The Battery Management System (BMS) monitors several key parameters in real time, such as State of Charge (SOC) and State of Health (SOH). SOC accurately reflects the remaining usable energy of the battery, while SOH assesses the overall condition and aging level of the battery cells. Precise monitoring of these indicators is crucial for maintaining efficient energy utilization and delaying premature battery aging.
To meet regulatory requirements for battery efficiency and environmental sustainability, automakers must strive to maintain optimal battery health throughout the entire vehicle lifecycle. For example, the California Air Resources Board has established numerous standards that clearly define this requirement.
The regulations stipulate that electric vehicles must maintain at least 80% of their driving range within 10 years (150,000 miles for models before 2030). This requirement will be implemented gradually starting with the 2026 model year and will remain strictly enforced after the 2031 model year. With similar standards being implemented globally, there is an urgent need to utilize more advanced integrated solutions in the Battery Management System (BMS) to improve detection accuracy. This article will delve into how integrated high-voltage resistor dividers, compared to discrete resistor chains, help the BMS better balance the battery pack and effectively extend battery life through more precise and space-saving voltage decay methods.
Operating voltage and signal conversion requirements of electric vehicle battery systems
Typical electric vehicle battery voltages are usually ≥400V, and the industry is moving towards 1kV or even higher voltages. Higher voltage batteries offer significant advantages, such as reduced maximum current requirements, thus maximizing energy efficiency. However, measuring such high voltages and transmitting them to the vehicle's systems requires signal conversion using an analog-to-digital converter (ADC). Generally, ADCs are powered by approximately 5V and cannot accept input signals exceeding this voltage.
To protect the ADC and other low-voltage components from the impact of high battery voltages, isolation amplifiers and other devices must be used to maintain electrical isolation between the high-voltage and low-voltage domains. Although the isolation amplifier acts as a bridge between the two voltage domains, its acceptable voltage range is similar to that of the ADC. Therefore, the battery voltage needs to be attenuated before the signal reaches the isolation amplifier. Typically, a resistor divider is used in this stage, which reduces the high-voltage signal to the lower voltage range within its full-scale range to meet the processing requirements of subsequent circuits.
Disadvantages of traditional discrete resistor chains
When handling high voltages greater than 400V, creepage distance and clearance are critical considerations to prevent arcing and ensure insulation safety. Traditional resistive voltage dividers theoretically require only two resistors, but to meet creepage and clearance requirements, long chains of resistors are often used for high-voltage attenuation, increasing the physical distance between high-voltage and low-voltage nodes. According to the IEC 60115-8 standard, there are limits on the maximum sustained voltage drop across each resistor; for example, a surface-mount resistor in a 1206 case is typically 200V, and one in a 0805 case is 150V. Because the actual battery voltage fluctuates around the rated value, more resistors are often used for safety reasons, typically forming long chains of 10 or more discrete resistors.
This design approach has numerous drawbacks. Even when using precision resistors, the inherent tolerance variations of each discrete resistor can lead to significant differences in the voltage division ratio, resulting in inaccurate voltage measurements. Discrete resistors are also highly susceptible to temperature variations and aging, causing changes in resistance values. Furthermore, the exposed solder joints at both ends of these resistors, without conformal coatings or other protective measures, are highly likely to generate additional leakage current and parasitic capacitance or inductance, undoubtedly increasing the cost of the solution. In long-chain discrete resistors, these adverse effects are further compounded. Over time, voltage detection accuracy continuously decreases, eventually leading to errors in the estimation of state of charge and battery health, resulting in flawed battery management decisions, such as inappropriate charge and discharge cycles, thereby shortening battery life and reducing the driving range of electric vehicles.
Advantages of integrated resistor voltage dividers
Meets high voltage processing requirements
Texas Instruments' RES60A-Q1 integrated resistor divider features a carefully designed wide-body SOIC package that fully complies with the creepage distance and clearance standards defined by the International Electrotechnical Commission (IEC) standard 61010. It can easily handle voltages up to 1.7kV, fully meeting the application requirements of high-voltage systems in electric vehicles.
Excellent performance and reliability
This device excels in both performance and reliability. By setting maximum limits for the initial ratio and timeout tolerance, it effectively ensures the accuracy of the voltage divider ratio even in the face of environmental factors such as aging or temperature variations. For example, the Texas Instruments RES60A-Q1 resistor divider specifies a maximum lifetime ratio of ±0.2% over its 10-year lifespan. This high reliability is crucial for electric vehicle applications with stringent performance consistency requirements.
Save space and cost
The integrated circuit packaging design cleverly eliminates lengthy discrete resistor chains, significantly reducing the size of the required printed circuit board. This integration not only simplifies complex circuit layouts but also reduces assembly costs associated with component count. Simultaneously, fewer exposed nodes significantly reduce the likelihood of errors due to leakage or parasitic effects, eliminating the need for additional conformal coatings and further lowering costs.
Advantages of differential signal conversion
Isolated amplifiers with differential outputs are highly favored because differential outputs perform exceptionally well in long-distance signal transmission, and for safety reasons, designers typically place low-voltage components away from high-voltage sources. Feeding such differential signals into a single-ended ADC requires differential-to-single-ended conversion. Previously, this required adding an integrated differential amplifier or configuring four discrete resistors around the amplifier. Now, by combining integrated resistors (such as the RES11A-Q1) with a high-precision amplifier (such as the OPA388-Q1), it is possible to construct a differential amplifier with a high common-mode rejection ratio, effectively reducing noise interference and minimizing other sources of error.
When designing high-voltage attenuation circuits for battery management systems (BMS), the shift from traditional discrete resistor chains to integrated resistor divider solutions such as the RES60A-Q1 offers significant advantages. When used in conjunction with auxiliary components like the RES11A-Q1 for differential signal conversion, these integrated devices work synergistically to help electric vehicles maintain optimal battery condition over extended periods, improving battery system performance and providing a solid guarantee for the efficient and reliable operation of electric vehicles. This drives the electric vehicle industry towards higher quality and more environmentally friendly development.
