How innovations in battery management systems can increase electric vehicle adoption

BMS protects the battery from damage, extends battery life through intelligent charging and discharging algorithms, predicts the remaining battery life, and keeps the battery in a healthy state. Lithium-ion battery cells present huge challenges that require sophisticated electronic control systems to handle. In addition, there is a significant risk of injury from fire and explosion. Therefore, BMS requires advanced devices to meet all performance, safety and cost indicators.

Generally speaking, the three main BMS challenges that every designer strives to overcome are maximizing driving range, reducing costs, and enhancing safety.

Solving one of these challenges may have an adverse impact on another. In this white paper, we explore several emerging trends that are addressing all three challenges simultaneously.

Working Principle and Industry Trends of BMS

The distributed BMS architecture (Figure 1) has a modular structure and usually includes three main subsystems: cell monitoring unit (CSU), battery control unit (BCU) and battery disconnect unit (BDU).

Figure 1 Typical BMS architecture

These subsystems have different industry names, as listed in Table 1, so it is helpful to set a benchmark for the various names and acronyms.

Table 1 Common acronyms for BMS subsystems in the industry

The CSU collects parameter information of all battery cells by detecting the voltage and temperature of each cell. The CSU helps compensate for inconsistencies between battery cells by performing cell balancing. The BCU must contain parameter information from the CSU and must also detect the voltage and current of the battery pack to perform battery pack management. Based on all the voltage, current and temperature data collected, the BCU is responsible for allocating the battery charging and discharging method with reference to the overall condition of each battery cell. The condition of the battery is continuously monitored by calculating the state of charge, power state and operating status. Intelligent protection control is also an important function of the BCU, as it must perform insulation monitoring, control contactors in the event of a collision or short circuit, continuously monitor temperature sensors and perform diagnostics to check whether all input parameters are indeed valid. The information is transmitted to the vehicle control unit or electronic control unit via controller area network (CAN) communication.

New battery chemistries

Lithium-ion can refer to a range of chemistries; but it ultimately makes up a battery based on the charge and discharge reactions of a metal oxide cathode and a graphite anode. Two of the more common lithium-ion chemistries are nickel manganese cobalt (NMC) and lithium iron phosphate (LFP).

NMC is the dominant chemistry because of its superior energy density, which has a direct impact on driving range. However, as demand for nickel and cobalt has surged in recent years, automakers are adopting strategies to cope with market turmoil. Nickel and cobalt are also rare and difficult to extract from the earth.

While LFP is still a minority chemistry and has a lower energy density, it has significant advantages. LFP does not contain the expensive and rare elements nickel and cobalt, so the cost is lower. It also has a long life cycle, so the battery life is longer. LFP batteries are also more stable, less prone to fire, and require less protection than nickel and cobalt batteries.

As a result, LFP is likely to become the dominant chemistry in the high-volume automotive segment, where range is less important than affordability, safety, or environmental friendliness (no cobalt and nickel). LFP requires very precise battery monitoring technology because it has a very flat discharge curve. Read What’s Next for BMS? Safer, More Affordable Electric Vehicles to learn how advanced semiconductors can be used to enable BMS architectures for emerging battery chemistries.

Meanwhile, some suppliers are investigating how to use lower-cost sodium-ion cells to compete with LFP.

Unlike traditional lithium-ion batteries that use liquid electrolytes, solid-state batteries use solid electrolytes composed of glass, ceramics, solid polymers or sulfides, hence the name. Several automakers are conducting solid-state battery research due to the inherent performance advantages of solid-state batteries: higher energy density; higher reliability and anti-aging characteristics; significantly faster charging speeds and, most importantly, higher safety. Liquid electrolytes become flammable at high temperatures. Solid electrolytes have higher thermal stability, which in turn limits the risk of fire or explosion.

Wireless BMS

Utilizing wires is the de facto method for deploying BMS today. In many cases, this is the most reliable way to achieve Automotive Safety Integrity Level D (ASIL D) compliance because functional safety features are built into the daisy chain wired communication protocol. However, wires also have their disadvantages: cable failures, warranty repairs, and battery cell replacements are costly.

One advantage of a wireless BMS (as shown in Figure 2) is the ease of battery pack assembly and production, which can save costs and improve production efficiency. Production line technicians only need to assemble the battery pack and get instant readings, while a wired BMS requires technicians to plug cables into each battery module.

Figure 2 TI wireless BMS technology

Another benefit of a wireless BMS is that cable harnesses and connectors can be one of the leading causes of battery pack failure. A wireless BMS reduces low voltage wiring and can potentially save the original equipment manufacturer (OEM) from significant warranty claims.

Wireless BMS helps reduce weight and more importantly, there is now more space in the battery pack. The increased space means that the battery manufacturer or OEM can add more battery cells to the battery pack. The increased number of cells and reduced weight will increase the driving range.

Wireless BMS can also help save component costs through its inherent isolation, so automakers don’t have to use transformers, capacitors or common-mode chokes to achieve isolation, saving costs.

TI's automotive-qualified CC2662R-Q1 SimpleLink™ wireless microcontroller (MCU) includes a 48MHz Arm® Cortex®-M4 processor capable of running a 2.4GHz proprietary wireless BMS protocol.

Advanced estimation of battery capacity and battery health

An accurate estimate of the remaining charge in the battery has a direct impact on the remaining range. Although battery cell manufacturers provide a rated capacity for the battery, it changes over time. Some of the important factors that contribute to battery capacity decay include temperature rise, cycling (usage), depth of discharge mode, and aging. Given these factors, a continuous estimate of battery capacity is required in order to accurately estimate the state of charge.

Accurately measuring the battery’s health will determine whether the driver must replace the battery or wait until a clear, dangerous battery failure event occurs.

Effective synchronization of voltage and current enables accurate state-of-charge, health, and electrical impedance spectroscopy (EIS) calculations to get the most out of the battery. For more information, see the technical article: How to Design a Smart Battery Junction Box for Advanced Electric Vehicle Battery Management Systems .

Cell Monitoring Unit (CSU) Details

A simplified version of the CSU is shown in Figure 3. The CSU operates closely within the actual cells of the battery pack, connecting to the wiring harness for the cell monitoring devices and ensuring that important battery pack data is efficiently transmitted back to the host BCU.

Figure 3 Simplified CSU system block diagram

Without a CSU, there is very little information available about the state of the battery pack. The diagnostic data output by the CSU enables health and state-of-charge estimates, which directly impact the safety objectives of the system. With high-precision monitors, these algorithms can provide drivers with very accurate estimates and get the most out of each charge. This is usually done passively and at high enough currents that thermal management becomes difficult to maintain and measure. Overall, deploying a sophisticated CSU in the battery pack determines the vehicle's charging cycle, providing a safer and better overall experience.

CSUs provide increasingly detailed cell state measurements, maximizing the aforementioned battery pack benefits. Safely and reliably synchronizing these measurements at the highest possible data rates enables ideal estimates for health and state of charge calculations. With the trend toward higher voltage battery packs above 400V, smart CSU designs facilitate the transmission of more and more cell data in the battery pack. The challenge to achieve more affordable HEV/ EVs is how to achieve these benefits with the lowest possible power consumption and external PCB components.

As LFP becomes more popular, smooth discharge curves require more accurate cell voltage measurement data to determine the usable range of electric vehicles compared to NMC (as shown in Figure 4). The Texas Instruments (TI) BQ79718-Q1 stackable battery monitor and cell balancer can measure 18 series-connected batteries. It provides cell voltage measurement with an accuracy of ±1mV and passive cell balancing with 300mA current capability. The device also supports simultaneous voltage and current measurement and the BQ79731-Q1 battery monitor for more accurate health and state of charge calculations.

Figure 4. Battery chemistry discharge curves (red = NMC, blue = LFP)

Comparison of Traditional and Smart Battery Junction Boxes (BJB)

BMS architectures are constantly evolving. Device innovations, driven by so-called battery pack monitors, are driving a shift toward a more modern architecture, the smart battery junction box (BJB). While traditional BJBs consisted of only mechanical components, smart BJBs introduce active silicon devices into the BJB to perform high voltage monitoring, current sensing, and insulation sensing, functions traditionally performed by the BCU.