Feature: Batteries


Te analogue front-end combines a


12-channel voltage-measurement data- acquisition system with a high-voltage switch-bank input. All measurements are done differentially across each cell. Te full-scale measurement range is 0-5.0V, with a usable range of 0.2-4.8V. A high-speed successive approximation ADC digitises the cell voltages at 14-bit resolution with oversampling. All twelve cells can be measured in under 142μs. Te MAX17843 uses a two-


scan approach for collecting cell measurements and correcting them for errors, which yields excellent accuracy over its operating temperature range. Accuracy of cell differential measurement is specified at ±2mV at +25°C and 3.6V. To facilitate design-in with this IC, Analog Devices offers the MAX17843EVKIT# evaluation kit with a PC-based graphical user interface for setup, configuration and assessment. • ISL78714ANZ-T from Renesas. Tis ISL78714 Li-ion BMS IC supervises up to 14 series-connected cells and provides accurate cell voltage and temperature monitoring, cell balancing and extensive system diagnostics. In a typical configuration, a master ISL78714 communicates with a host microcontroller through a serial peripheral interface (SPI) port, and up to 29 additional ISL78714 devices linked by a robust, proprietary two- wire daisy chain; see Figure 5. Tis communication system is highly flexible and can use capacitor or transformer isolation, or a combination of both at up to 1Mbits/s. Initial voltage measurement accuracy


is ±2mV with 14-bit resolution over a range of 1.65-4.28V from 20˚C to +85˚C; post-board-assembly device accuracy is a tight ±2.5mV over a cell input range of ±5.0V (the negative voltage range is oſten needed for bus bars). Tis BMS includes three cell-balancing


modes: manual, time and auto. Auto- balance mode terminates balancing aſter a host-specified amount of charge has been removed from every cell. Among the integrated system diagnostics for all key functions is a watchdog shutdown


28 June 2022 www.electronicsworld.co.uk


device in case communication is lost. • BQ76PL455APFCR and BQ79616PAPRQ1 from Texas Instruments. Te BQ76PL455A is an integrated 16-cell battery monitoring and protection device designed for high-reliability, high-voltage industrial applications. Te integrated high-speed, differential, capacitor-isolated interface supports up to sixteen BQ76PL455A devices, communicating with a host through a single high-speed UART interface via a daisy-chain with twisted- pair cabling at up to 1Mbits/s; see Figure 6.


Te 14-bit ADC uses an internal


reference with all cell outputs converted in 2.4ms. Te BQ76PL455A monitors and detects several different fault conditions including overvoltage, undervoltage, overtemperature and communication faults. It supports passive cell balancing with external n-FETs, as well as active balancing via external switch-matrix gate drivers. Tis BMS easily handles strings with


fewer than its maximum of 16 cells. Te only restriction when doing so is that the inputs must be used in ascending order, with all unused inputs connected together with the input to the highest used VSENSE_ input. For example, in a 13-cell design, inputs VSENSE14, VSENSE15 and VSENSE16 are not used; see Figure 7. Other ICs, such as the Texas


Instruments’s BQ79616PAPRQ1, offer ring configuration and bidirectional communication, enabling the system to continue monitoring the SOH and safety of the battery pack; see Figure 8. If there is a fault, open or short between


two of the battery-monitoring ASICs in this configuration, the control processor will be able to continue communicating with all the ASICs by switching the direction of messaging backward and forward. Tus, if normal communication encounters a fault, the system can maintain availability using the fault tolerance of the ring communication feature, with no loss of voltage and temperature information from the battery modules. For designers looking to experiment with the BQ79616PAPRQ1,


Texas Instruments provides the BQ79616EVM evaluation board. • LTC6813-1 from Analog Devices. Tis is an automotive-qualified, multicell battery-stack monitor that measures up to 18 series-connected battery cells, with a total measurement error of less than 2.2mV via its 16-bit delta-sigma ADC with programmable noise filter; see Figure 9. Note that this is more cells than some of the other ICs can support directly. All 18 cells can be measured in under 290μs, and lower data acquisition rates can be selected for better noise reduction. Multiple LTC6813-1 devices can be


connected in series, thus permitting simultaneous monitoring of long, high- voltage battery strings. Te LTC6813-1 supports two types of serial ports: a standard four-wire SPI and a two-wire isolated interface (isoSPI). Te non- isolated four-wire port is suitable for shorter distance links and some non- automotive applications; see Figure 10. Te 1Mbit/s isolated serial


communications port uses a single twisted pair for distances of up to 100 meters with low interference susceptibility and emissions, since the interface is designed for low packet error rates even when the cabling is subjected to high RF fields. Tis daisy chain’s bidirectional capability ensures communication integrity even in the event of a fault, such as a broken wire along the communication path. In its two-wire configuration mode,


isolation is achieved through an external transformer, with standard SPI signals encoded into differential pulses. Te strength of the transmission pulse and the threshold level of the receiver are set by two external resistors, RB1 and RB2; see Figure 11. Te resistor values are chosen by the designer to allow a trade- off between power dissipation and noise immunity. Te LTC6813-1 can be powered directly


from the battery stack it is monitoring, or from a separate isolated supply. It also includes passive balancing for each cell, along with individual duty-cycle control using pulse-width modulation.


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