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

Introduction: As the old saying goes, "Murphy's Law": "Anything that can go wrong will go wrong." Over the last 50 years, electronic systems based on components have been fully developed to offer advanced monitoring and control functions with exceptional reliability. People often worry about reliability due to potential dangers to human life, followed by the high losses caused by failures and decreased user satisfaction. However, nothing is ever perfect, so there is always a need to keep improving reliability to create safe, durable electronic systems. When system reliability has to be guaranteed and there’s no other option, the best but most expensive approach is to use fully redundant circuits. The same circuit performs the same function simultaneously, and some form of result voting always produces the safest outcome. In many such systems, if a faulty circuit is detected, it gets automatically removed and replaced with an identical backup circuit. This is the ideal topology for long-term reliable operation. On the other hand, the consequences of failures don’t always justify the high cost of full redundancy. Such systems rely solely on the inherent reliability of each component used. The failure of a single component can severely damage the system or permanently compromise its accuracy. A design with this property carries a lot of risk but can be implemented at the lowest cost. In highly reliable systems, an intermediate method is fault monitoring, where the circuit monitors various system components and reports any anomalies. Since anything can happen at any time in the circuit, the more components that are monitored, the better. The response to the detected fault can vary from system downtime (like a normally closed emergency stop switch on a train) to some simple service alarm (similar to the dashboard "fool lamp" in a car). This article explains how to improve the long-term reliability of a high-voltage lithium-ion battery pack using the LTC6801 fault monitoring IC. In applications like electric vehicles, uninterruptible power supplies, medical instruments, and even power tools, the use of batteries as a power source is an ongoing trend, each with varying degrees of reliability expectations. Long-life battery power challenges: For electric vehicles and many other types of portable devices, batteries have become a major non-traditional energy source. Lithium-ion batteries are very popular because their energy density allows them to be smaller and lighter than batteries of other chemical compositions with the same energy density. For high-power applications, such as electric vehicles, hundreds of batteries are stacked to form a high-voltage power supply that produces less current and uses thinner and lighter wires. In this type of automotive application, the safety of the driver is the first priority, followed by the satisfaction of the owner. Therefore, there are clear reasons for achieving safe and reliable long-term operation. To achieve this, the power of each battery must be continuously monitored to maintain an optimal level for years of use. In the simplest case, the circuit is required to measure the voltage of each cell in the battery pack. This measurement is typically performed by an AD converter that passes the information to a microcontroller. The controller carefully manages the charging and discharging of all batteries so that the battery does not operate beyond a tight range, and exceeding this range can greatly shorten battery life. In a system with hundreds of individual cells, an integrated measurement circuit can save a significant number of components. The LTC6802 from Linear Technology is such an integrated functional component. With a built-in 12-bit ADC, it can measure and report voltages on up to 12 cells and two temperature sensors. Any number of batteries can be stacked on top of each other, and each set of measured voltages consisting of 12 batteries is serially transferred to a main microcontroller. These measurement devices and controllers form the heart of the battery management system. Careful control of the state of charge of each cell is extremely important to extend the usable life of the battery, but this may not be enough to satisfy the increasingly demanding automotive customers. In the case of sensitive electronics, the car presents a harsh and dangerous operating environment. To achieve long-term satisfaction without worry, it is necessary to conduct a “what-if” analysis of the system. Some issues to consider might include: - What happens if one wire that connects the battery is disconnected? - What happens if the voltage measurement accuracy shifts? - What happens if the internal register bits remain at a certain value and always indicate a good battery voltage reading? - What happens if the measurement IC is somehow damaged by a severe system voltage transient? The most latent problem could cause the controller to erroneously determine that a battery or a battery pack is in perfect condition, while in reality, the battery or battery pack was not measured correctly. Afterwards, these batteries may be completely discharged or dangerously overcharged, but the system is not aware of it at all. There needs to be 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 the fault monitoring circuit in parallel with the measuring device to function as a basic function of the review system. The circuit in Figure 1 shows a solution for a battery pack consisting 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 is designed with careful consideration of many potential system failures and is also easy to use. An important design requirement is to allow the device to operate automatically without any software. The only external requirement is the power supply (provided by the battery pack itself) and an enable clock signal. Without the clock input enabled, the LTC6801 stays in a static low power state, drawing only a few uA of current from the battery pack. The enable clock can be provided by a system controller or any other source of oscillation such as the LTC6906 silicon oscillator. Upon receipt of the clock signal, the device automatically wakes up and begins monitoring all of the batteries. The internal circuitry of the LTC6801 provides not only simple comparator functionality but also several other features: - REGULATOR: Voltage Regulator - MUX: Multiplexer - REFERENCE: Benchmark - SELF TEST REFERENCE: Self-test benchmark - DIGITAL COMPARATORS: Digital Comparator - DECODER: Decoder - UV/OV FLAGS AND CONTROL LOGIC: UV/OV tag and control logic - "GOOD": "Good" Figure 2 is a block diagram of the basic components of the LTC6801. A 12-bit delta-sigma AD converter (ADC) filters and digitizes the voltage of up to 12 cells and two temperature sensors. A 5V regulator and a precisely trimmed 3V ADC voltage reference are built in. All operating characteristics of the device are set by bonding the device pins to a 5V regulator, 3V reference, or V-. No external components are required. Settable overvoltage and undervoltage threshold ranges further enhance the reliability of the LTC6801, ensuring that any deviation from normal operating conditions is quickly detected and flagged. This additional layer of monitoring ensures that even if a primary measurement device fails, the LTC6801 can still provide critical data to prevent catastrophic system failures.

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