Cover story
Low voltage battery monitor floats into high voltage electric vehicles
By Christopher Gobok, Product Marketing and Operations Manager, Analog Devices N
ot surprisingly, of all the electronic subsystems in an EV, manufacturers and consumers alike focus on the heart of the
EV, the battery system. The battery system includes the rechargeable battery itself, lithium-ion (Li-Ion) being the current standard, and the battery management system (BMS), which maximises battery usage and safety. BMS solutions by Analog Devices are the standard for monitoring them.
BMS Monitoring
A BMS’s primary function is to monitor the state of a battery or, in the case of EVs, a very large pack or stack of batteries. A BMS typically monitors individual cell and pack voltages, currents, temperatures, state of charge (SOC), state of health (SOH), and other related functions, such as coolant flow. The obvious safety and performance benefits afforded by BMS, accurately monitoring these parameters generally translates to a better driving experience, where drivers are well-informed of real-time battery conditions.
BMS measurement circuits, must be precise and fast, have high common- mode voltage rejection, consume low power, and securely communicate with other devices. Other EV BMS responsibilities include recovering energy back into the battery stack (that is, regenerative braking), balancing cells, protecting the battery stack from dangerous levels of voltage, current and temperature, and communicating with other subsystems (for example, chargers, loads, thermal management and emergency shutdown).
Multiple BMS monitoring topologies
are used by auto manufacturers to meet their need for accuracy, reliability, ease of manufacture, cost and power requirements. For example, the distributed topology shown in Figure 1 emphasises high accuracy with local smarts, high manufacturability with series-connected battery packs, and minimum power consumption and high reliability via low power SPI and isoSPI interfaces for inter-IC communications. The LTC2949 is a high-precision current, voltage, temperature, charge, power, and energy meter specifically designed for EVs. By measuring these key parameters, system designers have the essentials to calculate real-time SOC and SOH, as well as other figures of merit for the entire battery stack. Figure 2 shows a block diagram of the LTC2949 used in a high-side current sensing configuration. Here, the LTC2949 utilises an adjustable floating topology, enabling it to monitor a very high voltage battery stack, unfettered by its own 14.5V rating. Power to the LTC2949 is supplied via an LT8301 isolated flyback converter with VCC
connected to the positive
battery terminal. For enhanced reliability, a dual communication scheme can be realised by connecting a second isoSPI transceiver to the top of the battery stack and creating a ring topology that can communicate in both directions. Isolated communication with the SPI master controller is implemented via an LTC6820 isoSPI-to-SPI signal converter. Analog Devices’ stackable LTC681x family of multicell battery monitors can be used to measure individual voltages of up to 6, 12, 15, or 18 series-connected battery cells. Together, the LTC681x and LTC2949 form a BMS’s analogue front end (AFE).
Ahead with Analogue
System designers will appreciate the LTC2949’s analogue performance and its
06 February 2021
www.electronicsworld.co.uk
seamless integration into practically any EV BMS. At the core of the LTC2949 are five rail-to-rail, low offset, sigma- delta (Σ-∆) ADCs to ensure accurate voltage measurements. Of the five ADCs, two 20-bit ADCs are available to measure the voltages across two sense resistors (see Figure 2) and infer the current flow through two separate rails with an impressive 0.3% accuracy; with less than 1µV of offset, the LTC2949 also offers exceptionally high dynamic range. The total battery stack voltage is measured with up to 18 bits and 0.4% accuracy. Two dedicated power ADCs sense the shunt and battery stack voltage inputs, yielding 0.9% accurate power readings. The last 15-bit ADC can be used to measure up to 12 auxiliary voltages — handy for use with external temperature sensors or resistive dividers. Using a built-in mux, the LTC2949 can perform differential rail-to-rail voltage measurements between any pair of the 12 buffered inputs with 0.4% accuracy. The LTC2949 consumes only 16mA when turned on and only 8µA when asleep. To simplify setup, the LTC2949’s five ADCs form three data acquisition channels. Each channel can be configured for one of two speeds, depending on the application. For example, two channels can be used to monitor a single shunt resistor: one channel for slow (100ms) high- precision current, power, charge and energy measurements, the other for fast (782µs) snapshots of current, synchronised to battery stack voltage measurements for impedance tracking or precharge measurements. Alternatively, two different sized shunt resistors monitored by two separate channels (again, as shown in Figure 2) allow users to balance accuracy and
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