Battery Management System for Electric Vehicle: Ensuring Safety and Reliability of Electric Vehicles
The automotive industry is really serious about Electric Vehicles this time! Unlike the past escapades that fizzled out with the launch of a few EV models that weren’t received well.
Governments, OEMs, tier-1 suppliers, and every other stakeholder are vying to make electric vehicles a worthy replacement to IC engine automobiles.
While we often get totally caught up in the glitz, we ignore one of the most important components of an EV, the Battery Pack. Cost-wise, it constitutes almost 40% of the vehicle cost. Battery pack comprises the Lithium-ion cells that power the EV drivetrain and along with that, a smart solution called the EV Battery Management System aka BMS.
A BMS can be found in almost every high-end electronic device that runs on battery; for example, your smartphone. How do you think the phone shows the battery percentage or the overall battery health? It’s the BMS at play that monitors every cell of the battery and uses its complex algorithm to calculate battery percentage, health etc. When we extrapolate the battery management aspect to an electric vehicle, the complexity gets several notches higher.
In this blog, we will learn more about battery management system for electric vehicles and also the evolution that this smart solution has gone through over the years.
Key Responsibilities of an EV Battery Management System
A battery pack of lithium-ion cells is akin to a small explosive. Keeping a close eye on parameters like voltage, charge, and temperature becomes highly important. An unchecked thermal runaway can cause explosion or fire which can be fatal for the vehicle occupants.
Following are the usual functions that a regular Electric Vehicle BMS performs.
- Cell Monitoring: When charging or discharging, the battery cells need to be monitored at all times. Any out-of-spec situation must be identified and reported along with the triggering of the safety mechanism. There are integrated circuits equipped with cell monitoring algorithms that perform this function. There is a chain of command where the cell monitoring circuit records the cell voltage and temperature and passes on the data to a cell management controller.
At this stage, some algorithms run to calculate the state of charge (SOC) and state of health (SOH).
- SOC is determined to ensure that the battery is never over or undercharged. SOC can also be seen as a fuel indicator of an electric vehicle as it indicates the energy remaining in the battery. Using this information additional algorithms can be run to determine the distance the EV would cover before the battery needs a recharge.
- The SOH is an indicator of overall health of the battery and gives an insight into the operating conditions of the battery. Based on this information, battery lifespan and maintenance schedule can be projected.
Diagnostics is another important dimension to cell monitoring. UDS based diagnostics software help the EV BMS to identify and report the error codes and diagnostic info. Such data points can be used to fix the issue or trigger ISO 26262 based safety mechanism.
- Power Optimization: The direct outcome of cell monitoring is the optimization of battery power. While the cell monitoring functionality determines SOC and SOH, the EV battery management system’s job is to keep the SOC and SOH parameters within the specified values. When the battery is charging, the EV battery management system determines how much current can be allowed in the individual cells. During the EV operation when the EV is discharging, the BMS ensures that the voltage level does not get too low. It communicates with the motor controller to ensure that.
- Safety of Electric Vehicle: Safety is the primary concern when EV power supply and battery management system is in the picture. An undetected thermal runaway can cause major mishap. As mentioned earlier, the BMS captures data such as voltage, temperature and current to optimize the power. Similar set of data is also utilized for ensuring safety. Standards like ISO 26262 have certain requirements that must be fulfilled to ensure that the BMS is developed as a fail-safe system. Another aspect of safety is the insulation of vehicle body/chassis from the battery pack to avoid electric shock to vehicle occupants.
- Battery Charging Optimization: The battery cells deteriorate over time. An intelligent EV BMS factors in this deterioration that results in change on battery parameters such voltage, current, etc. For instance, a battery cell gets slightly damaged by heat and starts getting charged at a lower voltage than the rest of the cells. Battery management system is responsible for identifying this fault and optimizing the charging process so that all cells are charged at the lower voltage. This reduces the stress on the overall battery pack and enhances its life. Of course, the BMS diagnostics will also store this issue as a trouble code so that it is fixed at a later stage. Moreover, oxidation in the battery terminals may lead to reduction in voltage and the BMS adapts to these changes to extract best performance from the battery.
Common Algorithms that Constitute an EV Battery Management System
Battery parameters such as voltage, current and temperature are actionable data points that need to be processed in order to derive certain metrics. For instance, battery voltage can be used to compute the distance an electric vehicle can cover before the battery gets exhausted.
A battery management system does several such calculations for which algorithms are written. Let’s understand two of these:
- Cell Balancing Algorithm: We discussed about how different cells in a battery pack can develop different capacities with time. A battery cell might reach a maximum of 3.7 volts compared to others that reach up to 4.2 volts. That cell will also be the first to discharge. As a result, the pack cannot be used at its max potential. The cell balancing algorithm comes to rescue in such circumstances. Two kinds of cell balancing technique are deployed- active and passive cell 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 the excess voltage of the stronger cells by connecting them with a load.
- Communication Algorithms: Different ECUs in an electric vehicle require battery parameters in order to work efficiently. For instance, the BMS communicates with the EV motor controller to ensure that it is drawing current in an optimized manner. Similarly, the BMS communicates with an external charging device to pass on the required current and output voltage information. It also controls when the charging starts and stops. If there is any out-of-spec issue, BMS cuts off the connection and stops the charging. In order to facilitate this communication, various communication protocols such as CAN, J1939, CHAdeMO, Bharat EV specifications, etc. are employed.
Electric Vehicle Battery Management Systems are evolving at a fast pace. The latest breakthrough is a wireless battery management system which gets rid of CAN BUS and SPI cables. As the battery packs are getting bigger, these wires increase the complexity and thus, elevates the hazards associated with so many wires clogging the BMS.
Various such innovative technologies are currently being developed with respect to Electric Vehicle BMS and associated components. These innovations will empower the ambitious EV program that OEMs, governments and other stakeholders are pursuing with all their might.