The key role of BMS (Battery Management Systems)

In an increasingly technology-driven world, where the energy transition is of paramount importance, the use of batteries has become essential. Whether in electric vehicles, renewable energy storage devices or in our everyday electronic devices (telephones, computers, toothbrushes...), they have invaded our environment. These different technologies require increasingly powerful batteries with high storage capacities, and their use must respect precise electrical and thermal conditions, at the risk of producing a thermal runaway. To limit the risk of failure, lithium battery packs incorporate battery management systems (BMS).

1 - Causes of battery pack failure

Thermal runaway is a particularly dangerous and, unfortunately, fairly frequent phenomenon. It is characterized by a rapid, uncontrolled rise in battery temperature. This phenomenon is triggered by a succession of stages:

- Initial trigger: usually an internal short-circuit, overload, physical damage, manufacturing defect or inappropriate use. This trigger will increase the battery's temperature.

- Internal chemical reactions: as the temperature rises, internal chemical reactions are triggered, producing even more heat, such as the decomposition of electrolytes or the reaction between electrodes.

- Significant rise in temperature: these chain reactions will lead to an even more rapid rise in temperature.

- Battery damage: once the temperature reaches a certain threshold, the battery structure can begin to degrade, potentially leading to cell rupture.

2 - BMS functionalities

A BMS, or Battery Management System, is an electronic device that supervises and manages the operation of rechargeable batteries, typically lithium-ion batteries. It plays an essential role in protecting, monitoring and managing the battery's state of charge and health.

Battery cell monitoring

The BMS monitors several battery cell elements.

Voltage

The BMS measures the terminal voltage of each individual cell in a battery pack to ensure that they remain within their optimum operating range. If the voltage is too high, i.e. overcharging, this can lead to thermal runaway. Conversely, too low a voltage, i.e. discharge, degrades the cell and thus facilitates the onset of thermal runaway.

Temperature

The BMS monitors cell temperature to prevent overheating and thermal runaway. A high temperature can lead directly to thermal runaway. Too low a temperature leads to degradation, facilitating the onset of thermal runaway.

The current

The BMS measures incoming and outgoing current to prevent overcharging and excessive discharge. The battery pack is dimensioned to supply or receive a maximum current depending on the ambient temperature. A high current can lead to overheating of the cells and a risk of short-circuiting.

Isolation

In some areas, the battery pack must be totally isolated from the rest of the product. This is particularly true of electric vehicles, where the battery pack and motor are isolated from the vehicle's mechanical chassis.

Protection

The BMS protects the battery against a number of dangerous hazards, such as overcharging, discharging, short-circuits and extreme temperatures.

Overcurrents

The BMS ensures that the battery pack current remains within the operating range defined by the manufacturer. The BMS provides overcurrent protection in several ways:

- By opening contactors when current exceeds thresholds.

- If fitted with a fuse or thermal circuit breaker, these systems open when thresholds are exceeded, disconnecting the battery pack.

- If it is also fitted with a thermal fuse, this will cut the current if the temperature exceeds a certain threshold.

Overloads and overdischarges

The BMS incorporates a device for constantly monitoring the battery pack voltage. When a cell reaches the threshold set by the manufacturer, the BMS can open the contactor to disconnect the battery pack from its application.

Short circuits

Short-circuit protection in a BMS is indispensable in a battery pack. The BMS continuously monitors the current flowing through the battery. As soon as it detects an abnormally high current, it reacts quickly to cut the circuit. Some BMSs use circuit breakers or fuses that trip to physically interrupt the circuit.

Extreme temperatures

Temperature has a direct impact on battery performance, safety and longevity. When temperatures exceed safety thresholds, the BMS can limit or stop charging or discharging the battery to reduce heat generation. Some batteries are equipped with cooling systems (fans, liquid cooling) which the BMS can activate to help reduce temperatures. The BMS can even disconnect the battery from the system to prevent damage or fire hazards.

Individual cells

As well as monitoring individual cells, the BMS actively balances the charge between cells to avoid overcharging or overdischarging individual cells. This is a very important point in battery management.

Battery wear and tear (SOH)

The BMS can integrate a function to evaluate battery pack performance and service life. This data is vital to enable battery packs to be reused in a second life for less demanding applications. Electric vehicle brands such as BMW, Nissan and Tesla replace batteries as soon as the percentage of capacity falls below 60-70%.

3 - The benefits of a BMS

Installing a BMS for battery packs offers several advantages, depending on the type of BMS chosen:

- Prevent risks that could lead to fire or explosion.

- Optimize battery performance.

- Increase pack life.

- Manage state of charge.

- Communicate with other systems.