How DC Fast Charging Really Works and an Intro to Charging Curves 

EV Life | 7/18/2019

Team ChargePoint

Deep Dive into DC Fast Charging

DC fast charging is great for when you need to recharge quickly, especially for brief stops on road trips or when you're low on your charge and pressed for time. We covered when and how to use it briefly in a previous post, to help you get the best DC fast charging experience possible. If you’ve ever used a DC fast charging station, you may have wondered what’s controlling the charging speed and why it varies. For the sake of keeping EV drivers and station owners informed and happy, here’s a little more about what’s happening behind the scenes—or, inside the charger (and vehicle). 

How do DC fast chargers protect my battery? 

DC chargers have the same built-in precautions as any other EV charger: they only deliver as much power as your car can handle. From start to finish, your car is communicating with the charging station, monitoring conditions such as the vehicle’s state of charge, the maximum electrical current available from the station and the battery voltage, and regulating the power flow accordingly. Because fast charging delivers so much more power than AC charging, it’s especially important that it not send more power than the car or battery can handle. 

Once charging gets started, your battery warms up and your car may ask for more power, enabling faster charging. As charging continues, your car will alert the charger when the safest top speed is reached and will continue to maintain that pace for as long as possible. Your car is actively in control throughout a fast charging session—and it may request a moderate speed to ensure safety and maximize battery life. In the long run, this is good for you, too.

Why isn’t my car always charging at top speed? 

Because the car is in control, every car fast charges a little differently. Each car has a different top fast charging speed, such as 55 kW for the Chevrolet Bolt EV and 125 kW for Teslas. That means not every car can charge as fast as every charger. Although the 350 kW fast charging stations that have been announced are exciting and will be useful in the future, cars on the road can’t charge that fast yet, so don’t expect your Outlander PHEV battery to fill up in seconds.

Keep in mind that battery temperature (batteries may charge more slowly until they warm up) and ambient temperature (extreme temps can slow charging) affect charging speed, too. Here’s how a typical DC fast charging session might look as charging ramps up, reaches top speed and slows down as the battery fills: 

Typical DC Fast Charging Curve

When you buy an EV, make sure to ask questions and do research about fast charging for that model so you know what to expect, from what connector to use to how long it will take to charge. This will help you have a stellar ownership experience and avoid any fast charging surprises. 

Now that you know how fast charging works and where the curves come in, you’re ready to get the most out of your EV. Find fast charging spots everywhere you need to go with the ChargePoint app, which lets you filter for the connectors that will work with your car. 

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