How to alleviate the anxiety of power battery life from the system perspective?

文—ED SPERLING

Source— Semiconductor Engineering

Compile | editorial office

The future development of electric vehicles will depend on where and how these cars are used, as well as major advances in battery materials, energy density and complex battery management systems.

Battery power consumption stored for a long time needs to be balanced and delivered in real time to where it is most needed. It's a huge challenge, from the mixture of various elements in the battery cathode and anode to the layout, shape and packaging of the battery and battery modules, any part in the vehicle today is being rebuilt. All of this must be monitored and managed electronically to ensure that individual batteries are still properly charged as they age.

Replacing the internal combustion engine with battery-powered motors is only the first step in a complex technological shift. Increasing the range of a vehicle on a single charge and reducing the time it takes to charge the battery can be a tricky challenge. Battery capacity has always grown at a rate of about 5% to 6% per year. Providing cars with denser storage or more batteries can make the ride longer and the growth rate faster, but the supply of materials needed to make these batteries is limited by geopolitical and environmental issues, so many companies are considering more alternatives.

The question is how to keep the battery charging more charging more resilient while also making fast charging more resilient. "Mixing silicon inside the anode (about 5 to 10 percent) is a solution on the market today, and they are also looking for ways to mix more silicon," said Felix Weidner, a senior engineer at Infineon. "During charging, these lithium ions are pumped into graphene, but a considerable amount of graphene is needed to make a lithium ion. The idea of this scheme is to obtain more energy density through silicon, because silicon can capture more lithium ions. But it also has the disadvantage that silicon is not very stable and diffuses easily. Otherwise you will use a 100% silicon anode. ”

Today, nickel and cobalt are also used to increase energy density and reduce the risk of fire. But the shortage panic caused by the war and limited sources has driven up battery prices and further boosted the price of electric vehicles, prompting global markets to actively seek out richer, easier-to-use new materials.

"Cost is what currently limits people to buying electric vehicles," said Venkat Srinivasan, director of the Argonne Center for Energy Storage Science And Associate Director of the Joint Center for Energy Storage Research. "We estimate that this year's battery costs about $130 per kWh, compared to 90 kWh for a typical electric car. We believe the goal is to reduce it to around $65 per kWh. But it looks like the rise in the average cost of batteries from this year is just the beginning. Although not all car companies, because some companies have signed long-term contracts because of cobalt and nickel shortages. ”

Figure 1: Global lithium-ion electric vehicle battery demand forecast (Source: Argonne National Laboratory)

Typically, just adding more batteries is an expensive option reserved for the luxury car market, which is the only one with a range of 500 miles. Since charging an empty battery module is faster than charging a nearly full battery module, more or larger batteries also have an advantage in terms of charging time. The larger the battery module, the more battery charge it is, and the faster it is to get another 200 or 300 miles of range, because the most time-consuming part of charging is when the battery is about to fully charge.

However, the range may also vary depending on other factors. For example, wind resistance has become a key issue in wheel design. Ambient temperatures and even mountainous terrain can affect range.

Consumer Reports' 2019 field tests showed that electric vehicles reduced their range by nearly half in cold climates. The following year, Car and Driver reported an increase in calories in the Tesla Model 3, with a 60-mile drop in range. The motor doesn't generate heat like an internal combustion engine, so it's basically like attaching a small oven to a battery. Air conditioning has a similar effect on hot weather.

The more frequently batteries are charged, the shorter the service life of these batteries. This usually manifests itself in the fact that the maximum range of power reserves decreases over time, similar to what happens when the battery in a mobile phone degrades over time. However, replacing the battery in a vehicle is much more expensive than buying a new phone. Because this requires maintaining a balance between the battery modules. This, in turn, forces automakers to think more like chipmakers, where the power budget is fixed and new electronics need to adapt to that budget.

"Our goal is to minimize the number of charge cycles because the cost of replacing batteries keeps consumers away," said David Fritz, senior director of autonomy and ADAS at Siemens Digital Industrial Software. "The less power the system consumes, the more efficiently the drive motor will be. We can use composite materials to reduce battery weight, but the effect is limited. The next big challenge is to understand what all electronics do and how much they consume, which can be controlled by shutting down these devices and putting them in a low-power mode. This had to be laid out before we jumped from L3 to L4 and L5.

Battery management

From a purely functional standpoint, battery management is the next big differentiator for automakers. While OEMs will continue to stand out with their gorgeous features, there is limited uniqueness and customization when it comes to electric vehicles' motors and transmissions.

Battery management is much harder than it sounds. Current battery management systems mainly consist of heating, cooling and determining the optimal charge percentage to extend the life of the battery. But that's just the beginning. In the future, battery architectures are expected to become more complex, potentially involving specialized batteries for different tasks and new materials. It is also expected that there will be a solution for undesirable units, almost like ECC memory and DRAM, to maintain the maximum range for a longer period of time.

Several indicators already exist for this purpose. One is the State of Charge (SOC), which is how much energy the battery has at any given moment. The second is the Health State (SOH), which is the percentage of available battery capacity.

"Health will change over time," notes Scott Winder, systems application engineer at Infineon. "There are several factors at play. First, when the charge increases to close to its maximum capacity, chemical changes occur within the battery. If fully charged, the reaction may not last that long. This is something that phone manufacturers have been fixing. In short-haul mode, it is usually possible to charge the battery up to 250 miles, but for long-distance travel, it can be charged to 300 miles. There are also thermal problems due to resistance. It's easier to charge the battery when the battery is empty, but harder when the battery is almost full. Thus, it is possible to start up faster and then reduce the charge depending on the temperature of the battery. ”

Since the battery may overheat and cause a fire, the battery also needs to consider the safety level. "Battery cell health is one of the most important metrics because when you have a car on fire, the problem stems from a single unit unless an accident damages multiple units," Winder said. "Maybe it wasn't charged properly or something malfunctioned so much that it heated to the point where the heat ran out of control." These units are usually sealed in metal boxes that are meant to try to control diffusion, but when there is excess energy, it eventually diffuses into the rest of the vehicle. ”

Companies focused on electric vehicles are particularly concerned about the heat inside batteries, one of the challenges of introducing new materials. Over time, everything needs to be tested under extreme conditions, and in automotive applications, the testing time can be as long as five years.

"Monitor the battery and determine the status of individual batteries," said Roland Jancke, head of engineering design methods at the Adaptive Systems Division at Fraunhofer IIS. "There will be backup units, but these systems need to decide when to switch and what the health is. So a management system and some kind of battery management chip are needed. Building the system requires a complete simulation in which faults can be injected into the entire battery pack, see what happens if one of the batteries fails, and see what the battery management system does. Does it monitor everything correctly? Is the diagnostics working correctly? Does it switch to another unit? ”

Battery architecture

Batteries are physically heavy and need to be installed into the vehicle in a way that improves vehicle handling. They also need to be arranged in a way that provides enough energy for critical functions while minimizing losses and avoiding battery overheating.

"Now, we're putting all the batteries in one battery pack at the bottom of the vehicle, and various OEMs are using a lot of conductive cooling, just passing the heat through the metal belt to the chassis," said Infineon's Winder. "There's also liquid cooling, using a conductive current. And, if the battery is dispersed so that parts are integrated into the chassis at different points, the concentration in one area is much lower. On the other hand, when the outdoor temperature is low, the batteries need to be heated so that they can operate at maximum efficiency. By distributing batteries, you need to heat them in various places. ”

But not all batteries are the same, and not all batteries are suitable for every task. Some systems in the vehicle are always on. Others should only be used occasionally. The response time of turning on the vehicle and viewing the reversing camera requires an almost instantaneous response, while a delay of one or two seconds in starting the infotainment system is often overlooked. Whether this evolves into a distributed battery architecture, using different types of batteries, or better control over existing batteries remains to be seen. But at this point, all options are being explored, including the possible use of hydrogen fuel cells as a backup.

"Batteries are ideal for light vehicles such as passenger cars," says Argonne's Sriivasan. "On heavy-duty trucks, ships and airplanes, the battery energy density is only that much. People are starting to turn their attention to hydrogen fuel, which is carbon neutral, or carbonless fuels like ammonia. As we start to look at decarbonization in the context of different industries, not just passenger cars, we're going to start to see other technologies come into play. In a range of applications such as passenger cars, mobile phones, laptops, watches, etc., there may be some kind of battery. ”

Figure 2: Global reserves of key minerals for batteries (Source: Argonne National Laboratory/USGS/U.S. Department of Energy)

Low power consumption design

Of course, another aspect of the power equation is to improve the efficiency of the chip and system inside the vehicle.

"People will be paying more attention to smarter, more efficient electronics," said Michal Siwinski, chief marketing officer at Arteris IP. "We've seen some of them. Five years ago, car chips were done somehow. Now, it's all a customized process. Some of them are regulation, others are the reality that electric vehicles will continue to exist. But even as batteries become more advanced, you still won't have hundreds, but thousands of different electronic subsystems and chips, all of which will be connected. This will definitely consume the power supply. ”

As with all complex electronics, one of the biggest challenges is figuring out how to divide and prioritize power.

"These changes amount to a transition from multiple discrete ICs to systems-on-chip," says Fritz of Siemens. The way we turn off the power in soCs is by clock gating and turning it on and off when needed. In cars, this may be important, but from the car company's point of view, it's almost against the rules. There are so many ECUs that are performing separate tasks that they cannot be turned on and off. We're working with seven different OEMs and they're all taking a very different approach. One of them is looking for an L4 automatic solution that requires about 4 kW. We were able to model the same solution and based on state-of-the-art technology, rather than some off-the-shelf power-hungry x86 system, we were able to reduce it to 40 watts. Once all peripherals are added, the overall system is 50 watts instead of 4 kilowatts. This has an impact on range and sustainability, with a single charge reducing co2 emissions by about 7 pounds. ”

summary

Battery chemistry, battery management, and battery design are becoming increasingly complex. The trend is still batteries that can be recharged quickly, last hundreds of thousands of miles of roads, and are both safe and relatively inexpensive. As with most complex electronics, some complex architectures require trade-offs and constant reflection on the power delivery and storage systems within the vehicle. So while features such as sound damping and slick displays are used to attract buyers, most people have never seen these technologies because there are a lot of innovations and experiments still underway.

"Automakers have been trying to separate engine starting power from infotainment and electronics and put it on separate electric buses," said Marc Swinnen, director of product marketing at Ansys' semiconductor business unit. "For electric cars, it's different. These large batteries are about 60 to 70 kWh. For electric cars, it's all about range, and one of the big problems is heat. This may explain why people in Michigan and Dakota are less enthusiastic about electric cars. ”

This remains a huge challenge for the entire EV ecosystem.