Maximize EV Battery Longevity with Integrated Resistor Dividers

What you’ll learn:

• Integrated high-voltage resistor dividers are shown to offer a more precise and space-efficient approach to voltage attenuation compared to discrete resistor chains.
• On-chip ratiometric voltage dividers can hold a maximum lifetime ratio of drift of ±0.2% specified over a 10-year lifetime
• Combined with a high-precision amplifier, dividers with matched ratios can produce a difference amplifier with an exceptionally high common-mode rejection ratio, which helps reduce noise and other errors.

In modern electric vehicles (EVs) and hybrid electric vehicles (HEVs), the battery-management system (BMS) serves as the brains behind the battery pack, responsible for ensuring that the battery performs well, operates safely, and lasts a long time. The BMS tracks parameters such as state of charge (SOC), which indicates how much energy is available, and state of health (SOH), which assesses the overall condition and aging of the battery cells. Monitoring these metrics helps maintain efficient energy usage and prevents batteries from degrading prematurely.

To meet regulations around battery efficiency and environmental sustainability, automakers must maintain high levels of battery health throughout a vehicle’s life. For example, the California Air Resources Board introduced standards mandating that EVs maintain at least 80% of their electric range for 10 years or 150,000 miles by model year 2030.

That’s a culmination of lesser requirements set to take effect as early as model year 2026, with stipulations to keep tightening the regulation after model year 2031. Similar standards are already in effect around the world, necessitating higher battery voltages and more advanced sensing within the BMS to improve accuracy.

In this article, integrated high-voltage resistor dividers are shown to offer a more precise and space-efficient approach to voltage attenuation compared to discrete resistor chains. This approach enables the BMS to better balance the battery pack and improve its lifetime. Figure 1 illustrates the battery cell’s measurement divider resistors typically used in an EV.

Application ABCs

The typical EV battery voltage is ≥400 V, with the industry trending toward higher voltages of 1 kV or more. This trend arose because EV battery chargers that output greater power can charge the vehicle faster, and power is a function of both current and voltage: P = IV. To reach a particular charging speed, this voltage/current relationship can be used to increase the voltage to minimize or hold flat current values while still delivering the necessary power.

Because heat degrades the lifespan of batteries and electronic components, minimizing current values is an advantage in EV designs, since increased current results in greater heat dissipation within the electric powertrain. In addition, less current makes it possible to use a lighter gauge of wire, reducing overall wiring harness weight and resulting in a vehicle that can travel further on the same charge.

Measuring voltage directly from the battery and communicating it to relevant digital vehicle systems requires signal conversion with an analog-to-digital converter (ADC), which is typically powered by a voltage around 5 V. Because an input signal >5 V could damage the ADC, protecting it and other low-voltage components from the relatively large voltage of the battery requires a device such as an isolated amplifier to maintain a barrier between the high- and low-voltage domains.

Despite being a bridge between two voltage domains, isolated amplifiers can only accept a voltage range similar to the ADC, necessitating attenuation of the battery voltage before reaching the isolated amplifier. A resistor divider is commonly used for this purpose, reducing the high-voltage signal into a lower-voltage full-scale range.

Figure 2 is a circuit diagram for DC bus measurement designed using long strings of resistors to attenuate the battery voltage to an acceptable level.

Disadvantages of Discrete Resistor Chains

When dealing with voltages greater than 400 V, creepage and clearance distances must be considered to prevent electrical arcing and ensure safe insulation. Although a traditional resistor divider requires only two resistors, when it comes to creepage and clearance, high-voltage attenuation often features long chains of resistors to increase the physical distance between a high- and low-voltage node.

Per International Electrotechnical Commission (IEC) 60115-8, the maximum sustained voltage drop across each resistor is limited; typically, 200 V for each 1206 case-size surface-mount resistor and 150 V for each 0805 case-size resistor. Since the voltage of a battery varies both above and below its rated value, excess numbers of resistors are used as a precaution, often resulting in chains of 10 or more discrete resistors.

This design method does include some drawbacks. Even with precision resistors, variations in the inherent tolerance of each discrete resistor can lead to significant discrepancies in the voltage division ratio, resulting in inaccurate voltage measurements.

Discrete resistors are also susceptible to changes in resistance resulting from temperature fluctuations and aging. The solder points on either end of such resistors are exposed, potentially causing additional leakage and parasitic capacitance or inductance unless a conformal coating or other form of protection is utilized, which would increase solution costs.

These effects can compound in a long chain of discrete resistors, further degrading voltage-sensing accuracy over time. This may cause SOC and SOH estimation errors, leading to suboptimal battery-management decisions such as incorrect charging and discharging cycles, and ultimately shortening battery life and weakening the EV’s range.

Performance and Reliability Advantages of Integration

Using modern semiconductor manufacturing process technology, it’s possible to build up a resistive layer of thin-film silicon chromium on a standard wafer of silicon substrate. This resistive layer is encapsulated by a silicon-dioxide insulative layer, enabling usage at extremely high voltages >1 kV.

Packaging an individual piece of this wafer, known as the die, into a standard plastic integrated-circuit (IC) housing will protect the die from external stresses. Because creepage and clearance are measured across the gaps between pins, manufacturers can place similarly sized die into smaller packages for lower-cost products, or into larger packages for increased distances based on design specifications.

This methodology provides significant advantages in terms of performance and reliability because the relative resistances for each wafer section that becomes a die are very tightly matched. Specified maximum limits for initial ratio and over-time tolerance help ensure that the voltage-division ratio remains accurate, despite the effects of aging or environmental changes such as temperature shifts.

For example, by leveraging this technology, the Texas Instruments (TI) RES60A-Q1 resistor divider has a maximum lifetime ratio of drift ±0.2% specified over a 10-year lifetime. Such reliability is important for applications where consistent performance is a priority.

The IC-packaged design eliminates the need for lengthy chains of discrete resistors, reducing the required printed-circuit-board footprint. This consolidation not only simplifies circuit layout, but it also lowers assembly costs related to component count. Fewer exposed nodes reduce the likelihood of errors from leakage or parasitics, eliminating the need for conformal coating, which also potentially knocks down costs.

Figure 3 is a circuit diagram for DC bus measurement in which the TI RES60A-Q1, RES11A-Q1, and AMC1311B-Q1 provide a way to measure the voltage that crosses the isolation barrier, achieving a full-scale range error <1%.

Differential- to Signal-Ended Conversion

Isolated amplifiers such as TI’s AMC1311B-Q1 with differential outputs are popular because differential outputs are ideal to carry signals across longer distances without degradation, and designers will often place their low-voltage components away from high-voltage sources for safety reasons. Feeding this signal into a single-ended ADC requires differential- to single-ended conversion through the addition of an integrated difference amplifier, or four discrete resistors configured around an amplifier as two resistor dividers.

For the same reasons that a discrete resistor divider may introduce error during attenuation, individual resistors can also introduce ratio drift in a discrete difference amplifier implementation. Integrated resistor networks consisting of two resistor dividers on the same die can also fit into plastic IC packaging. This provides protection from stresses and ensures that the two included dividers also have a tight tolerance between their ratios.

In conjunction with a high-precision amplifier, dividers with matched ratios can produce a difference amplifier with an exceptionally high common-mode rejection ratio, which helps reduce noise and other errors.

Integrated Solutions Lead to Extended Battery Health

Transitioning from discrete resistor chains to an integrated solution offers numerous advantages when designing high-voltage attenuation circuits for a BMS. When coupled with complementary integrated components for differential signal conversion, these devices enable electric powertrains with higher voltages and accurate SOC and SOH readings. The end result is that EVs are able to maintain battery health over extended periods.