COVER STORY
Battery Management Systems play a critical role in current and next generation battery packs
T
he world is moving rapidly to electrification, most noticeably in Electric Vehicle (EV) powertrains, but also in material handling and off highway machines and many other applications. This is driven heavily by consumer demand but also by national and international regulations to reduce our carbon footprint.
The electrified systems powering vehicles, machines and the green buildings and industries of the future will require energy storage systems that can deliver power reliably whilst remote from an electrical grid connection or when connected to intermittent energy harvesting sources such as solar and wind power. A practical energy storage solution for most applications will be a rechargeable battery, typically a Lithium based system. To maximise the efficiency, lifetime and to ensure the safety of the battery, a good battery management system (BMS) is crucial.
Matthew Johnson, Divisional Marketing Manager at Anglia takes a look at a highly integrated BMS solution from Analog Devices and gives an insight into a new product development which is already gaining acceptance with major automotive manufacturers and will debut on upcoming EV platforms.
Typically the BMS oversees many critical functions of the rechargeable battery pack including but not limited to protecting the battery from operation outside its safe parameters, controlling environmental variables such as temperature, monitoring charge and discharge states, calculating capacity, authenticating the battery is the correct fi tment and reporting all of this data to a host system. It is clear to see why the BMS is a critical part of any rechargeable battery powered system and careful consideration must be given to its design and the selection of components. Fortunately, Analog Devices have vast
Figure 1: Typical BMS front end solution
experience in the field of battery powered systems and have a wide range of innovative products available for designers to choose from to optimise their battery pack solution. Analog Devices’ portfolio offers a family of parts for achieving a well maintained BMS; LTC681x cell monitors, LTC2949 fuel gauge and the LTC6820 to provide isolated communications to these devices over the ruggedized communications standard of isoSPI.
Analog Devices’ stackable LTC681x family of multicell battery monitors can measure individual voltages of 6, 12, 15, or 18 series-connected battery cells with a total measurement error of just 1.2 to 2.2mV. The LTC681x devices cell measurement range of 0V to 5V makes the family suitable for use with the majority of battery chemistries available on the market. Cell measurement of all connected cells is achieved in less than 290µs, lower data acquisition rates can also be selected for high noise reduction. The LTC681x can be powered directly from the battery pack or from an isolated supply. The battery monitor provides passive balancing and PWM duty cycle control for each cell in the pack and includes the ability to perform measurements on redundant cells. Figure 1 demonstrates cell monitors used in combination with the LTC2949 in a low-side current sensing configuration, where the isoSPI communication lines are parallel with the bottom LTC6811-1 battery monitor. To enhance reliability, a dual communication scheme can be implemented by connecting a second isoSPI transceiver to the top of the battery stack therefore creating a ring topology that can communicate in both directions. Isolated communication with the SPI master controller is achieved using the LTC6820 isoSPI-to-SPI signal converter. The LTC2949 device accurately interprets the charge and energy flowing in and out of a battery pack by simultaneously monitoring the voltage drop over up to two sense resistors and the battery pack voltage. The device can be configured to sense current on the high or low side of the system and can achieve an impressive 0.3% current and voltage measurement accuracy.
Utilising low offset Sigma Delta true average ADCs ensures the device takes accurate measurement of voltage and current with minimal power loss. The continuous integration of current and power also ensures lossless tracking of charge and energy delivered or received by the battery pack. Energy and charge accuracy of 1% can be realised with the LTC2949 device.
Modular battery packs require a wiring harness. This wiring harness must be tolerant of electromagnetic
8 November 2020 Components in Electronics
www.cieonline.co.uk
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