Fault monitoring in a non-perfect battery management system - News - Global IC Trade Starts Here.

Introduction: As Murphy's Law famously states, "Anything that can go wrong will go wrong." Over the last fifty years, electronic systems based on components have evolved to deliver highly advanced monitoring and control functionalities with exceptional reliability. Reliability concerns often arise due to the potential risks to human life, followed by costly failures and reduced user satisfaction. However, perfection is elusive, and there remains a constant need to enhance reliability to create safer, more durable electronic systems. When system reliability is paramount and there’s no room for error, the most effective but expensive approach is to implement fully redundant circuits. Two identical circuits perform the same function simultaneously, with some form of result validation ensuring the safest outcomes. In many such systems, when a faulty circuit is detected, it is automatically bypassed and replaced by its identical backup. This is the ideal topology for long-term reliable operation. On the other hand, while the consequences of failure might not justify the high cost of full redundancy, most systems rely on the inherent reliability of their individual components. A failure in a single component can severely compromise the system or permanently affect its accuracy, assuming significant risk but offering the lowest cost solution. In highly reliable systems, an intermediate approach involves fault monitoring, where the circuit monitors various system components and alerts for any anomalies. Given the unpredictable nature of circuits, the more components monitored, the better. Responses to detected faults can range from system shutdown (like emergency stops in trains) to simpler service alarms (akin to warning lights in cars). This article explores how to improve the long-term reliability of a high-voltage lithium-ion battery pack by utilizing the LTC6801 fault monitoring IC. In applications like electric vehicles, uninterruptible power supplies, medical equipment, and even power tools, batteries are increasingly becoming the primary power source, each with varying reliability demands. Long-life Battery Power Challenges: For electric vehicles and numerous portable devices, batteries have emerged as a significant non-traditional energy source. Lithium-ion batteries are particularly favored due to their high energy density, allowing them to be smaller and lighter compared to other chemistries with similar capacities. For high-power applications such as electric vehicles, hundreds of batteries are stacked to form a high-voltage power supply that generates lower currents and uses thinner, lighter wires. In automotive applications, passenger safety is paramount, followed by customer satisfaction. Therefore, ensuring safe and reliable long-term operation is crucial. To achieve this, the power of each battery must be continuously monitored to maintain optimal performance over years of use. In the simplest scenario, the circuit is required to measure the voltage of each cell in the battery pack. This is typically done using an analog-to-digital converter (ADC), which sends the data to a microcontroller. The controller carefully manages the charging and discharging of all batteries to ensure they operate within a strict range, as exceeding this range can drastically reduce battery life. In systems with hundreds of individual cells, an integrated measurement circuit significantly reduces the number of components. The LTC6802 from Linear Technology is such an integrated solution, featuring a 12-bit ADC capable of measuring and reporting up to 12 cell voltages and two temperature sensors. Multiple batteries can be stacked, with each set of 12 measurements being transferred serially to a main microcontroller. These devices and controllers form the core of the battery management system (BMS). Precise control of each cell's state of charge is vital to extending battery lifespan. However, this alone may not meet the increasingly demanding expectations of automotive customers. Cars present harsh and potentially hazardous operating environments for sensitive electronics. To ensure long-term satisfaction, a "what-if" analysis of the system is essential. Some considerations include: - What happens if a connection wire to the battery is disconnected? - What happens if the voltage measurement accuracy shifts? - What happens if internal register bits stay stuck at a certain value, falsely indicating good battery voltage readings? - What happens if the measurement IC is damaged by a severe system voltage transient? The most insidious issue could lead the controller to mistakenly conclude that a battery or battery pack is in perfect condition when, in reality, the measurements are unreliable. Subsequently, these batteries might be fully discharged or dangerously overcharged, unbeknownst to the system. There's a need for something to "monitor the monitor" to achieve a higher level of reliable operation. Battery Management System (BMS) Fault Monitoring with the LTC6801: An alternative to the fully redundant measurement method is to connect a fault monitoring circuit in parallel with the measuring device to act as a basic review system. Figure 1 illustrates a solution for a battery pack of 12 lithium-ion batteries using an LTC6802 measuring device and an accompanying LTC6801 fault monitor. This solution is implemented for a battery pack consisting of 12 lithium-ion batteries. The LTC6801 was meticulously designed to account for many potential system failures and is also user-friendly. A key design requirement is to allow the device to operate autonomously without any software. The only external requirements are power (from the battery pack itself) and an enable clock signal. Without the clock input enabled, the LTC6801 remains in a static low-power state, drawing only a few microamperes from the battery pack. The enable clock can be provided by a system controller or any other oscillation source, such as the LTC6906 silicon oscillator. Upon receiving the clock signal, the device automatically wakes up and begins monitoring all the batteries. Figure 2 is a block diagram of the LTC6801's basic components. It features a 12-bit delta-sigma ADC that filters and digitizes the voltages of up to 12 cells and two temperature sensors. A 5V regulator and a precisely trimmed 3V ADC voltage reference are built-in. All operational characteristics of the device are set by connecting the pins to a 5V regulator, 3V reference, or V-. No external components are required. The LTC6801 offers adjustable overvoltage and undervoltage thresholds, providing additional safeguards against potential failures. Its digital comparators and decoder ensure accurate monitoring, while UV/OV flags and control logic manage the system's response to detected anomalies. The "GOOD" status indicator confirms normal operation, providing peace of mind to users. This fault-monitoring IC not only enhances the reliability of the BMS but also ensures the longevity and safety of high-voltage lithium-ion battery packs across various applications.

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