How to Design a Battery Management Unit?

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How to Design a Battery Management Unit?

The battery management unit is part of the battery management system and is installed on the battery module (pack). The functions of BMU include providing real-time monitoring function of voltage and temperature of a single battery (single cell), thermal management and equalization ability, and communication with the main control module of superior battery cluster through CAN bus to form a highly flexible BMS. It is an important part of ensuring the safety and optimal use of the battery. The content is complex and the relationship is compact. It is necessary to understand the connection between the functional modules, to better design it. Through today’s article, we will take a closer look at its function and design principle.

What is a Battery Management Unit?

The main function of the BMU is to collect the voltage and temperature of the battery cell and implement the battery balancing strategy. Information collection communicates with the second level through the communication link, usually using CAN or Daisy chain communication mode. In addition, BMU is also known as the slave control of BMS. The slave control is a very basic BMU battery management unit, usually responsible for the management of the battery package. On the one hand, it should monitor and collect the operation information inside the battery package in real-time, including temperature, voltage, current, SoC, SoH, etc. On the other hand, the battery balancing strategy should be implemented. In addition, the collected battery pack information will communicate with the second stage through the communication link.

Why We Need Battery Management Units?


BMU is an electronic device used to manage and monitor the status of battery packs. The benefits it can bring include:

Battery capacity calculation: It can calculate the battery capacity, judge the health state of the battery, and predict the life of the battery.

Battery temperature monitoring: The battery temperature can be monitored to avoid overheating.

Current monitoring: It can monitor the current of the battery, maintain the normal working state of the battery, and prevent problems such as over-current and short circuit of the battery, so as to prevent accidents during the use of the battery.

Voltage monitoring: It can monitor the voltage of the battery to ensure that the voltage of the battery is within a certain range, to ensure the working and charging state of the battery and avoid the overcharge or over-discharge of the battery.

Warning and protection: When the battery pack is abnormal, the battery management unit BMU will issue an alarm and take corresponding measures, such as inverter or charger shutdown, to prevent any battery pack damage or short circuit caused by fire and other risks.

How to Design the BMU and What Should You Pay Attention to When Designing?

Battery Types

Different types of batteries, such as lithium-ion, lead-acid, and nickel-metal hydride, have varying charging and discharging characteristics. Therefore, the design of the battery management unit must be adjusted according to the specific characteristics of the battery. Factors to consider include the rated voltage, charge/discharge rates, temperature range, and charge/discharge curves.

Data Acquisition System

Data acquisition includes single-cell voltage data, single-cell temperature data, and current data. These data serve as the foundation for BMS analysis and must be processed effectively to enable other function evaluations. Different data acquisition methods may have varying data update rates and precision, which should be planned during hardware design based on actual requirements, taking into account the capabilities of the hardware chips. Acquired data typically has some level of latency and requires validation through methods such as setting the sampling rate (typically 5-10 times the signal’s highest rate), debouncing, and filtering to eliminate outliers, ensuring a certain level of reliability.

Fault Detection and Protection

It is essential to establish corresponding fault measures based on relevant cell parameter values. Typically, different levels of fault severity are defined, such as Level 1 warnings, Level 2 power reduction, and Level 3 high-voltage cutoff. Specific faults are associated with unique fault codes and common fault determinations include:

  • High or low-temperature fault: Exceeding the temperature range within which the battery can operate safely over extended periods.
  • Excessive temperature differential warning: The temperature difference between individual cells exceeding the normal allowable range.
  • Extreme high or low-temperature faults: Reaching temperature levels outside the operational range where the battery cannot function.
  • High or low single-cell voltage fault: Reaching the critical voltage range for individual cell operation.
  • Extreme overvoltage or undervoltage fault: Exceeding the voltage range within which the battery can be safely reused.
  • Excessive total voltage warning: Reaching the critical total voltage range for battery use.
  • Overcurrent during charging or discharging fault: Exceeding the normal current limits for battery operation.
  • Communication faults: Include single-cell loss, acquisition chip loss, sub-control loss, and external link component loss. Differentiate warnings based on data loss conditions.
  • Relay sticking or failure to close: The relay not acting as per instructions.
  • High-voltage interlock: Ensuring proper connections of high-voltage electrical components.
  • Insulation detection: Low battery insulation levels with a risk of leakage.
  • Fast-charging socket high-temperature warning: Socket temperature exceeding the warning threshold.

By addressing different cell parameter values and potential fault scenarios, the establishment of corresponding fault measures ensures the safe operation of the battery system in various conditions while mitigating potential risks.

Thermal Management

Battery thermal management involves thermal simulation, creating a thermal-electric coupled model, and analyzing the thermal management of the entire vehicle battery pack under various driving cycles and design conditions. The BMS acts as the specific control component for managing battery temperatures. It includes cooling and heating measures, with temperature thresholds for activating and deactivating devices, additional protective measures, and adjustment times to regulate the battery pack’s temperature. The corresponding measures include:

  • Cooling: Liquid cooling, air conditioning.
  • Heating: PTC (Positive Temperature Coefficient) heating elements, ACB (Air Circuit Breaker) heating.

Balancing Management

Balancing management ensures the uniformity of individual cells in the battery pack, keeping cell voltage differentials within a safe range. This can be achieved through active or passive balancing. Effective balancing control circuits help redistribute charge among cells, prolonging the battery pack’s lifespan and enhancing overall system performance.

Communication Interfaces

The battery management unit needs to communicate with the BMS system for remote monitoring and control of the battery pack. This communication involves various protocols, such as CAN bus, RS485, Modbus, and different interfaces to ensure fast and reliable data transmission.

Data Storage

Based on client requirements, specific data storage design is necessary, typically including:

  • Historical data storage: Serving specific functions, this provides temporary storage for historical data. If a TBOX (Telematics Control Unit) is used, certain data that needs to be retained can be transmitted to the TBOX for storage, facilitating analysis of battery usage history and supporting data for algorithms.
  • Shutdown data storage: When the BMS is powered down, it stores crucial information such as the current SOC and high-level faults.

How to Design the BMU and What Should You Pay Attention to When Designing?

Highlights of MOKOEnergys Battery Management Unit Design

The subordinate BMU, responsible for the individual cell management layer in the BMS, is composed of battery monitoring chips and their associated circuits. It is responsible for collecting various types of information from individual cells, calculating and analyzing the battery’s SOC and SOH, actively balancing individual cells, and uploading abnormal cell information to the master controller.

When constructing a battery BMU solution, MOKOEnergy possesses the following features:

Single-cell voltage detection: Sampling accuracy of ±2.6mV, with a maximum collection series of 14S for each AFE (Analog Front End).

Temperature detection: NTC temperature is directly acquired by AFE, enhancing reliability. It features a 16-bit ADC sampling resolution, ensuring temperature sampling accuracy. Each AFE supports 7 temperature detection points.

Communication: BMU supports bidirectional loop daisy-chain communication with a maximum communication rate of 2.66M. It also supports CRC verification, ensuring effective communication and robustness.

Online balancing: BMU utilizes resistive energy-consuming balancing with a maximum balancing current of 200mA. Odd and even channels can be simultaneously activated for balancing, improving balancing efficiency.

Diagnostics: BMU supports hardware diagnostics for line disconnection, balancing faults, and communication issues.

Secure interface design: BMU supports hot-plugging, surge protection, and ESD protection. The connector design includes a physical keying mechanism to prevent incorrect cable connections. The connector’s flame-retardant rating is UL94V-0 to minimize fire hazards associated with the BMU.

Due to MOKOEnergy’s meticulous design of various components in our BMS, our products have been recognized by customers from multiple countries. In addition to the hot electric vehicle market in recent years, our BMS is also widely used in energy storage systems, renewable energy systems, portable devices, and other applications. In the future, with the joint efforts of our 70 R&D staff, we will still follow the pace of the times and continue to innovate.


BMUs monitor, control, and protect battery packs, ensuring safety and performance. Their design focuses on data acquisition, fault protection, thermal management, equalization, and data storage.

MOKOEnergy’s BMS has several highlights in its BMU design that provide strong support for the performance of the BMS. As electric vehicles and renewable energy systems become more popular, the importance of BMUs will continue to be emphasized, and MOKOEnergy continues to innovate and improve its BMU technology, so please feel free to contact us with any questions you may have.


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