Power Battery Management System Hardware Design Technology

Electric vehicles (EVs) are vehicles that are fully or partially driven by electric motors. Currently, there are three main types of electric vehicles: pure electric vehicles, hybrid electric vehicles, and fuel cell vehicles. The common power sources used for electric vehicles today are lead-acid batteries, lithium batteries, and nickel-hydrogen batteries.

Lithium batteries have high individual cell voltage, high specific energy, and high energy density, making them the batteries with the highest specific energy available today. However, because of their high energy density, lithium batteries can cause safety accidents if misused or abused. A Battery Management System (BMS) can address this issue. When the battery is overcharged or undercharged, the management system can automatically disconnect the charging and discharging circuits, and its balancing function ensures that the voltage difference between individual cells is maintained within a small range. Additionally, it provides protection against over-temperature, over-current, and includes remaining battery capacity estimation functions. This article introduces the design of a microcontroller-based battery management system.

1. Hardware Structure of the Battery Management System
   The hardware circuits of the system can be divided into three main modules: MCU module, detection module, and balancing module.

1.1 MCU Module
The MCU is the core of the system control. The MCU used in this design is from the M68HC08 series, model GZ16. This series of MCUs adopts the enhanced M68HC08 central processing unit (CP08). The MCU has the following features:

8 MHz internal bus frequency
16 KB built-in FLASH memory
2 16-bit timer interface modules

Clock generator supporting 1 MHz to 8 MHz crystal oscillators
Enhanced Serial Communication Interface (ESCI) module
1.2 Detection Module
The detection module includes voltage detection, current detection, and temperature detection.

1.2.1 Voltage Detection Module
In this system, the microcontroller detects both the total voltage of the battery pack and the voltage of individual cells. There are two methods for detecting the total voltage of the battery pack:

Using a dedicated voltage detection module, such as a Hall voltage sensor.
Using precision resistors to construct a voltage divider circuit.
Because using a dedicated voltage detection module is costly and requires a specific power supply, a voltage divider circuit is used. For a 10-series lithium manganese battery pack, the voltage range is 28 V to 42 V. Using a 3.9 MΩ and 300 kΩ resistor for voltage division, the voltage signal range becomes 2 V to 3 V, corresponding to AD conversion results of 409 and 1023.

For individual cell voltage detection, flying capacitor technology is mainly used. The flying capacitor schematic diagram is shown in Figure 1, where a protection circuit for the last 4 cells in the battery pack is illustrated. A four-channel switch array allows for the voltage of any of the last four cells to be captured by the microcontroller. The microcontroller outputs a drive signal to control the MOSFET’s on/off state, thus protecting the charging and discharging of the battery pack.

1.2.2 Current Sampling Circuit
In current sampling, the parameters of the battery are important for overcurrent protection. The current sampling circuit is shown in Figure 2. During battery discharge, copper wire is used to detect the current signal. The voltage signal detected is amplified by a differential amplifier and converted into a 0 to 5 V signal sent to the microcontroller. If the discharge current exceeds a certain limit, the voltage detected by the microcontroller will trigger the transistor to act, changing the gate voltage of the MOSFET to cut off the discharge circuit. For example, for a 36 V lithium manganese battery, the overcurrent protection is set to 60 A. The resistance of the copper wire is approximately 5 mΩ, and when the current reaches 60 A, the voltage across the copper wire is around 300 mV. To improve accuracy, the voltage is amplified by 10 times before being sent to the microcontroller for detection.

1.2.3 Temperature Detection
During charging and discharging, a portion of the energy is released as heat. If this heat is not dissipated promptly, it could cause the battery pack to overheat. If a single nickel-hydrogen battery exceeds 55°C, its properties will deteriorate, causing imbalances in the charging and discharging of the battery pack and potentially resulting in permanent damage or explosion. To prevent this, it is necessary to monitor the temperature of the battery pack in real time and manage heat dissipation.

A thermistor is used as the temperature sensor for temperature sampling. A thermistor is a temperature-sensitive semiconductor resistor whose resistance decreases as temperature increases. The temperature-resistance characteristic of the thermistor can be approximated by the following formula:

1.3 Balancing Module
Common balancing methods for battery packs include shunt methods, flying capacitor balancing, and inductive energy transfer methods. In this system, a charge balancing method involving full charging to single cell charging is used. The schematic diagram is shown in Figure 3. Q4 controls the switch for full charging of the battery pack, and Q2, Q3, and Q5 control the switches for charging individual cells. For a 10-cell lithium manganese battery pack, the voltage across the primary transformer coil is 42 V, and the secondary coil voltage corresponds to the rated voltage of the battery (4.2 V). Initially, Q4 is turned on, and Q2, Q3, and Q5 are turned off. The voltage of individual cells increases, and once a cell reaches its rated voltage of 4.2 V, the voltage detection chip sends a drive signal to close Q4 and open Q2, Q3, and Q5, entering the single-cell charging stage. Uncharged cells continue charging to reach their rated voltage, while fully charged cells maintain their rated voltage.

2. SOC Capacity Detection
   In lithium-ion battery management systems, common methods for SOC (State of Charge) calculation include open-circuit voltage method, coulomb counting method, impedance measurement method, and combined lookup table method.

Open-circuit Voltage Method: This is the simplest method, where the SOC is determined by the open-circuit voltage of the battery, which has a known relationship with the remaining capacity.

Coulomb Counting Method: This method measures the charging and discharging current of the battery, integrates the product of current and time to calculate the total charge added or removed, and estimates the SOC based on the total energy input and output.

Impedance Measurement Method: This method uses the battery’s internal resistance, which has a linear relationship with SOC, to estimate the SOC by measuring the battery’s voltage and current parameters.

Combined Lookup Table Method: In this method, the remaining SOC is calculated based on a lookup table that correlates the battery’s voltage, current, and temperature.

In this design, considering the circuit integration, cost, and MCU performance, the coulomb counting method is chosen. The total charge released by the battery as it discharges from 42 V to 32 V can be represented as C. The ratio of charge released by current i over time t is represented by η.

The remaining charge (CRM) is calculated by integrating the discharge or charge amounts, and updating the value of CRM over time based on the current.

3. Experimental Results
   The battery management system was used to conduct charging and discharging tests on a lithium manganese battery pack. Figure 4(a) shows the discharge test with a current of 8 A. When the battery voltage drops to 32 V, the discharge MOSFET is turned off. Figure 4(b) shows the charging test, and after 4 hours, balancing is completed.

This battery management system, based on the M68HC08GZ16 microcontroller, successfully collects signals for the individual voltages, current, and temperature of the battery pack. After balancing the charging capacity, the voltage difference between individual cells does not exceed 50 mV. The overall system operates well and meets the needs of electric vehicle power battery pack applications.