A common question for the EVgo Charging Crew revolves around charging speed - how fast will my vehicle charge? How long will my session take? Can I help the charger go faster? Why does charging speed slow down as I charge? How does my vehicle decide on the charging rate?
This article will answer all of these questions and provide a little more information on the different factors that impact your charging experience. There's a lot of information here, so we've included some quick links throughout the article if you want to jump to certain information.
We also know that most questions about charging speed with EVgo involve fast chargers, so let's address a common point of confusion first. Ultimately, one thing is always true when it comes to fast charging:
Your EV controls how quickly it can fast charge.
You can check your owner's manual for more details on exactly how much energy your vehicle will accept at any point during a charging session. That said, your vehicle may not always charge at the Maximum Charge Rate on your car's nameplate. Charging speed can be impacted by one, or a combination, of these factors:
- Optimal times to fast charge based on battery
- When your battery impacts charging speed
- Possible loads in use while charging
- Understand why charging speed slows based on battery level
- Understand Current (C) and Voltage (A) limits
Can you help your vehicle charge faster?
Your vehicle will always determine its charging speed, but these actions can avoid common reasons for slower charging speeds:
- Fast-charge when your battery level is between 20-60% full
- Turn off your vehicle while it charges
- Use a charger whose energy source is not in use by any other vehicle or device
Understand the levels of EV charging stations
As the industry has evolved, EV battery sizes have increased to provide greater driving range. The power of charging stations has also evolved and increased to fill these batteries as fast as possible. Before we get into the details of fast charging, let's break down the 3 levels of EV charging to get a general expectation on charge times:
AC Level 1 (L1)
When a driver plugs their car into a standard wall outlet, that’s Level 1 charging. It’s the most basic type of charging. And it’s the slowest, taking 20+ hours to fully charge a battery. But for people who don’t drive a lot each day, Level 1 charging is a convenient way to charge at home because it doesn't require installing an AC Level 2 charger.
AC Level 2 (L2)
Level 2 charging involves a 208-240 volt circuit (like the kind used for electric dryers). They charge much faster than L1 chargers, taking about 5-6 hours. Many drivers might access Level 2 chargers where vehicles are parked for a significant amount of time, like at home or at work.. You can also find Level 2 charging stations within public charging networks, often located at malls and shopping centers.
DC Fast Charging
If AC Level 1 and Level 2 chargers are like “dial-up Internet,” then DC Fast Charging is like “fiber internet.” When charging on Level 1 or 2, electric vehicles convert AC power from the grid to DC power to recharge the battery. DC Fast Chargers do this conversion internally, using a much larger grid connection, and deliver DC power directly to the vehicle — resulting in a much faster and more powerful charge.
As the industry has evolved, EV battery sizes have increased to provide greater driving range. The power of fast chargers has also evolved and increased to fill these batteries in the fastest possible times. Fast chargers today are available from 25kW to 350kW for passenger EVs—with even higher-powered chargers for heavy duty electric vehicles like semis-trucks. It is important to understand the maximum power at which your EV can be charged and the power of the charger you plug into for the best charging experience.
Optimal time to fast charge based on battery
With your vehicle's battery level, or "State of Charge" in mind, you can optimize your charging sessions. Think of it like a fuel gauge. Batteries charge fastest when they are nearly empty—when they have a low SoC.
The best time to use a fast charger is when the battery is 20-60% full.
If your battery is above or below this level, many vehicles will slow the rate of charging to help preserve the battery.
Since you are unlikely to get the full benefits of fast charging speeds, we recommend using an L2 charger outside of this range for more cost-effective charging.
Your battery's impact on charging speed
When most people imagine a car battery, they might imagine one big block sitting inside the car. In reality, inside a “battery pack” are hundreds — and often thousands — of smaller “battery cells.” (The Tesla Model S has up to 7,104 battery cells!) As a result, when a battery charges, those thousands of cells are actually what’s being charged.
The battery's State of Charge (SOC)
Your State of Charge describes how full your battery is, in terms of percentage.
The temperature of your battery
Batteries don’t like to be too cold or too hot, and the charge rate slows down in both cases. However, it is difficult to know how your EV manages battery temperatures and something you’ll need to learn from experience.
Battery deterioration
Batteries can deteriorate over time and lose capacity as they do. This also affects their charge rate, which decreases as well.
The duration of your fast charge
When a fast charge session starts at a low state of charge, say 20%, a warm battery will charge at as fast a rate as the EV and charger can deliver. However, as the battery fills and heats up over the course of a longer charging session, the charge rate often slows.
Possible loads in use while charging
A few factors can increase the 'load' on the charger, which will often decrease the charging speed.
Using your vehicle while it charges
To ensure your battery receives the full amount of energy while charging, the vehicle should be completely off. If you leave your vehicle on while it's charging, some of the charging energy can be diverted for loads such as air conditioning or heating, lights, radio, or other accessories in use. The thermal management system also uses some of the charging power to heat or cool the battery. This is why sometimes the kW display on the charger may be a few percent more than that of what your in-dash displays indicate.
Outside factors that impact the charger
This typically affects your home charging station if multiple devices are pulling electricity from the same source. However, it is possible that a busy public charging station could see a slower charging rate than one where only one vehicle is charging.
Understand why charging speed slows based on battery level
To understand why charging speed slows, we first need to understand the vehicle's battery. When most people imagine a car battery, they might imagine one big block sitting inside the car.
In reality, inside a “battery pack” are hundreds — and often thousands — of smaller “battery cells.” (The Tesla Model S has up to 7,104 battery cells!) As a result, when a battery charges, those thousands of cells are actually what’s being charged.
It can be helpful to imagine sitting in a movie theater:
When the theater is empty, it’s easy to find a seat right away. But as the theater fills up, we have to take a few moments to find a seat and climb over people (without knocking over their popcorn).
That’s what happens with battery cells at the molecular level. When the battery cells are nearly empty, it’s easy to “find a seat” to charge. But as the battery cells fill up, it takes more time to find (and navigate) the empty cells. Generally, above 80% full is when it’s hardest for electrons to find a seat in your battery’s movie theater.
So what does this mean?
Your charging speed will slow down throughout the course of your charge. And every vehicle decides that “slow down rate” differently. Every manufacturer determines this in order to keep your vehicle’s battery healthy and increase longevity.
Understand Current (C) and Voltage (A) limits
The amount of power (kW) you receive is the product of Voltage (V) and Current (A). Both your vehicle and the charger have voltage and current limits, and both can impact the charging speed. This is also know as the Charge Curve.
As an example: EVgo’s 50kW chargers provide 125A up to 400V (125x400=50,000) and then decrease the current down to 100A at 500V. EV batteries tend to operate in the mid-300 to low 400V range; however, some operate in the low to the mid-300V range.
As your EV battery charges, the voltage increases. For EVs with batteries in the lower voltage ranges, a 50kW charger will deliver less than 50kW due to the current limit of 125A and voltage of less than 400V.
To learn more, check out this article from our blog: 5 Things That Affect Your Charging Speed