|Why EV Battery Design Is So Difficult|
October 3, 2019 - By: Ann Steffora Mutschler
Automotive batteries always have been treated as plug-and-play parts of a vehicle, but that approach no longer works in electric vehicles. In fact, the battery is now a differentiating factor, and it is the heaviest and most expensive component.
What used to be a relatively simple component has been replaced by a variety of sensors to measure complex static thermal and aging effects, as well as dynamic thermal and self-healing effects. In addition, batteries now must be included in the overall architecture of a vehicle because they add a number of challenges that can affect the operation the entire system.
“Everyone recognizes the battery is the most important and challenging component of an EV because it determines the crucial factors such as the range, how much the vehicle is going to cost, safety, convenience of recharging the vehicle,” said to Bryan Kelly, an R&D engineer at Synopsys. “Limited range, high cost, battery issues and spotty charging infrastructure are the main challenges of the battery electric vehicle. The battery provides the energy, but something needs to direct and control it for the purposes of operating everything else in the EV.”
This is where system-level considerations come into play. “Compressors, pumps, motors, auxiliary power modules and the onboard charging module — there are many types of power electronics needed,” Kelly said. “Also, along with the fact that things are switching on and off, which naturally points the finger at the power semiconductors, range is always an issue. So bigger batteries are being built. They are higher-power, and the power electronics need to deal with that. They need to switch faster, because the faster you switch the less power you’re going to waste, and it’s all about efficiency. That’s the bottom line.”
Autonomous vehicles make this much more complicated. “You really do need to take the whole system approach earlier on and consider the battery inside of the system,” said Jim Patton, manager of applications engineering at Synopsys. “This is driving a greater need for simulation. Autonomous capabilities are very power-hungry. You have a lot of cameras, a lot of image processing, a lot of additional components that are on the vehicle consuming electrical power. That needs to be taken into account because it’s a another huge drain on the vehicle.”
And all of this needs to be set in the context of safety. “You must simulate fault capabilities so you can automatically insert open circuits and short circuits, and programmatically run a large number of fault scenarios to say, ‘What happens if my alternator fails? Am I still able to limp my autonomous vehicle off to the side of the road? You need to be able to handle that fault and have a mitigation strategy in place for handling those kind of faults. You want to anticipate the worst case scenarios, and then switch power from non-lifesaving modes and maintain power to your brakes and things that really matter,” Patton said.
One of the big differentiators in EVs today is the charging speed. “Ten years ago when I started working on this field, battery packs cost more than $1,000 per kilowatt hour,” said Puneet Sinha, director of new mobility solutions for the mechanical analysis division of Mentor, a Siemens Business. “For the battery in the Tesla Model 3 or the Chevy Volt, today the cost is approximately $150 per kilowatt hour. The cost has come down tremendously for batteries.”
Alongside of those price drops, there needs to be a way to charge those batteries faster. That changes the fundamental design of the battery.
“The cell development has to be done in a way that the cells can accept fast charge without having a tremendous loss of age, in addition to not having any bad events happening to them,” Sinha said. “It’s about the material chemistry in the cell. More than that, how you design the electrode matters also at the battery pack level. These considerations are important because when you’re fast charging, you’re dumping 150 to 200 kilowatts of power, and when you’re charging you are going to create a lot of heat. So companies are looking into how to do the cooling of the battery pack while they are charging the same pack.
Dean Drako, CEO of Drako Motors, understands these issues. The company designed the battery for its GTE luxury electric supercar.
The GTE’s battery was designed from the ground-up for megawatt power output, as well as cooling capabilities to withstand track-level performance on the world’s most challenging circuits, the company’s website says. “With 90 kWh of energy capacity and the ability to output 1,800 continuous and 2,200 peak amps, GTE’s battery is designed to supply GTE’s four motors with 900 kW of uninterrupted power. An internal massively-parallel cooling architecture integrates numerous cooling channels surrounding each individual battery cell to quickly dissipate heat lap after lap.”
Drako said the company has been working very hard on this, and that the design of the lithium ion battery was part of the thought process of the entire vehicle architecture. “We picked a very different design point than the other manufacturers, which are just trying to get you range or other things, because we are going for the sports car market. We wanted the car to be able to race on the track, and that takes a lot of power and design, and a lot more cooling to keep the battery from overheating. As the battery discharges and as it charges, some of the energy goes into heat, which is just the fact of the chemicals doing what the chemicals do.”
The challenge in designing the battery and the battery management systems was that the acceptable operating temperature range for a battery is very narrow, and operates in difficult conditions. So if one cell in the battery is off and overheats, it could start a chain reaction.
“This is why people like to use regular standard cells, because the manufacturers have done a lot of testing and a lot of work to make sure they don’t blow up,” Drako said. “As a result, what you have to do in this battery pack is include a ton of temperature sensors, voltage sensors, current sensors, sensing everything all the time. There’s literally a fuse for every battery and microcontrollers controlling this whole thing. And it overheats, so you have to pump liquid through the whole thing to keep it cool. Designing the battery pack is horrifically hard to get the reliability, the safety, the robustness, the power out of it.”
Mentor’s Sinha agreed this involves much more than just pulling an EV into an air conditioned garage to charge. “Many of our customers are looking into these aspects. If you look at this system, the cell is important, but it’s a small part of the whole problem. You can have the most optimized cell in the world, but it is of no use if you’re not doing the right thermal management for the battery pack while it is being charged. You don’t want to dump too much energy because you don’t want to waste it. So it’s managing the system in the right way so that you can achieve the charging time in less time without wasting energy or without taking the energy from the grid and dumping it. All these are critical issues when it comes to DC fast charging.”
At the same time, all of these issues feed into the entire vehicle architecture, he said. “These are the things that very few humans, if any, can chalk out on a piece of paper and say, ‘If this happens, I need this. If that happens, I need that.’ So as you’re looking for these hundreds of permutations and combinations, this is where specialized software comes into play so you can look into all these considerations such that from the beginning, if you do this, this happens; or if you do that, that happens. What are my options? What can make it cost effective? Then you make the right decisions. This is where we interact a lot with our customers in delivering the right design software so when you’re thinking of the vehicle architecture, you can define what is the right architecture for the vehicle depending on what the usage profile is.”
Each stage is important, and they are connected. “That is the big difference between how you engineer and consume internal combustion (IC) engines versus electric vehicles,” Sinha said. “IC engine vehicle technology has matured so much that they are siloed. You can develop an engine in a silo, and then just plug it in. You can develop the vehicle body and find the right seats and plug that in the cabin. Everything can be siloed or compartmentalized. This is not the case with EVs.”
Where’s the data?
In some areas of semiconductor design today, there is an overabundance of data. The opposite is true for battery design.
“Data is so hard to obtain, and I’ve been literally banging my head on this issue for years trying to get data and work with customers and manufacturers to get data,” said Synopsys’ Kelly. “As a result, [the Synopsys tool] doesn’t possess artificial intelligence. What it does, with a minimal set of data, is guess what the performance will be.”
That creates a reasonable model that can be plugged into the system-level simulation and run against things like typical European drive cycles that are in the tools, with the ability to the plug in the initial state of charge, what the pack is made up of, and so forth. This allows the engineering team to look at the voltage and current variations across cells, energy consumption, state of charge behavior, and even efficiency and range.
These models are critical to electric vehicle design, said Synopsys’ Patton. “Clearly, the battery is fundamental to an electric vehicle, and it’s probably one of the most, if not the most challenging component. That’s where you see the competitive pressure coming to bear because right now the battery makes the difference in an electric vehicle. It’s difficult to balance weight and power. You can add a lot more batteries, and thus, a lot more power, but every time you do you’re adding to the weight and you have to counterbalance that.”
There is also a lot of attention paid to the chemistry of the batteries. “You see a lot of different chemistries and a lot of different approaches to the cell layout and so forth, but sometimes what’s often overlooked is that there’s a lot of challenges beyond just the chemistry,” Patton said. “The power electronics are the things that actually enable the electrification connecting the battery to the traction motor, or to the rest of the vehicle, so those power electronics are critical, as well. You really need to maximize efficiency there.”
The HVAC system is another area that needs improvement. “It’s up to a 40% burden on the battery because you have to cool the battery,” he said. “Then, with the heating and cooling of the cabin, you see that some electric vehicles struggle in the winter in northern climates. It just really puts a drain on the battery when you turn the heat on, so this is a direction our customers are pushing to try to better handle the entire system.”
All of this add up to big system-level challenges.
“When you approach things, you have all these different interacting devices in system design, and things are so complicated now that you really don’t know what you’ve got until you simulate it,” Kelly said. “You need the big picture because certain interactions you can’t mathematically really calculate anymore. There are just too many moving parts, so to speak. Simulation is no longer a luxury. You must have it. In addition, the battery itself is very nonlinear with respect to temperature and the state of charge, which is a reflection of the internal energy that’s still available in the battery to power things or whatnot. There are other nonlinear effects going on, as well. So on a system level, everything is driving in unexpected regions of operation. There are a lot of variables. You no longer can even throw a dart at a side of a barn and guess where it is. You really need the simulation to understand it.”
These challenges could open the door to more advanced technologies, such as on-chip monitoring, where tying it to thermal guardbanding can allow users to eke more life out of a battery, said Stephen Crosher, CEO of Moortec. “Whether you’re trying to get an extra 15 minutes of playback on the phone, or you’re trying to keep the electronics draw on the batteries down because it’s an electric vehicle, anything we can do to improve battery life will overall incrementally add to the user experience.”
They also open the door to a variety of new materials that have never been considered in automobiles, which are being used around the battery.
“Traditionally, people have used silicon to do power electronics, but most recently, they’ve started to move into wide bandgap semiconductor materials that have wonderful properties that make them better for high voltage, high power electronics,” said Graham Bell, vice president of marketing at Silvaco. “Silicon carbide and gallium nitride are the new guys on the block.”
The growth projections for SiC reflect the need for these kinds of material changes.
“The CAGR for the next 5 or 7 years is approximately 30%, mostly driven by hybrid electronics and EV vehicle electronics,” Bell said. “This covers a bunch of different pieces for the infrastructure, including the power unit, power converter from the wall, to the DC power to charge the battery. There’s also some power conversion electronics they’re going on. Another area of use are the control electronics that drive the servo motors in the car. And, of course, there’s the other related to actual power regulation, the sort of metering up the current at the right way, the right time, to the different parts of the car, including the motors and all of the other electronics,” he said.
At the end of the day, the next 10 years will be a thrill ride for all in the semiconductor and automotive ecosystem to see how the automotive industry changes, and which companies will come out as winners, Mentor’s Sinha said.
“When we look at the kind of activities happening in the world, it is clear the companies that can understand, master and give the right importance to the connectivity, among different disciplines — electrical, electronics and mechanical, together from the beginning, not continuing with the siloed way of doing things that the automotive industry has largely done in last hundred years — those are the companies that have better chance to win, whether it’s a human-driven electric vehicle, or a Level 4 or Level 5 electric vehicle,” he said. “Connectivity among these different domains in terms of how we’re designing these vehicles, how you are producing these vehicles and how you are going to consume the experience of these vehicles — all of his has to be part of the discussion in a connected fashion, not in a siloed way.”