Addressing BMS Battery Pack Current and Voltage Measurement Requirements

As the transition from nonrenewable to renewable energy sources accelerates, batteries are becoming a prominent energy storage device. Their use spans harvesting energy from solar panels and wind turbines to storing power in electric vehicles (EVs).

As battery technology continues to evolve and cells are manufactured with higher power and energy densities, it is equally important to improve the performance of the battery management system. A BMS, seen in a block diagram in Figure 1, is responsible for making a battery pack safe, reliable, and cost-efficient while providing accurate estimations about its status.

 

Figure 1: Typical BMS block diagram

 

In general, a BMS performs these functions:

• Cell balancing: The individual battery pack cells need to be monitored and balanced to redistribute charge between cells during charging and discharging cycles.
• Temperature monitoring: The individual cell temperatures and battery pack temperatures at several locations need measuring to ensure safe operation with maximum efficiency.
• State-of-charge (SoC) and state-of-health (SoH) estimations: In addition to individual cell voltage measurements, accurate current and voltage measurements of the entire battery pack enable the BMS to accurately estimate the battery pack’s SoC and SoH. Accurate estimation is important to improve battery efficiency and safety. In EVs, the SoC and SoH of a battery pack compute the exact driving range and dictate the battery pack’s charging and discharging profiles.
• Isolation monitoring: This safety-critical function checks the resistance between the high-voltage bus lines and chassis to ensure that there is sufficient isolation between the two.
• Contactor control: A BMS algorithm controls pre-charge and safety contactors that detect any fault outside or within the battery pack. 

In this article, we'll learn about the requirements for battery pack current measurement and analog-to-digital converters within BMSs. 

 

Understanding BMS Battery Pack Current Measurement Requirements

A battery pack, as shown in Figure 2, typically has two operating modes: charging mode and discharging mode.

 

Figure 2: Operating modes in a BMS

 

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

In discharging mode, the battery pack provides power to an external load.

For example, in EVs, the battery pack provides power to the electric motor, which converts the electrical energy to mechanical energy and propels the automobile. Therefore, in discharging mode, current flows in the opposite direction from charging mode, out of the HV+ terminal.

Generally, a BMS measures bidirectional battery pack current both in charging mode and discharging mode. A method called Coulomb counting uses these measured currents to calculate the SoC and SoH of the battery pack. The magnitude of currents during charging and discharging modes could be drastically different by one or two orders of magnitude.

As an example, the charge current in EVs has a typical range of 0 A to 100 A, whereas the discharge current can peak at 2,000 A.

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

 

Table 1: Battery pack current-measurement requirements in EV BMSs

 

Shunt-based current measurements, on the other hand, are the preferred option to achieve accuracy levels across such a wide current range. Closed-loop Hall modules could be an alternative, but they are very expensive compared to shunt-based solutions.

Low-side shunt-based current measurements are common for monitoring a battery pack’s charge and discharge currents in a BMS. However, one of the challenges of shunt-based measurements is how to handle thermal dissipation across the shunt. With improvements in shunt technology, the shunts now come with much smaller resistance values to minimize thermal dissipation and offer very high accuracy with excellent over-temperature and lifetime drift performance.

For EV BMS battery pack current measurements, shunts range anywhere from 25 µΩ to 100 µΩ.

 

Understanding ADC requirements in BMSs

One of the most established ways to accomplish highly accurate shunt-based current measurements with a wide dynamic range is to use a high-resolution delta-sigma (ΔΣ) ADC.

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

 

Figure 3: Shunt-based current measurement in BMSs

 

A shunt is typically placed on the battery pack’s HV– terminal, with the ADC measuring the shunt current referenced to this same HV– terminal. Since the shunt has a very low resistance value, the voltage drop across the shunt is very small. Therefore, the ADC should be able to measure small bidirectional voltage drops at high accuracy and dynamic range.

Table 2 lists ADC performance requirements for current measurements.

 

Table 2: ADC requirements in EV BMSs

 

Because the shunts drift over temperature, designers often place a thermistor close to the shunt to measure the shunt temperature and compensate for temperature variations that would result in inaccurate current measurements. In addition to measuring the battery pack current, taking accurate voltage measurements of the battery pack is also important for accurate SoC and SoH estimations. 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, four-channel, simultaneous sampling ΔΣ ADC.

 

Figure 4: Using the ADS131B04-Q1 in a BMS

 

The HV– terminal serves as a ground reference to the high-voltage side of the BMS. Therefore, the AGND and DGND pins of the ADS131B04-Q1, as well as the low side of the shunt, thermistor, and resistor-divider network, all connect to the HV– terminal. One side of the thermistor and one side of the bottom resistor of the resistor-divider network also connect to this same HV– terminal.

With an integrated low-drift reference, a low-noise programmable gain amplifier, a special global-chop offset removal feature, and the front end required to measure bidirectional currents, the ADS131B04-Q1 can provide a one-chip high-performance option to measure:

• Battery pack current with high resolution and accuracy, using a low-side current shunt resistor.
• Battery pack voltage, using a high-voltage resistor divider.
• Shunt temperature, using a thermistor.
• Auxiliary measurements, such as the supply voltage, for diagnostic purposes.

As demand for batteries to store energy continues to increase, the need for accurate battery pack current, voltage, and temperature measurements becomes even more important. The low offset and gain errors over temperature and low noise of ADCs enable BMSs to monitor and control battery packs more efficiently, resulting in improved system safety and reliability.

 

All images used courtesy of Texas Instruments

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