damaged electronic components: During the riding and charging process of an electric bicycle, it is prone to be interfered by external static electricity, such as static electricity generated by human contact or clothing friction. The integrated circuits and microcontrollers in the BMS are highly sensitive to static electricity. The high voltage and large current generated by the instantaneous static discharge may directly break through the insulation layer of the components, causing permanent damage to the components and making some or all functions of the BMS fail.

2. Communication failure: The BMS needs to communicate with devices such as the motor controller and charger to achieve precise management of the battery. Common communication interfaces such as CAN and RS-485, when subjected to electrostatic surge impacts, will generate transient overvoltage and overcurrent on the communication lines, interfering with the normal communication signals and causing data transmission errors, packet loss, and even communication interruption. This will affect the BMS's accurate judgment and control of the battery status, and subsequently impact the performance and safety of the electric bicycle.

3. Causes of fire hazards: When the battery protection circuit in the BMS fails due to an electrostatic surge, it may be unable to promptly detect and control abnormal situations such as overcharging, overdischarging, and overcurrent of the battery. During abnormal operation, the battery is prone to heating, expansion, and even catching fire and exploding, seriously threatening the life and property safety of the rider. The new national standard has strengthened safety requirements such as fire prevention and flame retardancy, and the fire hazards caused by electrostatic surges cannot be ignored.

Strategies for dealing with electrostatic surges

1. Grounding Design: Good grounding is the foundation of electrostatic surge protection. In the design of BMS, it is necessary to ensure that the grounding plane on the PCB board is complete and has an adequate area, providing a low-impedance discharge path for electrostatic charges. The metal casing of the BMS should be reliably connected to the vehicle frame. The vehicle frame serves as the reference point for grounding of the entire vehicle, effectively conducting electrostatic charges to the ground. For example, use metal screws and conductive pads to connect the BMS casing to the vehicle frame to ensure the reliability of the grounding connection.

2. Shielding measures: Shielding the sensitive circuits and communication lines in the BMS can reduce interference from external electrostatic fields and electromagnetic fields. For communication cables, twisted-pair cables with shielding layers can be used, with one end grounded to effectively suppress common-mode interference. In PCB design, the sensitive circuit area is surrounded by ground copper foil to form local shielding, preventing static electricity from coupling into the circuit.

3. Selection of circuit protection devices 

◦ TVS Diode: TVS (Transient Voltage Suppression Diode) is a commonly used surge protection device. At positions in the BMS where it is susceptible to surge impacts, such as the power input end and communication interfaces, TVS diodes are connected in parallel. When a surge voltage arrives, the TVS diode quickly conducts, directing the surge current to the ground, thereby protecting the subsequent circuits. For example, at the 48V power input port, a TVS diode such as Leiditech SMDJ58CA can be selected. Its reverse working voltage is higher than the normal operating voltage of the power supply, and it can withstand higher surge energy. 

◦ ESD Protection Diode: ESD (Electrostatic Discharge) protection diodes are specifically designed to handle electrostatic interference. At positions such as the I/O interface and key input of the BMS, ESD protection diodes are connected in series or parallel. For example, models like Leiditech SD05C, SD0581D3W, etc., feature low capacitance and fast response, and can effectively suppress electrostatic discharge without affecting the normal transmission of signals. 

◦ Varistor: Varistors can be used to absorb high-energy surges. At the power input of the BMS, varistors are used in conjunction with TVS diodes, such as MOV (Metal Oxide Varistor). When the surge voltage exceeds the threshold of the varistor, its resistance value rapidly decreases, allowing the surge current to be dissipated. However, varistors have certain aging issues, and their lifespan and reliability need to be considered when selecting them.

4. PCB Layout and Routing Optimization 

◦ Shorten the circuit length: Try to minimize the length of the sensitive signal lines and reduce the induced static charges on the lines. For instance, design the connection lines between the microcontroller in the BMS and the surrounding circuits to be simple and compact, avoiding overly long traces. 

◦ Separate the strong and weak electrical circuits: Arrange the strong electrical lines (such as power lines) and the weak electrical lines (such as signal lines) separately to prevent surges on the strong lines from being coupled to the weak lines through electromagnetic induction. During PCB routing, maintain sufficient distance between the strong and weak electrical circuits and isolate them through the grounding plane. 

◦ Increase decoupling capacitors: Near the power pins of the chip, place decoupling capacitors appropriately, such as a combination of 0.1μF and 10μF capacitors. Decoupling capacitors can effectively filter out high-frequency noise and transient interference on the power lines, and reduce the impact of electrostatic surges on the chip.

BMS circuit board 485 communication surge damage repair case

In the market feedback for a certain brand of electric bicycles, multiple BMS circuit board failure issues have been reported. After testing, the faults were concentrated at the 485 interface where the BMS communicates with the motor controller. In actual usage scenarios, especially during rainy weather or during charging plug-in and unplugging processes, communication interruptions occurred frequently. Further inspection revealed that the 485 communication chip and some surrounding circuit components had been burned out. 

Based on the initial analysis, this is because the 485 communication line was subjected to an electrostatic surge impact in a complex usage environment. In the original design of this BMS, the 485 interface merely simply configured two ordinary resistors for current limiting, lacking effective surge protection devices. When facing instantaneous high voltage and large current electrostatic surges, it was unable to provide sufficient protection. 

In response to this issue, Shanghai Lebao Electronics has proposed a comprehensive improvement design. At the 485 interface, a multi-channel integrated TVS protection device developed by Lebao itself was added, such as the SM712 suitable for the RS485 communication interface. It is specifically designed to protect RS-485 applications with asymmetric working voltages (-7V to 12V), capable of absorbing the highest level of repetitive ESD discharge exceeding the IEC61000-4-2 international standard of 30KV. It can safely dissipate 19A of 8/20μs waveform surge current (IEC61000-4-5) at extremely low clamping voltage. At the same time, a self-recovery fuse PPTC was connected in series at the communication line entrance. When an abnormal large current occurs in the line, the resistance value of the PPTC will increase rapidly, limiting the current and providing overcurrent protection. And it can automatically return to the normal working state after the fault is resolved. 

In the PCB layout, the 485 communication lines were optimized. The 485 communication lines were separately planned in a dedicated area, kept away from other strong electrical lines and interference sources, and a grounding copper foil shielding layer was added to reduce the impact of external electromagnetic interference on the communication lines. The routing of the communication lines was also shortened as much as possible and kept simple to reduce line impedance and the accumulation of induced charges. 

The improved BMS circuit board has undergone rigorous laboratory tests, simulating various severe environmental conditions such as contact discharge of ±15kV and air discharge of ±25kV, etc. Under these extreme test conditions, it can operate stably, with normal 485 communication, and no longer experienced chip burnout or communication failure issues. In actual road tests, different climate conditions and complex road scenarios were selected, and electric bicycles equipped with the improved BMS were placed for long-term cycling tests. Through a large number of samples and long-term verification, the reliability of the BMS has been significantly enhanced, effectively solving the fault problems caused by 485 communication surges, and achieving the design goal of 100% safe operation.

The implementation of the new national standard for electric bicycles has placed higher demands on the safety and reliability of BMS. Static surge protection is a crucial aspect among them. Through reasonable grounding design, shielding measures, selection of circuit protection devices, as well as optimization of PCB layout and wiring, the risk of static surges to BMS can be effectively reduced, thereby enhancing the overall performance and safety of electric bicycles. We hope that the above suggestions will be helpful to all engineers when designing the BMS for electric bicycles, and jointly promote the safe development of the electric bicycle industry.