The 2023 Audi E-Tron and 2024 Subaru Solterra each offer a peak DC fast charging rate of 150 kilowatts. The Subaru’s battery size is about three-quarters of the Audi’s. If they both offer the same charging speed and one has a smaller battery pack, the Subaru should be able to charge faster. But it doesn't.
In two independent tests, the E-Tron took 30 minutes to reach 80% from zero, whereas the Solterra needed 38 minutes. Not only did the E-Tron take less time to charge, but it added more energy in the same period. So what gives?
Before getting to the answer, we must learn some fundamentals of charging an electric car. EV powertrains are generally simpler than internal combustion engine ones, but that doesn't mean the engineering behind them is. Many factors go into an electric car's charging capabilities. Battery cell chemistry, charging protocols, thermal management, and ambient conditions are all key attributes determining how long you'll spend at the station.
In the real world, your charging times might differ from the manufacturer’s listed numbers. And all these factors will play a role in some capacity. Worse yet, some specifications are straight-up misleading and don’t reflect realistic conditions at all.
If this story has any goal, it's to say that you shouldn't expect a manufacturer's peak charging rate to always be reproducible. Like the curve in which an EV charges, there is a learning curve to understanding how this process functions and what affects it.
How Does DC Fast Charging Work?
You've just arrived at the empty movie theater early. It seems as if everyone else had your idea, though. People are piling in. But right now, it's effortless to find empty seats. People are still heading into the theater, but some of the rows are now filling up. People are starting to climb over others to find a place to sit down.
As the last few stragglers are making their way in, they have difficulty finding spots. The room is now packed, and people are no longer trying to get in.
Equating a battery pack to an empty movie theater or parking lot is one of the best ways to visualize what is happening while charging a battery. Replace people with the negatively charged particles called electrons, and it should be pretty straightforward. But like anything involving chemistry and circuits, the reality is usually far more complex. For instance, here's a brief overview of what's actually happening when you're charging.
When charging an electric car's battery pack, the charger itself (often known as an EVSE, or Electric Vehicle Supply Equipment) supplies current to the battery. The electric current drives a process where electrons are pushed from the positive side (cathode) to the negative side (anode) of the battery's cells. Since electrons are negatively charged, they will "want" to flow to the positive side. When discharging, the process reverses: electrons flow from the negative to the positive side, generating power to drive the vehicle.
A battery naturally wants to discharge, then. When you're charging a battery to a high state of charge, you're putting the electrons somewhere where they don't want to be. Therefore, the charge rate will eventually slow down as you're topping the battery off. This is why your car will charge rapidly at lower percentages but slower at higher ones. But this is merely one of many factors that result in a reduced charge rate.
Thermal Management And Charging
While the battery chemistry plays a significant role in the rate at which an EV charges, heat transfer is another force. Any wire has resistance. When an electric current flows through a wire, some energy will be lost as heat. The more current, the more heat generation. When DC-charging a battery pack, the cells will likewise begin to heat up. Some heat is actually suitable for DC charging, but too much for too long can accelerate battery degradation.
If the battery becomes too hot, the car must tell the charger to reduce the current to protect the cells. If it's warm outside, the car will simply have to work harder to keep the battery temperature in an optimal range. If the battery temperature nears or goes outside the optimal range, then the current in the pack must be reduced. Therefore, the charging rate will be reduced.
Conversely, a cold ambient temperature can result in a reduced charge rate as well. The colder the battery, the more viscous the electrolyte (the liquid between the cathode and anode) will be. If the electrolyte is warmer, it'll be easier for particles to flow throughout. However, the particles will face more difficulty traversing if the electrolyte is colder and more like honey. Temperatures on any extreme can inhibit the rate at which an EV can charge.
This is why some EVs feature a battery preconditioning feature. If you’re en route to a charger, it’ll make sense to warm up the battery so it’ll accept the maximum possible current. Not every EV has this feature, but it’s nevertheless fair to see a reduced charge rate in colder months. But if you’re charging in the Rub’ Al Khali desert, the car’s cooling system will have to work extra hard, and you’ll likely see a reduced rate as well to protect the battery.
Why Is My EV Charging Poorly?
The other side of the debacle is the charger itself. Unless you’re driving a Tesla or Rivian, the charger was probably not made by an automaker. Rather, there are a few manufacturers like SK Signet, ABB, and BTC Power that supply their hardware to various charging networks that often primarily work on the software side. ChargePoint is one of the few networks that builds its own chargers in-house. And all of these options have attributes that can affect your time charging.
The most obvious would be utilizing an underpowered charger. If your car has a peak charge rate of 250 kilowatts, and the charger is a 62.5 kilowatt ChargePoint unit, then you'll be limited by the charger itself. The peak rate you will see should be just 62.5 kilowatts at most.
Another problem is that chargers often have to share power between multiple stations. You may have noticed faster charging speeds when you have the station to yourself; there’s a reason for this. An electric car station might have a maximum power capacity, primarily designated by the grid connection via the on-site transformer.
If a station has a max power capacity of 400 kilowatts and all the stalls are empty, your depleted BMW iX could accept up to 195 kilowatts. But if four depleted BMW iXs are plugged in, each would accept 100 kilowatts, as the power would need to be shared.
The other issue to consider is the mismatch in EVSE and EV voltages. This is a bit complex and deserves a detailed breakdown on its own, but 800V EVs require special hardware to charge on lower-powered EVSEs, like 480V Superchargers. To supply energy from a 480V charger to an 800V battery, the EV needs an onboard voltage boost converter to step up the voltage to 800V and reduce the current. This hardware is typically expensive and adds weight and complexity.
To determine the voltage of the charger, you may need to look for the FCC compliance label on the unit itself. But most of the time, if it’s a high output unit (350 kilowatts), you can safely assume it’ll charge an 800V EV without invoking the boost converter. Just be considerate around V3 and older Superchargers and low-output units (anything under 60 kilowatts). But then again, a low-output charger would not be particularly fast anyway.
Right now, higher-powered EVs like the Lucid Air and Hyundai Motor Group’s e-GMP cars will charge at a reduced rate when utilizing lower-voltage chargers. Their voltage boosting process will only deliver around 50 kilowatts to the battery, a similar rate to that of the Chevrolet Bolt. Operating with the lower voltage chargers is a constraint for 800V EVs.
Manufacturer-Designated Charging Curve
Whenever an automaker mentions charging, the usual number it shares is the peak charge rate. For instance, when Tesla says the Model 3 can charge at 250 kilowatts, it's just saying that the car can accept up to 250 kilowatts at some point in a charging session. These numbers usually reflect ideal conditions, so someone charging in a blizzard in the arctic tundra might not ever see that 250-kilowatt figure in a charging session.
However, the extent to which an EV can sustain its peak charge rate is really up to the manufacturer's discretion. Some cars hit (or don't even hit) the peak rate for just a brief moment and then massively taper off the power coming from the EVSE. Others take a different approach. Some code the in-vehicle ECUs to sustain the peak charge rate for an extensive period.
If you’re new to the world of electric vehicles, you’ve probably heard veterans talk about “charging curves.” What this means is an EV will charge faster at a low state of charge and slower at a high state of charge. Plugging into a fast charger at 60% won’t get you going very quickly. But some EVs can sustain the high rate for a long time while others cannot.
EV charging curves typically follow one of two trends: "steep" or "flat." A steep charging curve refers to a graph with a slope that either reaches or comes close to the manufacturer's peak number and then quickly drops off. If an EV reaches a peak of 250 kilowatts but then drops to 100 kilowatts by the 50% state of charge mark, it would have a steep curve. A flat charging curve will have a horizontal slope for some time. If a car has a peak of 250 kilowatts but sustains that to around 50%, then it would have a flat charging curve.
The answer to why EVs have different charge curves depends on a lot of factors. Sometimes the battery management system can't quite handle the thermal load. Sometimes charging can also be limited by specific components, like the battery cells or insufficient high-voltage cabling. The battery cells may have poor insulation, so they get hot quickly. But just by looking at the charging curve, there is no way to know the root cause precisely.
E-Tron vs. Solterra: When 150 Doesn't Equal 150
Going back to the Audi E-Tron and Subaru Solterra charging question, the answer could be a mixture of many factors. But here's what we do know: the Audi E-Tron generally holds a really flat charging curve with an average of 138.9 kilowatts from 10 to 90%. As for the Solterra, it doesn't have a very flat charging curve. Based on EVKX's data analysis, it averaged 71.4 kilowatts from 10 to 90%.
The real issue lies within the marketing. If two automakers say their cars can accelerate to 60 mph in 5 seconds, you'd expect similar results when testing both. But when it comes to charging, it's the wild west. Pair deception with a lack of understanding, and it's a nightmare for customers.
For the first-generation E-Tron, Audi isn't exaggerating about its 150-kilowatt charge rate. The E-Tron can charge at or very close to 150 kilowatts for up to 80 % state of charge. With the Solterra, you might be able to attain 150 kilowatts in ideal conditions, but it likely won't sustain it for very long. In fact, by 80%, the Solterra's charge rate is in the 30-kilowatt range. And this isn't a one-off event. Plenty of owners report incredibly reduced charging rates on forums.
Through this lens, Subaru and Toyota are being a little unclear about their cars’ capabilities. You'd think the car will have similar charging performance to other options marketed with 150 kilowatts, but the Solterra and bZ4X won't.
Conclusion
The answer as to why the E-Tron charges better than the Solterra isn't straightforward. Electric car batteries are complex pieces of engineering developed upon foundations of electronics, chemistry, and heat transfer. There's no way to determine why a car will charge poorly unless you take it apart and scrutinize every component. Does the Subaru Solterra perform badly due to a flawed thermal management system? Inadequate battery cells? An artificially limited charging profile to promote battery longevity?
These are all possibilities, but this doesn't address the overarching issue. Simply put, like zero to 60 mph times, some automakers will be conservative with their estimates and others won't. Unless all manufacturers publish their charging curves or share an average (the integral) instead of a peak rate, buyers must do some independent searching.
Nevertheless, in the world of automotive marketing, 150 equals 150. But more scrutinized testing prove that 150 and 150 might be more like 138.9 and 71.4.
Andrew Lambrecht is a contributor at InsideEVs and an industrial engineering student at Clemson University. He also contributes to Forbes Wheels and Business Insider, covering the transportation sector.
