Electric Vehicle Battery Management System (EV-BMS)

The auto industry is really serious about electric vehicles this time! Unlike past escapades which failed with the launch of a few electric vehicle models which were not well received.

Governments, OEMs, Tier 1 suppliers and all other stakeholders are vying to make electric vehicles a worthy replacement for combustion engine automobiles.

Although we often get totally caught up in the glitz, we ignore one of the most important components of an EV, the battery. In terms of cost, it represents almost 40% of the cost of the vehicle. The battery includes the lithium-ion cells that power the EV drivetrain and with that, a clever solution called the EV Battery Management System aka BMS.

A BMS can be found in almost all high-end battery-operated electronic devices; for example, your smartphone. How do you think the phone displays battery percentage or overall battery health? It is the in-game BMS that monitors each battery cell and uses its complex algorithm to calculate the battery percentage, health, etc. When we extrapolate the battery management aspect to an electric vehicle, the complexity goes up several notches.

In this blog, we will learn more about battery management system for electric vehicles and also the evolution that this smart solution has undergone over the years.

Key Responsibilities of an EV Battery Management System

A lithium-ion cell battery is like a small explosive. Keeping a close eye on parameters such as voltage, load and temperature becomes very important. Uncontrolled thermal runaway can cause an explosion or fire that can be fatal to vehicle occupants.

Here are the usual functions that a regular EV BMS performs.

  • Cellular monitoring: When charging or discharging, the battery cells should be monitored at all times. Any out-of-specification situation must be identified and signaled with the triggering of the safety mechanism. There are integrated circuits equipped with cell monitoring algorithms which perform this function. There is a control chain in which the cell monitoring circuit registers cell voltage and temperature and transmits the data to a cell management controller.

    At this point, some algorithms run to calculate the state of charge (SOC) and state of health (SOH).

    • The SOC is determined to ensure that the battery is never overcharged or undercharged. The SOC can also be considered as a fuel indicator of an electric vehicle because it indicates the energy remaining in the battery. Using this information, additional algorithms can be run to determine how far the electric vehicle has traveled before the battery needs to be recharged.
    • SOH is an indicator of overall battery health and provides insight into battery operating conditions. Based on this information, battery life and maintenance schedule can be projected.

    Diagnosis is another important dimension of cell monitoring. UDS-based diagnostic software helps the BMS EV identify and report fault codes and diagnostic information. These data points can be used to solve the problem or trigger a security mechanism based on the ISO 26262 standard.

  • Power optimization: The direct result of cell monitoring is the optimization of battery power. While the cell monitoring functionality determines SOC and SOH, the job of the EV battery management system is to keep the SOC and SOH parameters within specified values. When the battery is charging, the EV battery management system determines how much current can be allowed into individual cells. During the operation of the EV when the EV is discharging, the BMS ensures that the voltage level is not too low. It communicates with the motor controller to make sure.
  • Electric vehicle safety: Safety is the primary concern when the electric vehicle power supply and battery management system is pictured. Undetected thermal runaway can cause a major accident. As mentioned earlier, the BMS captures data such as voltage, temperature, and current to optimize power. A similar set of data is also used to ensure security. Standards like ISO 26262 have certain requirements that must be met to ensure that the BMS is developed as a fail-safe system. Another aspect of safety is the isolation of the vehicle body/chassis from the battery to prevent electric shock to vehicle occupants.
  • Optimization of the battery charge: Battery cells deteriorate over time. An intelligent EV BMS takes into account this deterioration which leads to a change in battery parameters such as voltage, current, etc. For example, a battery cell is slightly damaged by heat and begins to charge at a lower voltage than other cells. The battery management system is responsible for identifying this fault and optimizing the charging process so that all cells are charged at the lowest voltage. This reduces stress on the entire battery and improves battery life. Of course, the BMS diagnostics will also store this problem as a trouble code so that it can be solved later. Additionally, oxidation of the battery terminals can cause voltage reduction and the BMS adapts to these changes to extract the best performance from the battery.

Common Algorithms That Make Up an EV Battery Management System

Battery parameters such as voltage, current, and temperature are actionable data points that must be processed in order to derive certain metrics. For example, battery voltage can be used to calculate the distance an electric vehicle can travel before the battery is depleted.

A battery management system performs many of these calculations for which algorithms are written. Let’s understand two:

  • Cell Balancing Algorithm: We discussed how different cells in a battery can develop different capacities over time. A battery cell can reach a maximum of 3.7 volts compared to others that reach up to 4.2 volts. This cell will also be the first to discharge. As a result, the pack cannot be used to its maximum potential. The cell balancing algorithm comes to the rescue in such circumstances. Two types of cell balancing technique are deployed- active and passive cellular balancing. In the active balancing technique, the stronger cells are used to charge the weaker cells so that their potential is equalized. Passive balancing works by discharging excess voltage from the stronger cells by connecting them to a load.
  • Communication algorithms: Different ECUs of an electric vehicle require battery parameters to work efficiently. For example, the BMS communicates with the EV motor controller to ensure that it draws current in an optimized manner. Similarly, the BMS communicates with an external load device to transmit the required output current and voltage information. It also controls the start and stop of the load. In case of non-compliant problem, BMS cuts the connection and stops charging. In order to facilitate this communication, various communication protocols such as CAN specifications, J1939, CHAdeMO, Bharat EV, etc. are used.

Way to go

Electric vehicle battery management systems are evolving at a rapid pace. The latest advancement is a wireless battery management system that does away with CAN BUS and SPI cables. As batteries grow, these wires increase in complexity and thus increase the risks associated with so many wires clogging the BMS.

Various such innovative technologies are currently being developed with respect to EV BMS and related components. These innovations will bolster the ambitious electric vehicle agenda that OEMs, governments and other stakeholders are pursuing with all their might.

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