DIMO Insights #5 - How Fast Can You Charge Your EV?

Introduction

Are you considering or already driving an electric vehicle (EV)? If so, you've probably heard about charging speed and how long it takes to power up your EV. But did you know that charging rates are not constant during a charging session? In this blog post, we'll break down some important research findings about EV charging speed and what it means for you as a driver.

For example, charging an EV more often on a long road trip can actually save drivers time! On one trip, a DIMO driver charged four times and the whole road trip was 11 hours. But when they charged six times on the same route, the whole trip was only 9.5 hours – all thanks to faster charging at lower states of charge!

Why Charging Speeds Aren't Constant

Imagine pulling up to a 100-kilowatt charging station with a 100-kilowatt-hour battery in your EV. It might seem logical to assume that it would take just one hour to charge your battery from 0% to 100%. However, here's where things get interesting.

To maximize your EV battery's lifespan, the vehicle's software slows down the charging process as the battery approaches full capacity. This slowdown is necessary to preserve the health of the battery, and it means that you won't always be charging at the maximum speed your charger can provide.

Predicting Charging Times

DIMO can use historical charging data to estimate how long it will take to charge your EV at specific charging stations. It does this by taking into account the actual charging capability of chargers and predicting the charging time based on your vehicle's battery management system.

For EV owners, DIMO can convert the state of charge (SoC) into kilowatt-hours of energy stored in the battery. This conversion allows for a standardized comparison of charging sessions across different EV models with varying battery sizes.

Understanding Charging Speeds with an Example

Let's take a look at an example charging session to illustrate the concept. If your EV has a 60 kilowatt-hour battery and you coast into a 120 kilowatt charger at 0% SoC, you would probably this example charging session, the vehicle started charging at 19% and ended at 92% SoC, and, due to the battery management system's rate limiting, it actually took 47 minutes (an overall average rate of about 56 kilowatts).

DIMO data can provide valuable insights into the time it will take to charge your vehicle at different types of level 3 chargers, which can vary in speed from 70 kW to 250 kW. This information can help you plan your charging stops more effectively. 

To give a sense of the slowdown at larger scale, this graph shows the estimated kilowatt rate of charge for tens of thousands of observed charging sessions:

Charging Timing Suggestions for Road Trips

Planning a road trip with your EV? The research also offers insights into optimizing your charging strategy. For example, it's essential to consider the slowdown in charging speed as you approach 100% SoC. Charging more often but to a lower state of charge may be more time-efficient on long journeys.

Here's a practical example of a road trip:

On the westbound journey, this EV was charged four times (starting at 26, 18, 36, and 15% SoC and ending at 86, 97, 87, 68% SoC respectively) and the trip took just over 11 hours.

However, on the eastbound journey, the EV was charged six times (starting at 19, 11, 8, 19, 14, 6 SoC and ending at 44, 80, 54, 50, 40, 59% SoC respectively) and the trip took about 9.5 hours. By capitalizing on the faster charging time at low states of charge, this driver saved more than an hour of charging time and shortened the overall trip length.

Level 3 Charging Around the US

We have also examined Level 3 charging stations across the US. To find out a charger's maximum capability, they focused on charging sessions when the vehicle's SoC was under 50%.

Understanding these charging dynamics can help you plan your trips better, optimize your charging stops, and make the most of your electric vehicle. With the right information, you can have a smoother and more efficient EV driving experience.