Lithium Batteries: BMS Theory

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BMS Theory | Importance of Management and Control

The Battery Management System (BMS) is a crucial component in ensuring the safety, efficiency, and longevity of lithium batteries. It is responsible for managing the power flowing in and out of the battery, balancing the cells, and monitoring internal temperatures. In this article, we will explore the importance of a high-quality BMS and the different methods of power interruption used in BMSs. Understanding the capabilities of a BMS can provide deep insights into the reliability and safety of the battery, making it an essential consideration when evaluating lithium batteries.

BMS Critical Role in Battery Function - Explained

It is essential to highlight the indispensable role of a high-quality BMS in the overall performance and durability of a lithium battery.

A Battery Management System is more than just a component; it's the central nervous system of a lithium battery. It meticulously manages the power flowing in and out, ensuring that the battery operates within its safe operating range.

The BMS continuously monitors the state of each cell, balances them to maintain desirable voltage ranges, and reports critical data. This vigilance prevents the battery cells from being overcharged or excessively drained, which are common causes of battery failure. Perhaps the most crucial function of a BMS is its role in safeguarding the battery from thermal and power extremes. It actively monitors internal temperatures and load, in cases of overheating or overload, can shut down discharge or charge processes to prevent damage. Advanced BMS systems go a step further, controlling the charging current in extremely cold conditions, and even activating heating in the battery to maintain optimum temperature ranges. This is particularly vital in environments where extreme temperature fluctuations are common, ensuring consistent battery performance over a wide temperature range.

Despite its importance, the quality of a BMS is often overlooked by consumers. The reality is stark: all power flowing to and from the battery passes through the BMS components. It's the battery's first line of defense. A subpar BMS may fail without warning, leading to a very hazardous situation. In the worst-case scenario, a poor-quality BMS can fail to prevent catastrophic events, posing serious safety risks. Therefore, when evaluating lithium batteries, it's imperative to consider the quality and features of the BMS. While these details might be glossed over in specification sheets, understanding the capabilities of the BMS can provide deep insights into the reliability and safety of the battery. A robust and advanced BMS is not just a feature; it's peace of mind, ensuring that your lithium battery is efficient, durable, and above all, safe.

BMS Theory | BMS Power Interruption Methods

The first consideration is the mechanics of how the BMS “disconnects” the battery from load and/or charging source. There generally are two types of BMS Power Interruption Methods: MOSFET-based and Contactor-based.


MOSFET-based BMS use MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistors) to connect and disconnect the battery power from the load and charger. MOSFETs are solid-state devices that are relatively inexpensive and have a very high cycle-life count due to the lack of moving parts.

In the rare occurrence of a MOSFET failure, it is typical for the component to fail 'short.' This implies that the MOSFET remains in a closed state until it reaches a point of complete thermal breakdown. This is not necessarily the safest failure mode for a battery and can have undesirable consequences. It is important to have several other safety mechanisms in conjunction with this system, like fusing, to protect the system in the event of a failure.

MOSFETs also have a distinct voltage drop across them, directly proportional to current, which generates substantial heat requiring proper thermal management in the form of heat sinks. MOSFETs are not typically designed for 100s of amps on their own, so a typical MOSFET-based design will require many MOSFETs in parallel to handle the power.

It’s also important to note that MOSFETs can have substantial flyback voltage caused by the deactivation of a FET under high current. Flyback voltage is a voltage spike that occurs when a current-carrying inductor is suddenly disconnected from a circuit, such as when a FET is turned off. The magnetic field generated by the inductor collapses, causing a rapid change in the current through the inductor. This rapid change in current induces a voltage across the inductor that is proportional to the rate of change of the current. The voltage can be many times higher than the supply voltage and can damage the FET or other components in the system.

Contactor-based BMSs

Contactor-based BMSs use contactors to connect and disconnect the battery power from the load and charger. Contactors are electro-mechanical devices widely used in electrical engineering for switching an electrical power circuit on or off. They function similarly to relays but are designed to handle much higher current loads, making them suitable for industrial and commercial applications.

Contactors integral to managing high-power circuits inevitably degrade with extensive usage, leading to diminished performance through mechanical wear and slower engagement, carbon accumulation on contacts increasing resistance, and the potential welding of contacts when directly handling high-power loads, causing a permanent closed state akin to a MOSFET failure.

This is why it's imperative to have a properly configured system. In such a system, the Battery Management System (BMS)'s activation of the contactor occurs infrequently, thereby sparing it from excessive use. By minimizing the operational demands of the contactor, the system ensures its longevity and reliability.

Particularly, latching contactors stand out as a highly reliable and efficient solution in this context. Unlike traditional contactors, which require continuous power to maintain their state (either open or closed), latching contactors retain their position (open or closed) without the need for constant power, thanks to a mechanical latching mechanism. This feature not only reduces wear and energy consumption but also contributes to the overall efficiency and reliability of the system, making latching contactors an ideal choice for applications where infrequent switching is a design criterion, such as in systems managed by a BMS.

There is still the concern about the potential welding of contacts when directly handling high-power loads, especially high loads related to abrupt charging of large capacitors contained within modern inverters. Fortunately, this is often solved by the use of a pre-charge resistor circuit (often found on high-quality BMS), which provides a higher impedance connection (to charge inverter capacitors) before engaging the contactors, balancing the voltage on either side of the main power contactors, reducing the likelihood of arcing. Many contactors also have auxiliary feedback circuits that allow the BMS to detect failures to engage or disengage.

In general, MOSFETs and contactors serve the same purpose - to connect and disconnect the battery power from the loads or charger. However, MOSFETs are solid-state devices that are relatively inexpensive and have significant current limitations, while contactors are mechanical and have much more power throughput capability with a lot less likelihood of failure. MOSFET-based BMSs are typically used in smaller, centralized, single-board BMSs where the pack power runs through the BMS PCBA. These tend to have lower power throughput limitations and are akin to lower cost batteries. Contactor-based BMS tend to be used on both centralized and distributed BMSs and are used for higher-power products.

In conclusion, understanding the pivotal role of a Battery Management System (BMS) in the performance, safety, and longevity of lithium batteries is critical for anyone involved in the selection, design, or use of battery-powered systems. The choice between MOSFET-based and contactor-based power interruption methods within a BMS can significantly impact the overall reliability and safety of the battery. This article has shed light on the complexities and considerations involved in BMS design, emphasizing the importance of selecting a high-quality BMS tailored to the specific needs of your application. By prioritizing advanced BMS features and understanding their operational mechanisms, users and manufacturers can ensure their battery systems are not only efficient and durable but also operate within the highest safety standards. Remember, a robust BMS isn't just a component of your battery system; it's the guardian of its safety, efficiency, and reliability.

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