A recent post by the U.S. Department of Energy proves what many of us suspected all along: that free charging leads to more use of a limited resource. With this non-Tesla charging data, we can be reasonably sure that not only does free DC fast charging use more power, but it also causes more congestion and lines at busier stations.
The study, published by the Vehicle Technologies Office, covers the period of 30 June 2020 to 30 June 2023. During this time, the dataset includes information from almost 2.4 million charging sessions. On average, drivers spent 54 minutes and the charger dispensed 29.4 kWh of electricity.
This figure is a little surprising, because you’d think that most people would only stay a few minutes and charge their vehicle at peak power for a good chunk of it. But, with an average of under 30 kW of power over all sessions, we’re not talking about much charging speed.
One of the reasons behind this is the prevalence of free charging programs. Out of all charging sessions, free sessions amounted to about 40% of the sessions in the dataset. On average, a paid charging session averaged about 42 minutes, while a free charging session was almost twice as long, at 78 minutes. Overall electricity used was also almost double (40.7 kWh vs 22 kWh).
Why Are Free Sessions So Long?
It all boils down to battery safety. EV batteries can charge quite fast when they’re under 50-60%, but after that the internal resistance to adding more charge gets higher. If you kept the rate of fast charging up, the battery would overheat, get damaged, and at worst catch fire. So, the onboard software in both the vehicle and the chargers work together to limit charging speeds.
Most EVs hit their first “elbow” or “shoulder” in charging speeds when they hit 50-60%, but it’s not a major drop all at once. Around 80%, charging speeds drop a lot more quickly. Finally, toward the very end, charging speeds drop to or even below the vehicle’s Level 2 (AC) charging speeds for safety and cell balancing.
EV drivers usually avoid charging over about 80% because of these slower speeds. If you discharge the vehicle as low as possible and arrive at the next station at 5-10% charge, the vehicle can be at its maximum charging speed for a lot longer. Charging up just enough to get to the next station along a highway means a shorter overall trip time.
Not only does doing things this way help you get where you’re going faster, but it also helps other drivers be less frustrated. If you’re in and out of there in 30-40 minutes instead of 80 minutes, the station will be free for someone else pull in, or it’ll be empty for them to arrive. This means shorter or non-existent wait times. But, when too many people charge for too long, there are more lines and frustration.
But, there are several reasons someone might want to charge up to over 80%, some valid and some not.
The obvious one is that someone might not have enough charge to get where they’re going without a full charge. Along most interstates, this isn’t necessary anymore, but when one ventures into the non-interstate hinterlands, charging is a lot less available. So, it may be necessary to charge to almost 100% just to get there, or to spend less time on a Level 2 station.
This can happen even more when people tow trailers. With higher energy usage, the vehicle’s effective range is lowered. So, you’ll need to charge up more even to make it to the next interstate station.
There are two very non-valid reasons for charging up to or near 100%, and they’re both related to free charging. The obvious one is that the charging is free, so why not? If you’re headed into a place with no free charging, it may even make sense on the surface to charge up to 100% and spend less money at the one that isn’t free for you. But, time is money, and this takes up a lot of extra time.
Another problem that comes up is that people don’t know about charging speed curves. If you don’t know that charging is super slow over 80%, it might make sense to go chill somewhere, like a restaurant or Walmart toilet paper fort, while the vehicle does what your cell phone does and get to 100%.
Solving This Problem
The biggest thing we can do is to get EV charging companies to stop offering unlimited free charging. When people get years or even a lifetime of free charging, the temptation to abuse that power goes up and up. Back when Tesla did this, some people even went home to power appliances off the car and even considered bitcoin mining. The zero price gives no incentive to take only what you need.
For some companies, this is an easy way to encourage EV sales. For example, Volkswagen (which also owns Electrify America) has great reason to give ID.4 drivers free charging. This gives them a sales edge over the competition. But, it comes at the predictable cost of saturating the charging network and keeping other people from being able to depend on it as readily for road trips, so it’s not a good idea in the big picture.
It’s possible to offer perks with limits, though. Free 30-minute sessions with payment after 30 minutes can help people get what they need and move on, for example. Another way to set limits would be to give a certain amount of electricity all in all instead of unlimited sessions. Both of these options would lead to conservation instead of getting stuffed at the all-you-can-eat buffet.
Another thing that’s badly needed is education. Dealers should be explaining things better. Even a simple “try to not charge over 80%, because it goes slow” or “use this trip planning software” would be a great way forward compared to just putting people in the seat and watching them drive off.
Whatever the industry does, it needs to happen. Free charging is causing a lot of problems, and now it’s not just anecdotal experience. We have the data now to show it’s a real menace to EV society. It’s time to move on.
Featured image by Jennifer Sensiba.