Guide to EV charging speed

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. Ultimately, however, one thing is always true:

Your vehicle controls how quickly it can 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:

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

 

 

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.

Still not sure? For some added technical context (and a simpler metaphor!), jump down to the section: Understand why charging speed slows based on battery level

state of charge (SOC) graphic

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.

battery management system (BMS)

 

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.

graphic showing AC affecting charging speed

 

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.

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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.

current charging voltage limits

 

 

To learn more, check out this article from our blog: 5 Things That Affect Your Charging Speed

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