Battery Management System Fault Monitoring Based on LTC6801

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Overview:
This article explains how to improve the long-term reliability of a high-voltage lithium-ion battery pack by using the LTC6801 fault monitoring IC. In applications such as 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.

The Challenges of Long-Life Battery Power For batteries and many other types of portable devices, batteries have become a major non-traditional source of energy. Lithium-ion batteries are very popular because the energy density of lithium-ion batteries allows lithium-ion batteries to be smaller and lighter than batteries of other chemical compositions with the same energy density. For high-power applications, such as electric vehicles, it is necessary to stack hundreds of batteries 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, followed by the satisfaction of the owner. Therefore, there are obvious 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 the case of 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. A few issues to consider are perhaps:

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 may cause the controller to erroneously determine that a battery or a battery pack is in perfect condition, and the fact is that the battery or battery pack is not measured in the correct way. Afterwards, these batteries may be completely discharged or dangerously overcharged, but the system is not aware of it at all. Need 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.

Figure 1: Combining battery measurement and fault detection to improve reliability.

The LTC6802 provides accurate measurements, while the LTC6801 checks the overvoltage/undervoltage condition of each cell.

The LTC6802-1 becomes the primary electronic component in the system by measuring and reporting the voltage of each cell in accordance with the instructions and applying a discharge current to the battery to distribute the charge per cell. Data is transferred to the controller over the SPI serial data link. At the same time, the LTC6801 also monitors each battery in the battery pack. Without the intervention of the system controller, the LTC6801 can periodically sample the voltage of each cell and perform simple undervoltage and overvoltage comparisons. If all conditions are normal, the LTC6801 provides a differential clock signal on the Status Output line. If anything is wrong, then the clock stops. As for the nature of the problem, the LTC6801 does not provide any information because it simply indicates that something is incorrect. Once this clock is stopped, the controller can perform diagnostics to determine what is going wrong.
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