Feature: Batteries
Figure 9: ADI’s LTC6813-1 supports the highest number of cells (18) and uses a 16-bit ADC to achieve 2.2mV accuracy and high-speed cell scanning
Figure 10: The LTC6813-1 supports a standard four-wire SPI interconnection for shorter distance links and some non-automotive applications
the system may be required to avoid hazardous events. Such safety-related availability requires the provision of basic functionality or a defi ned “exit” path for a specifi ed time period – despite the defi ned fault conditions – and the safety system must tolerate a fault for that time period. T is fault tolerance enables the system to continue functioning longer with an acceptable level of safety. Key sections of ISO 26262 “Functional Safety for Road Vehicles” provide guidance for system developers regarding safety-related availability requirements.
IC solutions Vendors have developed BMS ICs that are designed to solve the problem of reading a single cell in a series string with accuracy, despite high CMV and the harsh electrical environment. T ese ICs not only provide the basic readings but also address multiplexing, isolation and timing skew issues. T ey meet the relevant safety standards and, if appropriate, are rated for ASIL-D approval for automotive applications – the highest and most stringent level. Automotive Safety Integrity Level
(ASIL) is a risk classifi cation scheme defi ned by ISO 26262 – Functional Safety for Road Vehicles standard. T is is an adaptation of the Safety Integrity Level (SIL) used in IEC 61508 for the automotive industry. Although the broad-brush functions
of these BMS devices are similar, they diff er to some extent in architecture, number of cells they can handle, scan
26 June 2022
www.electronicsworld.co.uk
Figure 11: The LTC6813-1 also offers a two-wire, 1Mbit/s, transformer-isolated serial communications port via a single twisted pair for distances to 100m, with both low EMI susceptibility and low emissions
speed, resolution, unique features and interconnection approach: • T e isolated CAN architecture is based on a star confi guration and is robust; a break in the communications wire in the isolated CAN architecture disrupts only one IC, keeping the rest of the battery pack safe. However, the CAN architecture requires a microprocessor and CAN for each IC, making this approach more complex and costly, whilst providing relatively slow communication speeds.
• T e daisy-chain architecture is generally more cost-eff ective, since its universal asynchronous receiver/transmitter (UART)-based daisy chain can deliver reliable and fast communication without the complexity of CAN. It most oſt en uses capacitive isolation, but may also support transformer-based isolation. However, a wire break in the daisy-chain
architecture can disrupt communication, so some of these systems off er workarounds and support limited operation during the wire break. Notable BMS ICs include:
• MAX17843 BMS from Analog Devices, a programmable, 12-channel battery- monitoring data-acquisition interface with extensive safety features; see Figure 3. It is optimised for use with batteries for automotive systems, HEV battery packs, EVs and other systems that stack long series strings of secondary metal batteries, up to 48V. T e MAX17843 incorporates a high-
speed diff erential UART bus for robust daisy-chained serial communication, supporting up to 32 ICs connected in a single daisy-chain; see Figure 4. T e UART uses capacitive isolation, which not only reduces the bill of materias cost, but also improves failure-in-time rates.
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