You can now help protect the environment without fear of being trapped in it. Governments around the world are offering generous financial incentives to offset the premium on electric vehicles, hoping to steer you away from buying internal combustion engine (ICE) cars. Some governments have already taken steps to require automakers to manufacture and sell electric vehicles, hoping that the market will eventually be dominated by them, while others are drawing a clearer line in the sand; Germany, for example, is already pushing for a ban on ICE cars by 2030.
Electric vehicle mover
If you don't already drive an electric vehicle (EV)—a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV), or an all-electric vehicle—then it's very likely you will soon. Range anxiety is a thing of the past. You can now help protect the environment without worrying about being stuck in one. Governments around the world are offering generous financial incentives to offset the premium on electric vehicles, hoping to steer you away from buying internal combustion engine (ICE) cars. Some governments have taken steps to require automakers to build and sell electric vehicles, hoping the market will eventually be dominated by them, while others are drawing a clearer line in the sand; Germany, for example, is already pushing for a ban on internal combustion engine cars by 2030.
For much of automotive history, innovation has focused on improving the fuel efficiency of internal combustion engines, reducing emissions, and providing a comfortable user experience. However, the vast majority of recent innovations in internal combustion engine vehicles are direct results of advancements in electronics—improvements in chassis systems, powertrains, autonomous and advanced driver assistance systems (ADAS), infotainment, and safety systems. Electric vehicles share many of the same electronic systems as internal combustion engine vehicles, and of course, the drivetrain itself. According to Micron Technology, the electronics account for up to 75% of the value of an electric vehicle, and this percentage is increasing as advancements in semiconductor technology continue to reduce the cost of various electronic modules and subsystems. Even non-traditional automakers, such as Intel, are seeking a piece of this pie.
Unsurprisingly, across all the electronic subsystems of an electric vehicle, manufacturers and consumers are focused on the core of the vehicle: the battery system. The battery system includes the rechargeable battery itself (lithium-ion), which is currently the standard, and the battery management system (BMS), which maximizes battery utilization and safety by monitoring the battery.
Bare metal server monitoring
The primary function of a Battery Management System (BMS) is to monitor the state of the battery, or, in the case of an electric vehicle, the state of a very large battery pack or battery array. A BMS typically monitors the voltage, current, temperature, state of charge (SOC), state of health (SOH), and other relevant parameters of individual cells and battery packs, such as coolant flow. Beyond the obvious safety and performance advantages provided by a BMS, accurate monitoring of these parameters often translates to a better driving experience, allowing the driver to have a full understanding of the real-time battery status.
For it to be effective, the BMS measurement circuitry must be accurate and fast, have high common-mode voltage rejection, low power consumption, and communicate securely with other devices. Other responsibilities of an EV BMS include recovering energy back to the battery pack (i.e., regenerative braking), balancing the battery, protecting the battery pack from dangerous levels of voltage, current, and temperature, and communicating with other subsystems such as chargers, loads, thermal management, and emergency shutdown.
Automakers use a variety of BMS monitoring topologies to meet their requirements for accuracy, reliability, ease of manufacture, cost, and power. For example, the distributed topology shown in Figure 1 emphasizes high precision of local intelligence, high manufacturability of series battery packs, and low power consumption and high reliability for inter-IC communication via low-power SPI and isoSPI interfaces.
The topology in Figure 1 includes an EV battery pack monitor (in this case, Analog Devices' LTC2949) for a low-side current sensing configuration, with the isoSPI communication line connected in parallel with the bottom battery monitor (LTC6811-1). For enhanced reliability, a dual-communication scheme can be implemented by connecting a second isoSPI transceiver to the top of the battery pack, creating a ring topology that allows communication in both directions. Isolated communication with the SPI master controller is achieved through an LTC6820 isoSPI to SPI signal converter. Analog Devices' stackable LTC681x series of multi-cell battery monitors can be used to measure the individual voltage of up to 6, 12, 15, or 18 series-connected cells, while a single LTC2949 is used to measure the overall battery pack parameters. Together, the LTC681x and LTC2949 constitute a comprehensive EV BMS monitoring solution—for some, this circuit may be more widely known as the analog front-end (AFE) of the BMS.
Figure 1. Distributed EV BMS monitoring topology using a battery monitor (LTC6811-1) and an electric vehicle battery pack (LTC2949).
EV battery pack monitors are high-precision meters for current, voltage, temperature, charge, power, and energy, specifically designed for EVs. By measuring these critical parameters, system designers have the essential elements to calculate the real-time SOC and SOH of the entire battery pack, as well as other quality factors. Figure 2 shows a block diagram of the LTC2949 for a high-side current sensing configuration. The LTC2949 employs an adjustable floating topology, enabling it to monitor very high-voltage battery packs, unaffected by its own 14.5 V rated voltage. The LTC2949's power supply is fed to the battery positive terminal via an LT8301 isolated flyback converter with a voltage rating of V.
At the heart of the electric vehicle battery pack monitor is a rail-to-rail, low-offset, Σ-Δ ADC that ensures accurate voltage measurements. Of the five ADCs available in the LTC2949, two 20-bit ADCs can be used to measure the voltage across two sense resistors (as shown in Figure 2) and infer the current flowing through two independent power rails with 0.3% accuracy; high dynamic range is provided with an offset of less than 1 μV. Similarly, the total battery pack voltage is measured with up to 18 bits and 0.4% accuracy. Two dedicated power ADCs sense the shunt and battery pack voltage inputs, producing accurate power readings of 0.9%. A final 15-bit ADC can be used to measure up to 12 auxiliary voltages, facilitating use with external temperature sensors or resistive voltage dividers. Using a built-in multiplexer, the monitor can perform differential rail-to-rail voltage measurements between any pair of the 12 buffered inputs with 0.4% accuracy.
To simplify setup, the monitor's five ADCs form three data acquisition channels. Each channel can be configured for one of two speeds, depending on the application, as shown in Table 1. For example, two channels can be used to monitor a single shunt resistor: one for slow (100 ms) high-precision current, power, charge, and energy measurements; the other for fast (782 μs) current snapshots synchronized with battery pack voltage measurements for impedance tracking or pre-charge measurements. Alternatively, monitoring two shunt resistors of different sizes via two independent channels (as shown in Figure 2) allows the user to balance the accuracy and power loss of each shunt. Simultaneously, a third auxiliary channel can perform fast measurements on optional buffered inputs or automatic cyclic (RR) measurements on two configurable inputs (stack voltage, chip temperature, supply voltage, and reference voltage).
Table 1. Configuration options for the three data acquisition channels of the LTC2949
Since State of Harmony (SOH) is a point in the battery (or battery pack) lifecycle and a measure of its condition relative to a new battery, it's crucial to use an accurate EV BMS monitor not only to maximize driving range but also to minimize unexpected battery failures. Speaking of battery life, the LTC2949 consumes only 16 mA when on and only 8 μA when asleep. When any of the monitor's three data acquisition channels are configured in fast mode (782 μs conversion time and 15-bit resolution), the monitor can synchronize its battery pack voltage and current measurements with the battery voltage measurements of any LTC681x multi-cell monitor to infer individual cell impedance, age, and SOH. With this information, stack battery life can be evaluated, as the weakest cell ultimately determines the SOH of the entire stack. [Buy electronic components in stock at Weiyang Mall]
Digital Advantage
The digital functionality of the electric vehicle monitor includes oversampling multipliers and accumulators that generate 18-bit power values and 48-bit energy and charge values, reporting minimum and maximum values, as well as alarms based on user-defined limits. This frees the BMS controller and bus from the additional task of continuously polling the monitor for voltage and current data, and performing calculations based on the results. By acquiring power samples at the oversampling ADC clock rate (pre-decimation filter), rather than multiplying by an average, the monitor can accurately measure power even when current and voltage variations far exceed its slew rate, with signals up to 50 kHz.
Figure 2. Typical connection of the LTC2949 floating EV battery monitor with high-side current sensing configuration. The monitor is powered via an LT8301 flyback converter with V-band connected to the positive terminal of the battery.
Because the monitor tracks minimum and maximum values of current, voltage, power, and temperature data, the bus and host can use clock cycles for other tasks instead of continuously polling the monitor. In addition to detecting and storing minimum and maximum values, the monitor can also issue an alarm when any user-defined threshold is exceeded, freeing the host controller and bus from polling tasks again. The monitor can also generate an overflow alarm after a specified amount of energy or charge has been provided, or after a preset amount of time has elapsed.
To ensure monitoring accuracy, the monitor offers programmable gain correction factors to compensate for the tolerances of the measurement components: two for the shunt resistors, one for the battery voltage divider, and four multiplexed inputs. These correction factors can be stored in an external EEPROM for factory calibration of the battery pack using a modular approach. The monitor can also linearize temperature readings from up to two external NTC thermistors by solving the Steinhardt-Hart equation with programmable coefficients; these readings can then be used to automatically compensate for the temperature of the shunt resistor readings. By continuously compensating for tolerances and temperature effects, monitoring accuracy is improved, and lower-cost external components can be used.
The standard SPI interface can be used for direct MCU connection, while the isoSPI interface provides a physical layer adaptation to standard chip-level SPI, unlocking the full potential of cost-effective distributed packaging architectures. Designed for high-voltage and high-noise systems, isoSPI provides secure, reliable data transfer up to 1 Mbps over cables up to 100 meters long using only a single twisted-pair cable and a simple pulse transformer. isoSPI is also less expensive than other onboard isolation solutions. Figure 3 illustrates how isoSPI can be used as the last element in a daisy-chain or addressable parallel configuration.
Figure 3. Architecture using isoSPI configuration.
in conclusion
Electric vehicles have become mainstream, leading to an inflection point for mass adoption. To remain competitive, system designers need to pay close attention to battery and BMS technologies, which have a profound impact on the end-user experience. EV battery pack monitors simplify the handling of various battery stack monitoring topologies and configurations. Monitors enable high-performance, safe, flexible, and reliable battery management systems at virtually any voltage and current level. Battery SOH and SOC can be accurately assessed immediately by accurately reading current, voltage, power, energy, charge, temperature, and time. Critical minimums, maximums, and alarms can be measured, calculated, and reported via the isoSPI interface. This reduces the need for host resources, bus design and testing, and software design. Some digital features include multipliers, accumulators, minimum/maximum registers, configurable alarms, and external component tolerance/temperature compensation. Monitors such as the LTC2949 are designed to operate independently or in conjunction with any multi-cell battery monitor, meeting the critical requirements of next-generation EV BMS while complying with stringent AEC-Q100 guidelines and ISO 26262 safety standards.
