How to meet current and voltage measurement requirements for BMS battery packs

With the accelerated transition from non-renewable to renewable energy sources, chemical energy storage is becoming an important energy storage device, the most important of which are batteries, whose uses include harvesting from solar panels and wind turbines energy, and storing electricity in electric vehicles (EVs).

Author of this article: Texas Instruments Krunal Maniar, compiled from Allaboutcircuit

With the accelerated transition from non-renewable to renewable energy sources, chemical energy storage is becoming an important energy storage device, the most important of which are batteries, whose uses include harvesting from solar panels and wind turbines energy, and storing electricity in electric vehicles (EVs).

As battery technology continues to evolve and the power and energy densities of battery manufacturing increase, it is equally important to improve the performance of battery management systems. The BMS (shown in the block diagram in Figure 1) is responsible for making the battery pack safe, reliable, and cost-effective, while providing an accurate estimate of its state.

Figure 1: Typical BMS block diagram

Typically, a BMS performs the following functions:

• Cell Balancing: The individual battery cells need to be monitored and balanced to redistribute charge among the cells during charge and discharge cycles.

• Temperature monitoring: Individual cell and battery pack temperatures need to be measured at multiple locations to ensure safe operation at peak efficiency.

• State of Charge (SoC) and State of Health (SoH) estimation: In addition to individual cell voltage measurements, accurate current and voltage measurements of the entire battery pack allow the BMS to accurately estimate the SoC and SoH of the battery pack. Accurate estimates are important for improving battery efficiency and safety. In an electric vehicle, the SoC and SoH of the battery pack calculate the exact driving range and determine the charge and discharge curves of the battery pack.

• Isolation Monitoring: This safety-critical function checks the resistance between the high-voltage bus and the chassis to ensure adequate isolation between the two.

• Contactor Control: The BMS algorithm controls the pre-charge and safety contactors to detect any faults external or internal to the battery pack.

In this article, we will look at the requirements for battery pack current measurement and analog-to-digital converters in a BMS.

Understanding BMS Battery Pack Current Measurement Requirements

As shown in Figure 2, a battery pack typically has two modes of operation: charge mode and discharge mode.

Figure 2: Operational Modes in BMS

In charge mode, the charging circuit charges the battery pack; current flows into its HV+ terminal.

In discharge mode, the battery pack powers an external load and current flows out of its HV+ terminal.

For example, in an electric vehicle, the battery pack powers the electric motor, which converts electrical energy into mechanical energy and drives the car.

Typically, the BMS measures bidirectional battery pack current in charge mode and discharge mode. A method called coulomb counting uses these measured currents to calculate the SoC and SoH of the battery pack. The current magnitudes during charge and discharge modes may differ by one or two orders of magnitude.

For example, charging currents in electric vehicles typically range from 0 A to 100 A, while discharge currents can peak at 2,000 A.

Table 1 shows the typical accuracy requirements for bidirectional battery pack current sensing in an EV BMS.

Table 1: Battery Pack Current Measurement Requirements in EV BMS

On the other hand, shunt-based current measurement is the preferred solution to achieve the level of accuracy over such a wide current range. Closed-loop Hall modules may be an alternative, but they are very expensive compared to shunt-based solutions.

Current measurements based on low-side shunts are often used to monitor the charge and discharge currents of battery packs in BMSs. However, one of the challenges of shunt-based measurements is how to deal with heat dissipation across the shunt. As shunt technology has improved, shunts now have smaller resistance values ​​to minimize heat dissipation and provide very high accuracy along with excellent over temperature and lifetime drift performance.

For EV BMS battery pack current measurements, the shunt has a range of 25 µΩ to 100 µΩ.

Understanding ADC Requirements in BMS

One of the most established ways to achieve high-accuracy shunt current measurements with a wide dynamic range is to use a high-resolution delta-sigma ADC.

As shown in Figure 3, a typical implementation consists of a ΔΣ ADC with at least 24-bit resolution, followed by a digital isolator.

Figure 3: Shunt-based current measurement in BMS

A shunt is usually placed on the HV- terminal of the battery pack, and the ADC measures the shunt current referenced to the same HV- terminal. Since the resistance value of the shunt is very low, the voltage drop across the shunt is very small. Therefore, the ADC can measure bidirectional voltage drop with high accuracy and high dynamic range.

Table 2 lists the ADC performance requirements for current measurements.

Table 2: ADC Requirements in EV BMS

Because shunts drift with temperature, designers typically place a thermistor near the shunt to measure the shunt temperature and compensate for temperature changes that can cause inaccurate current measurements. In addition to measuring battery pack current, accurate voltage measurements on the battery pack are also important for accurate SoC and SoH estimation. For this measurement, a resistor divider network scales down the high voltage at the HV+ terminal.

Figure 4 shows the technical implementation of a typical BMS application circuit using the Texas Instruments (TI) ADS131B04-Q1, a 24-bit, quad-channel, simultaneous sampling delta-sigma ADC.

Figure 4: Using the ADS131B04-Q1 in a BMS

The HVC terminal is used as the ground reference for the high voltage side of the BMS. Therefore, the AGND and DGND pins of the ADS131B04-Q1 and the low side of the shunt, thermistor, and resistor divider network are connected to the HVC terminal. One side of the thermistor and the side of the resistor at the bottom of the resistor divider network are also connected to the same HVC terminal.

With an integrated low-drift reference, low-noise programmable gain amplifier, special global chopper offset cancellation, and the front-end required to measure bidirectional current, the ADS131B04-Q1 enables high-performance battery measurements on a single chip, including:

• High-resolution and accurate battery pack current using low-side current shunt resistors.
• Battery pack voltage using a high voltage resistor divider.
• Shunt temperature, use a thermistor.
• Auxiliary measurements for diagnostic purposes, eg mains voltage.

As the need for batteries to store energy continues to increase, the need for accurate battery pack current, voltage and temperature measurements becomes even more important. The ADC’s low offset and temperature-dependent gain error compensation and low noise allow the BMS to monitor and control the battery pack more efficiently, improving system safety and reliability.

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