Q&A: Karthik Ganesh, battery engineer at Mercedes-Benz Research and Development
The battery pack of an electric vehicle is by far the most poorly understood vehicle system.
Owing to its electrochemical nature and its relatively recent adoption in commercial electric vehicles, the optimal design of a battery pack is tricky and often overlooked.
With a multitude of components in the system including the cells, bus bars, relays, fuses, connectors, sensors, PCBs, and the battery housing and its structural components, it is not very easy to understand the electromagnetic behavior of the system and the interaction and corruption of data and power signals within it.
The use of a software tool with the capability to solve Maxwell's equations is only half the battle; the fundamental solution is to be able to represent/resolve the components in a high enough fidelity that enables us to understand their behavior under high frequency input.
Automotive IQ's Battery Management Systems conference is a dedicated forum on this topic taking place in Berlin, Germany in September. Ahead of the event, we asked one of the speakers, Karthik Ganesh, battery engineer at Mercedes-Benz Research and Development in India, for his thoughts on the key issues facing those working on BMS, including those on the effect of electromagnetic interference on battery systems.
What should be taken into consideration when dealing with electromagnetic interference?
The problem of electromagnetic interference was first recognized and addressed in 1933 by the International Electrotechnical Commission, but the regulations and methods employed to study it and control it have gained relevance with the advent of modern electronics in the last thirty to forty years.
The introduction of the electric vehicle to the commercial personal transport space has seen the incorporation of more electronics in the modern electric car than ever before. Each of these subsystems operates at different frequencies – for instance, the switching frequency of the DC-DC converter is in the range of 20 to 100kHz, the CAN bus operates between 250kHz and 1MHz and the microprocessor clock frequency for the Battery Management System could very well be in GHz.
The switching behavior of power converters, the noise current flow in electric motors, unshielded cables or bus bars and microprocessor clock frequencies all contribute to electromagnetic noise. It is imperative to understand not only the role of different components as electromagnetic sources, victims or even as paths but also to understand how the battery system is being affected at high frequencies.
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What are the most recurrent challenges you face when designing these systems?
The design of systems with a focus to minimize their electromagnetic footprint has been in existence for several decades now. However, the battery system is relatively new; the electromagnetic interference of this system has very limited literary precedence.
The limited understanding of the implications of high frequency on the battery could prove to be detrimental in designing the systems around it; the system not only suffers electrical effects but also electrochemical and thermal effects.
The corruption of data streams to and from the battery management system through the CAN line could lead to charging and discharging issues of the battery, improper cell balancing, failed current derating because of faulty temperature sensor data and even failure to open and close relays in case of a catastrophic accident.
However, the electrochemical impacts could be much more severe – from accelerated aging because of cell electrical degradation to high temperature zones within the cells due to thermal impacts of high frequency charging, thus compromising battery pack safety. Even if it were not for safety issues, the mere cost of the battery pack justifies an optimal design so that customers do not have to replace it earlier than they need to.
How does the powertrain electrification affect the vehicle’s architecture? What new concepts should be kept in mind?
Though the design and styling of the electric car may not be substantially different from that of a conventional internal combustion car, the powertrain architecture of the electric car witnesses a radical departure from its gasoline counterpart.
Right from a single electric motor with no transmission to hub motors for each wheel with an efficient control system ensuring tandem operation, a variety of architectures are available for the electric vehicle.
However, the battery tends to be the central element around which the vehicle architecture is developed. With the transition in Tesla's Model 3 motor from AC induction type to a more efficient permanent magnet motor, it is clear that the future might see previously unconventional motors like the switched reluctance DC motor.
There are also ideas around having a single box that houses all the power electronics modules in it to reduce its electromagnetic footprint; some other ideas and concepts include the full integration of the motor-generator, transmission system and all the power electronic devices.
Use of a hybrid battery-supercapacitor architecture for electric vehicles to handle drive cycle transient power requirements is also not out of the question.
What is your point of view regarding the future of transport in electric drives?
It is very evident from the data collected that the main reasons for low sales of electric cars are the high initial cost of batteries, range anxiety and inability to predict when the battery would need to be replaced.
Most researchers predict that lithium-based batteries are going to be around for another decade or two at least. However, major automotive companies are researching on how to increase the energy density of batteries through improved materials or solid-state electrolyte batteries and even metalloid-based and organic batteries.
Studies are being performed to understand the optimal charge-discharge pattern of batteries and ideal temperatures of operation of batteries to prolong their life.
Graphene is being explored as a super-material that improves not just the energy density but also life, safety and performance of batteries and supercapacitors alike. Of course, batteries that are ultra-fast-charge-capable, last for years and are clean will be inevitable, given the amount of effort being put in for the transition to electric vehicles. Improving the efficiency of over-the-air chargers is vital to the battery ecosystem as well.
It is very probable that the future witnesses more than a few transitions in the battery space in the next decade. While the battery predominantly plagues electric-car sales, the electric motor proves to be an area of interest to automotive companies; research is underway to improve the performance, thermal management and efficiency of electric motors.
For instance, the AC induction motor is powerful but it has lower efficiency because of the current requirement to activate the magnetic currents inside it; however, the permanent magnet motor is not as powerful but significantly more efficient.
In the future, the transition will be from conventional motors to motors with a non rare-earth metal that is powerful enough for the vehicle.
Thermal management is another avenue for manufacturers to contribute to motor efficiency; the traditional way of motor cooling is likely to be replaced by hollow-conductor cooling where coolant is forced inside the microfluidic channels of the hollow coils of the motor to dissipate the heat.
Use of this technique might also force all motor coils to be a more nestable cross-section rather than the conventional circular cross-section.
Precision control and motor torque vectoring will be improved for twin motor units; this will improve the performance and efficiency of the motors both in the torque-provision mode while propelling the vehicle forward and resistance-provision mode during regenerative braking.
More research will be done on reducing the weight of the motor while increasing the rev range; further, a lot of research will be focused on in-wheel hub motors because of their light weight, excellent performance and small footprint.
What improvements are needed in the BMS in order to handle energy demand efficiently?
The ubiquitous Battery Management System is the brain of the battery pack and is crucial to ensure the proper performance and safety of lithium-ion batteries. The evolution of the BMS from a mere battery performance monitoring device into an intelligent device that takes care of the different modes of battery operation and optimizes battery life and performance has been rapid.
The modular topology of the BMS is by far the most efficient by design; however, the distributed system has its own advantages if implemented properly. The ability to collect information from each of the cell – current, voltage and temperature, along with a prediction of its electrical state and internal cell core temperature is very valuable; for this purpose, a coupled electro-thermal model-based BMS algorithm would be used for joint state estimation.
This helps in moving away from traditional charging patterns to more optimized charging patterns for the pack. Active cell balancing algorithms will need to be the norm so that the round-trip efficiency of batteries will be improved greatly.
Inclusion of an electro-thermal model would also serve as a redundant SOC estimation method. The current methods of estimating SOH are either too simplistic and sensitive to disturbances, or too demanding in terms of the parameters required for modeling.
Machine learning and genetic algorithms have already been employed to predict cell performance parameters. Use of techniques that use fuzzy logic and independent cell toggling eventually leading to age-equalization in addition to Extended Kalman Filters or AC impedance estimation methods would greatly increase the prediction of cell life. Further, the BMS and its sensors need to be EMC compliant to make sure that data corruption is eliminated.
What do you think will be the future trends in battery electronics?
With a complete transition to electric vehicles, automotive manufacturers will be looking to make battery packs in the 400V to 1000V range. The traditional battery electronic components, such as the thermal management system controller, current, voltage and temperature sensors for the cells and the battery module, along with the high voltage interlocks and the isolation monitoring system, will not just need to be more accurate but also more modular.
With the increase in number of cells, these sensors would need to be smaller, cheaper, wireless and more accurate with the capability of being integrated into an IC along with its counterparts. For instance, the cell parameter monitoring circuit of the future could house a cell switch to aid age-equalization, along with voltage, current and temperature sensors; this would be in the form a tiny IC that can be coupled with an individual cell and communicates over the air with the Battery Management System.
The algorithms for predicting the battery life, optimize battery performance and improve battery safety all require storing and processing extensive amounts of data; to facilitate this, the battery management system would need to be a full-blown computer that measures, processes and computes the data in addition to communicating it over the air to the main vehicle control unit or a cloud-based system for diagnostics.