For many that grew up around ICE cars, charging an electric vehicle is a whole new experience. While putting gas inside the tank is straightforward and takes a few minutes, ‘filling up’ an EV battery is more complex and takes longer. One aspect of charging that every electric car owner should know about is the charging curve. Keep reading to find out everything about EV charging curves.
What is a charging curve?
Your charging curve is the rate at which your charging speed changes. The fact is that when you use a DC fast charger, the charging speed is not constant from 0 to 100 percent.
While several factors affect your charging speed, including the weather, your battery charging rate is faster when it is near zero percent and slower near 100 percent.
To illustrate, assume you are pouring a liquid into a bottle with a tapering neck. Naturally, as the liquid level gets near the top, you will reduce the pour rate to prevent spilling the liquid on the floor.
In real life scenario, if you have a car rated to accept 120 kW but find a 150 kW charger, your battery will charge. However, you will not be able to take advantage of the charger’s total capacity because your peak charging rate will be 120 kW, the limit of your car. Even maintaining the peak rate is impossible because your charging speed will drop.
This peaking and dropping of speed make up what is known as the charging curve.
The charging peak differs from one EV to another. For example, the Audi e-Tron GT peaks at 260 kW. According to data released by some fast-charging network operators, the e-Tron GT will quickly hit the peak rate. However, by 50 percent, the rate has started to drop, and you would likely be receiving 100 kW at 75 percent. By the time you get to 95 percent, the speed would be somewhere around 50 kW, which while still remarkable, is far from where you started from.
Interestingly, some manufacturers use software to cap the charging speed and usable capacity. If the car can receive updates over the air, like Tesla, both may be moved up or down through a software upgrade.
Why does electric cars’ charge speed slow down after 80 percent?
Many public DC fast chargers slow down after reaching 80 percent. While this may increase the charging time, the purpose is to protect your battery. There are two reasons behind this phenomenon.
First, note that your EV battery is made up of hundreds or thousands of cells. When you plug in to charge, the energy goes into each of these cells. As more cells get ‘filled up,’ it takes longer to locate empty cells to charge. This becomes worse around the 80 percent mark and you would notice a steep drop in charging speed.
The second reason is the temperature. As your battery level moves toward full capacity, it gets hotter and the strain on it increases. To reduce the strain and temperature, EV makers slow down the charging speed. Every manufacturer determines to what extent to reduce the charging speed to preserve the battery health and make it last longer.
How does the charging curve impact charging time?
The effect of the charging curve is easy to see. If you check an EV’s specification, you will notice the manufacturer usually quotes the charging time from 0 to 80 percent on Level 3 or DC fast charging. For example, the Volkswagen ID.4 has an 82 kWh capacity battery (77 kWh usable) which takes about 38 minutes to charge to 80 percent. However, individual tests showed that to charge to 100 percent takes about 65 minutes. This means the battery takes almost as much time between 0 to 80 percent as from 80 to 100 percent.
The charging curve differs from one EV maker to another but generally, moving from 80 to 100 percent happens at a slower speed.
This observable charging behavior has some interesting applications. For example, if you are on a long trip and need to stop over to charge, it makes more sense to stop charging at 80 percent and resume the journey than to charge to the full at each stop. In fact, it would be rather selfish to tie down a public charging station as you try to go from 95 to 100 percent when other EVs owners are waiting in the queue.
Should an EV be charged to 100 percent?
Going by the recommendation by many EV makers, the answer is no. First, stopping your charging without getting to 100 percent helps prolong your battery’s useful life. Also, you can save your time, and of those queuing for a charge behind you by not waiting to get to 100 percent.
However, you can help your battery more by using Level 2 charging as much as possible. This you can easily do by installing your own home EV charging station.
Electric vehicles take longer to charge than to fill up an ICE with gas. However, how long you spend charging depends on your EV’s charging curve and what percent you will stop charging.