If there’s one thing we could do now to hasten the transition to electric vehicles, it’s this: Build a robust public EV-charging infrastructure. While the media has focused on vehicle performance and range, consumers have always been clear that they want electric cars to do essentially everything their old vehicles do—including long overnight trips.
To those who don’t yet own an EV, a robust infrastructure may seem unimportant. Studies, after all, show that in developed markets, as much as 90 percent of all charging takes place in the home. It turns out, however, that the remaining percentage of charging is critically important. Drivers of delivery trucks and taxis, residents of apartment buildings, students on their way to college, families on vacation, and countless others have learned that driving an EV can be a struggle where public charging is scarce or unreliable. A 2022 survey by Forbes, for example, indicated that 62 percent of EV owners were so anxious about EV range that they had at times curtailed their travel plans.
This is no secret to policymakers. A recent brief from the International Energy Agency indicates that in China, investing in charging infrastructure is considered four times as effective for EV success as providing subsidies to EV buyers.
These are issues we’ve been grappling with for decades. Back in 1992, we cofounded AC Propulsion, which offered the tZero, a high-performance electric sports car whose basic technologies and design were later incorporated into the original Tesla Roadster. In the years since, we’ve thought a lot about how to make vehicles that people actually want to own and drive.
The 1997 AC Propulsion TZero was a groundbreaking electric vehicle featuring technical innovations that were later incorporated into the Tesla Roadster.PeteGruber/Wikipedia
When we’ve asked potential EV owners what’s limiting EV adoption, they often point to limited access to charging stations—especially to fast public charging. The operators who own these charging stations have said it as well, and they also cite the high cost of equipment—a DC fast-charging station with four ports can cost between US $470,000 and $725,000. If equipment costs were lower, they say, they would install more recharging stations. It could be a virtuous circle: The recharge businesses would do better, EV owners would benefit, and more people would consider buying an EV.
The question is, can EV charging be done more economically and efficiently? More specifically, is there a way to reduce recharge station complexity and bring down the high cost of fast-charge stations—and, in so doing, significantly boost EV penetration without sacrificing safety?
The answer is yes, and here’s why.
How EV charging works
Before we explain our solution, let’s review some fundamentals, starting with the most basic. A charging station is a physical location that has one or more charging ports, each of which can charge a single EV. Each port may have multiple types of service connectors to support different EV standards.
The function of the port is to convert AC power from the grid into DC, which is then applied to the battery. The recharge current must be controlled so that the following criteria are met at all times: The voltage of the battery cells must not exceed a critical limit; cell temperatures must not exceed a preset threshold; and current drawn from the electric utility must remain below a certain value. If the first two are not met, cells may be damaged or catch fire. If the third is not met, the charger or utility may be overloaded, causing a breaker to trip or a fuse to blow.
The key safety feature of existing EV chargers is an isolation link [in teal]. Within this circuit, a high-frequency transformer provides physical separation between grid power and the electric vehicle’s battery. The isolation link is inside the vehicle’s onboard charger for Level 2 charging. For Level 3 (fast charging), the link is located inside the charging station. Chris Philpot
In addition to these requirements, the charger must protect users from electric shock. That’s not always easy. Chargers operate in rugged environments, usually outdoors, with greatly varying levels of humidity and where contaminated water may be present. Equipment may also be damaged or even sabotaged.
The time-tested way to prevent electric shock is to use electrical grounding. Grounding is exactly what it sounds like: a direct physical connection to the earth that provides a path for electric current. When such a path is present, stray electrical currents—in a chassis, for example—travel directly to the ground, avoiding any people who might be standing close by. In an electric car that’s charging, the green ground wire in the charging cable becomes the path to ground. (Because an electric car has rubber tires, the car itself can’t serve as a path.)
What happens if such a path is not present? If the ground connection in an electric car charger is broken or compromised, the charge port must have a backup solution. Today, that solution is something called galvanic isolation. In galvanic isolation, no direct conduction path is permitted between certain sections of an electrical system.
If an EV charger does not have an isolation link, and the ground circuit is broken and if a current path exists between the battery and the vehicle body, a person touching the vehicle could receive a potentially deadly electric shock [top illustration]. However, with the simple and inexpensive “double ground” circuit designed by Wally Rippel [bottom illustration, in teal], a detector circuit confirms that the ground is intact before closing contactors that enable current to flow. Chris Philpot
The hardware for a charger’s galvanic isolation is called an isolation link, and it works by physically and electrically separating two circuits, so that a difference in potential won’t result in current flow from one circuit to the other. In the case of EV charging, the two circuits are the electric grid on the one hand, and the vehicle battery and its associated circuitry on the other.
This isolation can be a literal lifesaver. Suppose an EV’s battery is leaking. The leaked fluid is conductive, and can therefore produce a current path between the battery circuit and the vehicle chassis. If the ground circuit happens to be broken, then, without isolation, the vehicle’s chassis would be at a high voltage. So a person touching the car while standing on the ground could receive a potentially lethal electric shock (see illustration, “A shock hazard”). With isolation, there wouldn’t be a shock hazard, because no current path would exist from the electric utility to the car body.
Only one component exists that can provide separation between two circuits while transmitting kilowatt levels of power—a transformer. The transformers that connect directly to low-frequency utility power are heavy and bulky. But for EV charging, where weight and size are critical, the transformers are much smaller—they’re not even half the size of a standard building brick. That’s because the charging stations convert DC power to high-frequency AC, using an inverter. The high-frequency AC is then applied to the small transformer, which provides the galvanic isolation. Finally, the output of the transformer is changed back to DC by a high-frequency rectifier circuit, completing the process (as shown in the “isolation link...” illustration).
We’ll get into the details of this power conversion in the next section, but this gives you an idea of how charging is done safely today, whether at a public charger or in a home garage by means of the car’s onboard charger.
Galvanic isolation costs a lot
Virtually every EV has an onboard charger (OBC), which performs the AC-to-DC conversion function, like a public fast charger does, when the vehicle is charging at home. As its name suggests, the OBC resides in the vehicle. It’s capable of providing power levels from about 5 to 22 kilowatts to the battery, depending on the vehicle make and model. Such charge rates are low in comparison with fast charging, generally only available at public chargers, which starts at 50 kW and can go up to 350 kW.
Today, all chargers—onboard and off-board—are galvanically isolated. The galvanic isolation is integrated into the power-conversion hardware, regardless of whether it’s in the car or in a public charger.
A single 300-kW port in a public charging station includes about US $90,000 of power electronics, of which about $54,000 is for the isolation link.
The hardware of an EV charger is basically a much larger and higher-power version of the switching power supplies that charge your smartphone or laptop. Earlier, we gave a basic idea about how power conversion in an EV works, but it’s actually a little more involved than that. For EVs, power conversion occurs in four stages (illustration, “A shock hazard”). In the first stage, AC power, either single-phase or three-phase, is converted to DC by an active rectifier. In the second, DC power from the first stage is converted to a high-frequency AC square wave (think of a classic sine wave but with a square shape rather than, well, a sinuous one) by a circuit known as an inverter. The reason for this high frequency is that in the third stage, a transformer converts the AC to a different voltage, and the high frequency allows this transformer to be much smaller and lighter than it would be for a lower frequency, like that of the power grid. Finally, at the fourth stage, a high-frequency rectifier converts the high-frequency AC back to DC, and then sends it to the vehicle’s battery. Collectively, stages two, three, and four make up the isolation link, which provides the galvanic isolation (see illustration, .
This isolation link is very expensive. It accounts for roughly 60 percent of the cost of the power electronics in a typical EV, and it’s also responsible for about 50 percent of the charger’s power loss. We estimate that the cost of the bill of materials and assembly of a galvanically isolated charging port is about $300 per kilowatt. So a single 300-kW port in a public charging station includes about $90,000 of power electronics, of which about $54,000 is for the isolation link.
Do the math: A charging station with four ports includes approximately $360,000 in power electronics, with more than $200,000 of that going for galvanic isolation. To get an idea of the total costs in a country, say the United States, multiply that 60 percent cost reduction of the power electronics per charger by the multiple ports at the more than 61,000 public EV-charging stations in the United States.
For an EV’s onboard charger, the isolation link adds not just cost but also bulk. The higher the charge capability, the greater the cost and size of the isolation system. That’s why you could never do fast charging with an OBC—the cost and size would be too great to include it inside the vehicle.
These are among the main reasons why we propose to eliminate galvanic isolation. Billions of dollars of capital and energy expenses could be saved. Hardware reliability would improve because the chargers would use about half as many components. Eliminating galvanic isolation—that is to say, eliminating stages two, three, and four of the charger hardware—would also greatly reduce the size of onboard chargers and enable them to handle fast charging, also known as Level 3 power. This is the highest charging level, providing 100 kW or more of DC current.
Tesla Motors unveiled its electric Roadster in Santa Monica in 2006.Glenn Koenig/Los Angeles Times/Getty Images
With the isolation link eliminated, we could then take the next step: having the vehicle’s onboard inverter supply power to the motor for driving and also to the batteries for charging. By having the car’s inverter do double duty, we would cut the remaining costs by half again.
None of this is a new idea. The original Tesla Roadster, which reached the market in 2008, and all of the products built by AC Propulsion successfully used non-galvanically isolated, integrated charging, in which the recharge function was carried out by the inverter. In those AC Propulsion vehicles, the nominal battery voltage was approximately 400 volts direct current, just as it is in most EVs today.
Can galvanic isolation be eliminated?
The requirements for eliminating the isolation link are not terribly complex or costly. Two issues in particular need to be addressed: the risk of electric shock and the compatibility between the utility and battery voltages.
First, let’s look at the shock hazard. Electrocution can occur if three conditions exist simultaneously: The vehicle isn’t grounded, power is applied to the ungrounded vehicle, and a current-leakage path has formed (see illustration, “A shock hazard”). A leakage path might be created if, for example, the battery’s electrolyte has begun leaking, forming a path between the battery and the car body. Because all EV charging systems include a ground connection, a leakage path is a problem only if the ground connection is broken or compromised.
All charging systems, both onboard and off-board, include components called safety contactors, which apply power to the battery only after various electronic checks have been carried out. These checks include ground verification, which tests whether the ground connection is intact. If the ground connection is missing or faulty, charging power won’t be applied to the battery.
EV CHARGING: LEVELS 1, 2, AND 3
LEVEL 1 charging uses standard single-phase 115-volt AC as the charger input. As such charge rates are limited to less than 2 kW. In order to fully recharge a 100-kWh battery, the total recharge time is about 80 hours.
LEVEL 2 chargers are what most EV owners have in their garages now. In the U.S., Level 2 uses single-phase 208 or 240 V AC as the charger input; in Europe, the input power is 380 V AC, three-phase. Maximum recharge rates are limited either by the current rating of the utility service or the onboard charger. At 6 kW, an overnight charge can typically provide an added range of 200 miles.
LEVEL 3, also called Fast Charging or DC Charging, uses an off-board charger that converts three-phase utility power to regulated DC for direct application to the vehicle battery, bypassing the onboard charger (see Figure 3). Maximum recharge rates typically start at 50 kW and now extend to about 360 kW. Chargers are under development that would support rates up to 600 kW, which translate to about 30 miles of added driving charge per minute of recharge.
For Level 2 charging—in a home garage, for example—the safety contactors are located in a module called the electric vehicle supply equipment. The EVSE is typically the size of a large shoebox and may be mounted on a wall or a post. In the case of public fast charging, the safety contactors are an integral part of the hardware.
What this means is that removing galvanic isolation won’t pose a shock hazard. If the vehicle is grounded and leakage causes the vehicle chassis to be at a high voltage, the resulting surge of current to ground will instantly trip breakers in the charger.
So the question then becomes: Can ground verification be trusted to be absolutely fail-safe? In other words, can we guarantee that power is never applied if the ground circuit is broken or compromised—even if components within the ground verification circuit have failed? Such an absolute guarantee is necessary from both moral and legal standpoints. Removing an existing safety factor, such as galvanic isolation, is unacceptable unless it is replaced by something that provides a net gain in safety.
We can do that. All it would take would be a relatively simple modification of the charger circuit.
Such a level of safety can be provided by a double-ground combined with ground-continuity detection (see illustration, “A ‘double-ground’ circuit prevents shock”). This double-ground method is based on—you guessed it—two ground wires. With this scheme, if one ground wire is severed, the other one ensures that the vehicle is still grounded. To further enhance safety, the broken ground would be detected and the power shut down, even if one ground wire was still intact.
Detection of ground-wire continuity is neither expensive nor complicated. One of us (Rippel) developed a prototype detection circuit about a year ago. The system uses two small transformers, one to inject a signal into one of the ground wires, and the other to detect the signal in the second ground wire. If the signal is not detected by the second transformer, the contactors—in the EVSE, for example—are opened so they can’t apply power. With this circuit, the overall system remains fail-safe in the event that one or more components fail.
The arrangement makes charging doubly safe, literally. Moreover, because the two ground circuits are mutually independent, no single failure can cause both grounds to fail. This lowers the probability of a ground failure: If the probability of a single ground failure is P, the probability of both failing is P2. Safety is further improved with the addition of a circuit that senses that the two grounds form a complete circuit; power is turned off as soon as one of the two grounds is damaged or broken.
Eliminating the risk of electric shock isn’t the only issue that we must deal with if we are to get rid of galvanic isolation. There’s also the issue of voltage—specifically, the need to prevent mismatches between the utility’s AC line voltage and that of the EV battery.
A voltage mismatch becomes a problem under one condition—when the input utility voltage exceeds the battery voltage. If this occurs, even for an instant, uncontrolled current can flow into the battery, possibly damaging it or causing a breaker to trip.
The solution to this problem is a device called a buck regulator (or buck converter). A buck regulator is similar, functionally, to a step-down transformer, except that it handles DC current rather than AC. In the event that the utility’s AC voltage exceeds the battery voltage, the buck regulator operates like a transformer and steps it down. In comparison with an isolation link of the same power rating, a buck regulator would cost less than 10 percent and the power loss would be less than 20 percent.
The future of public EV charging
At this point, we hope you appreciate why the existing four-stage scheme for both onboard and public EV charging is unnecessarily complicated and expensive. Three of the four stages can be completely eliminated. This would leave a single active-rectifier stage, followed, if necessary, by a low-cost buck regulator. To enhance safety to levels as high as if not higher than existing EV charging gear, we would add a double ground with ground-continuity detection. We call this improved approach direct power conversion.
Using the DPC approach could cut equipment costs by more than half while improving energy efficiency by two to three percent. That’s precisely what we need at this stage of the EV revolution, because it would make EV charging stations more affordable for operators, and enable thousands more such sites to be built in just a few years, rather than a decade or more. It would also make EVs more attractive to people who’ve resisted buying an EV because they’re put off by the feeble state of the charging infrastructure.
It’s time to simplify the EV recharging process and make it more cost effective. But that surely won’t happen without a discussion of galvanic isolation in the technical community. So let the discussion begin! We’re convinced that eliminating the isolation link should be the first step toward the robust charging infrastructure that the EV transition so desperately needs.
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