Updated: Mar 31
Everyone who has charged an electric vehicle at a fast charging station knows the problem...
The first 80% is extremely fast, and the remaining 20% seems to take forever.
Also, the typical car manufacturer’s claim that you can charge 50% of your vehicle in less than 30 minutes is misleading. Strictly speaking, it may be accurate, but the second 50% will take much longer.
The simple answer is: because of the constant voltage phase (CV phase).
But, maybe you’ve noticed that Tesla cars seem to struggle with these issues less than other manufacturers.
Is it true? Why is Tesla doing to prevent this problem?
In this article, we want to give you a simple overview and a technical explanation of this phenomenon. We’ll touch on how batteries and some electronic components work and how understanding the CV phase can help us charge EVs more smartly.
What is the state-of-charge (SoC)?
Before diving into the physics, let’s quickly define the term SoC. State-of-charge is one of the most commonly used terms when describing the charging of electric vehicles.
When talking about an SoC of 80%, we mean that the vehicle battery has received 80% of its total capacity. For instance, 80 kWh of a total 100 kWh that a battery pack can store.
You’ll often read that the CV phase starts with an SoC of 80%. However, that is not entirely true. It depends on the electric components, the battery, and other factors. But 80% is a pretty good average.
What is a capacitor, and why does it matter?
A vehicle has a large battery that stores energy. A battery works similarly to a capacitor. Even the definition sounds like a battery: a capacitor is an electronic component that stores electric charge.
More precisely, when using a capacitor in an electric circuit, electrons are “stored” on one side of the capacitor. These electrons cannot pass through the capacitor. Let’s call this the “charging phase.”
Once you disconnect the capacitor from the energy source and connect it to an electric device such as a light bulb, the electrons slowly flow from one side to the other.
How does the CV phase work in capacitors?
It takes a certain amount of time for the capacitor to store the maximum amount of electrons during the charging phase. Let’s say 10 seconds. This time depends on many factors, such as the size of the capacitor.
Now for the critical point:
If we measure the voltage at the beginning of the charging process, it will show zero volts (0V).
At the end of the charging time (when the capacitor is fully charged), the voltage will reach its maximum. Let’s say 1 volt, for example.
In between the start and finish of the charging stage, the voltage increases along a curve, as shown below.
We see that the voltage stays nearly constant during the last phase. This is the constant voltage (CV) phase. When the voltage remains almost constant, it is “difficult” to store more electrons. Therefore the charging process slows right down.
How does a battery work?
Batteries and capacitors are similar as both can store and release energy. However, batteries can keep the energy when they are disconnected. This makes their potential applications different.
The only reason we are comparing batteries and capacitors is that they both share the CV phase characteristic.
In contrast to capacitors, the batteries store energy in a chemical form. The chemical unit, called the cell, contains three main parts; a positive terminal called the cathode, a negative terminal called the anode, and the electrolyte.
This anode and cathode also exist in capacitors and have similar functionality. Consequently, we see a similar effect. Towards the end of the charging process, the voltage in the battery remains nearly constant. This constant voltage reduces the charging speed (CV phase). The battery charges a lot slower towards the end than at the beginning.
(Important note: Batteries of electric vehicles always need DC power.)
Why is the CV phase an important factor for charging electric vehicles?
Well, first of all, it is essential to know that only because the first 80% SoC took only 30 min, it doesn’t mean that you can leave with 100% SoC in a further 7 minutes. It will definitely take longer.
For drivers, it’s important to plan this time during long-distance trips or even consider not charging 100% in one go. Instead, plan a second stop when the battery is below 50%.
If you operate multiple vehicles, such as fleet vehicles, you have to pay even more attention to this as it has more significant implications when charging multiple vehicles.
Here is a simple example: A 100 kW charger will not charge a 100 kWh vehicle battery in 1 hour. It will more likely take 1.5 hours. We have explained how to deal with the charging of electric fleet vehicles in a previous article.
How does understanding the CV phase help with EV charging?
Once you understand how the CV phase works, you can use it to your benefit.
Here are three ways how you make sure you benefit:
Do your research on vehicles and manufacturers
Some manufacturers have tested the effect of the CV phase more than others. Many have optimized their battery cells to reduce the effects of the CV phase or have simply over-equipped the vehicle with more batteries without telling you. Why does that matter? If your vehicle has 100 kWh in its spec, but the battery pack is 120 kWh, you’ll hardly ever feel the CV phase. Your battery cells are simply not charging at 100%. That’s an expensive method but works fine.
Plan your charging equipment correctly
Now that you know a DC fast charger will never charge at full power during the entire charging process, you can plan accordingly. You might install more charging stations as the vehicles will probably block the chargers during the CV phase. If, in addition, you use smart charging software for electric vehicles, you can actively manage the power between the charge points and take advantage of it.
Use slow chargers when possible
As most EV batteries are designed for slow and fast charging, you’ll likely notice the CV phase much less when charging slowly. Why? As you reduce the power, the duration of charging will increase from beginning to end, and you’ll still have a CV phase, but it’s typically a lot shorter. Another advantage is that slow AC chargers are more affordable, so that you can install many more at the same site.
When charging electric vehicle batteries to total capacity, the last 20% or so is much slower than the first 80% due to the constant voltage (CV) phase.
This can cause problems, especially for drivers or fleet managers.
To get around the CV phase problem you can combine charging stations with smart charging software to monitor and control the various charge points.
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