How does the battery management System BMS work in electric vehicles?

The battery system of an electric vehicle is often made up of hundreds or even thousands of cells (also known as battery cells). Although there are so many cells working together, the overall performance of the battery system is actually determined by the worst cell, which is often referred to as the 'barrel principle'. Therefore, high performance, long life and safe operation can only be achieved by accurately monitoring each cell, accurately evaluating each cell and efficiently managing each cell to ensure that all cells are in consistently good condition [1]. Here we will talk more about how BMS are monitored, evaluated and managed.

Accurate monitoring of battery management systems

It is well known that management is based on a comprehensive knowledge of real information. Sensitive perception is therefore a prerequisite for optimal control of the management system. Only with full knowledge of the battery's usage can the battery be better managed. The batteries management system monitors the voltage of each battery in the system and recognises even small changes of just 1mv. At the same time, temperature sensors are installed in every area of the system, allowing the thermal field of the battery system to be fully sensed. During charging and discharging, the battery input and output currents are recorded in real time, providing a useful basis for assessing the battery's condition. The management system also monitors the insulation characteristics of the battery to ensure that the high voltage of the battery is adequately isolated from the body to ensure safe operation. At the same time, the battery pack management system diagnoses and verifies these monitoring functions in real time, and if a failure of the monitoring system is detected, a redundancy plan is activated to ensure that the batteries management system's sensing capability is always responsive.

Accurate assessment of the battery management system(BMS)

Next, the batteries management system can make an in-depth assessment of the state of the battery system based on the monitored information, which includes the following main aspects.

1. charge status: this can be simply understood as the remaining charge available in the battery. You may have encountered a similar situation when using a mobile phone - it shows 20% charge left, but then suddenly shuts down. If the battery management system of an electric vehicle does not accurately calculate the remaining charge, the consequences will not only be the inability to make phone calls or swipe through friends. Incorrect battery information can lead to drivers misjudging the range and the vehicle breaking down while driving. It is therefore important to accurately assess the state of charge.

2. Power Boundary: This is the analysis of the maximum power that can currently be output by/into the battery, thereby 'informing' the vehicle or charging device of the battery's maximum capacity at this moment in time, ensuring high performance while also taking into account longevity.

3. Health status: This provides a basis for battery maintenance and replacement by assessing the battery's life state.

4. Fault status: The batteries management system alerts the user to the severity of the fault and analyses the optimal treatment strategy.

Efficient management of the battery management system

Once accurately monitored and assessed, the batteries management system is ready to implement specific management measures.

In the beginning we mentioned the 'barrel principle' (the overall performance of the battery is determined by the worst cell), which means that it is vital to maintain the consistency of the battery.

The consistency of a battery is determined by two factors: on the one hand, the 'innate' consistency, which is mainly achieved through advanced manufacturing technology and processes, and on the other hand, the 'acquired' consistency, which needs to be maintained by a strong batteries management system.

1. Consistency of charge state

The battery management system needs to ensure that all batteries maintain a consistent state of charge at all times. A good battery pack should look like a neat square on parade, keeping the same pace as it moves forward. At GM, we use a highly reliable battery equalisation technology that, as soon as it identifies a charge difference between cells, immediately releases energy from the highly charged cells, regulating all cells to the same pace in time.

For example, once a battery has been fully charged (reached 100% charge) during the charging process, charging of the other batteries in the same system must be stopped immediately, otherwise the fully charged battery will be overcharged, leading to irreversible damage or even a safety hazard. However, stopping charging at this point means that most of the batteries are not fully charged and the capacity of the battery pack cannot be fully utilised. Therefore, when a battery is close to 100% charge, the batteries management system will advance the equalisation function so that the charge current of the highest charged battery is less than that of the other batteries, thus ensuring that all batteries are fully charged at the same time.

2. Temperature consistency

Batteries age at different rates at different temperatures and if temperature consistency is not maintained, differences between batteries will become apparent over time and these differences cannot be repaired by equalisation techniques.

Therefore, GM has designed an advanced battery liquid cooling solution [2] with multiple heat transfer methods to maintain the appropriate temperature range for battery operation while ensuring minimal temperature variation between batteries.

*Fighting low temperatures: You may have had similar experiences with mobile phones that quickly run out of power in cold winter outdoor conditions. Similarly, when the battery temperature is too low, the range of the vehicle is shortened. So, when the battery is in a cold environment, the batteries management system controls the switch of the liquid cooling pipeline to a heating circuit, which warms up the coolant and sends heat inside the battery system.

*Fighting high temperatures: When the battery is in a high temperature environment, the batteries management system will control the liquid cooling pipeline to switch to the heat sink circuit, where heat is carried out of the battery system by the coolant and released through the heat sink. When the battery is in extreme heat and needs to be cooled quickly, the battery management system will control the liquid cooling pipeline to switch to the air conditioning circuit, using the air conditioning to cool the coolant, thus achieving rapid cooling.