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Jun 04, 2023

Does DC Fast Charging Really Reduce Your EV's Battery Capacity?

If you have an electric vehicle that you regularly fast-charge, perhaps you

If you have an electric vehicle that you regularly fast-charge, perhaps you should take a look at the implications it has on battery degradation.

Fast-charging your EV sounds great since a 350-kilowatt rapid charger can take a big-battery EV like a Lucid Air Pure to an 80 percent state of charge in 15 minutes. Although it's not as quick as filling up a traditional gasoline vehicle, DC fast charging reduces the time it takes to fill up an electric vehicle.

But does the convenience of charging an electric vehicle come at the cost of battery degradation? Well, let's find out.

To better understand EV battery longevity, it's important to know what fast charging is and how it works. Broadly speaking, an electric vehicle's battery can be replenished using three different methodologies called Level 1, Level 2, and Level 3 charging. The first two types rely on alternating current (AC), while Level 3 charging, also known as DC fast charging, requires direct current.

The important distinction here is that Level 1 and Level 2 chargers use the vehicle's onboard charger to convert the current from AC to DC, as the lithium-ion battery on your vehicle can't take alternating current directly.

When it comes to Level 3 fast chargers, though, they can just pump juice directly into the battery without the need for the onboard charger. This allows DC charging to push huge amounts of current and voltage to the battery pack without being constrained by the capabilities of the vehicle's onboard charger.

Lithium-ion batteries convert chemical energy into electrical energy, and, hypothetically, this reaction should go on forever. However, we all know that lithium-ion batteries don't last forever. But what is the exact reason for this degradation?

Well, several chemical reactions occur inside a lithium-ion cell when it's charged or discharged. Some reactions work towards generating electricity, while others consume lithium ions that reduce the capacity of the battery. In other words, with every charge-discharge cycle, an EV's lithium-ion battery will lose some capacity; this actually happens and does not fall under the category of EV charging myths.

That said, it's important to understand that these reactions occur at different rates based on several environmental conditions, and there are steps you can take to make the battery last longer.

Therefore, most battery manufacturers provide a range of temperatures at which the batteries perform best. This range changes based on the battery chemistry, but in most cases, it lies between -4 and 140 degrees Fahrenheit for discharging and between 0 and 45 degrees Fahrenheit for charging.

This operational range shows that batteries can be charged in a lower range of temperatures, and charging them in extreme conditions, both cold and hot, can cause problems as these conditions increase the rate at which unwanted reactions occur, consuming lithium ions and reducing capacity.

Now that we know why the capacity of a lithium-ion battery goes down, we can try to understand what happens inside the battery when it's fast-charged.

Fast charging uses a high-voltage current to charge the battery. Lithium ions are pulled from the cathode with greater force and are moved to the anode when charged. This causes cracks in the cathode and also generates dendrites on the electrodes. Due to these cracks and dendrite buildup, the capacity of lithium-ion cells reduces, and they also increase the resistance of the battery.

The internal resistance of a battery increases when it's fast-charged. Due to this increase in resistance and the high current input during fast charging, excessive heat is generated inside the batteries. This high temperature reduces the capacity of the lithium-ion batteries.

When a lithium-ion battery is fast-charged using high currents at low temperatures, a phenomenon known as lithium plating occurs at the anode. Due to this, lithium atoms don't intercalate inside the anode. This results in inert lithium metal (which can't generate electricity) on the surface of the electrodes.

Looking at the list of degradation mechanisms given above, it is evident that fast charging is bound to reduce the life of an electric vehicle. That said, EV battery packs are designed to prevent battery damage. Therefore, before we conclude that fast charging is bad for electric vehicles, let's understand how their battery packs are designed to counter degradation.

EV battery packs consist of several lithium-ion cells that are connected to create modules. Several modules are connected to create the pack, and its battery health is managed by the Battery Management System, also known as a BMS.

The BMS is basically a computer connected to several sensors that monitor individual cell voltage, current, and temperature. It then analyzes this data to ensure that each cell is operating optimally.

If the cells inside the battery pack are too hot, the BMS will increase cooling to reduce the overall pack temperature. If it detects a high cell voltage or current during DC fast charging, it will regulate both parameters to prevent battery damage.

The BMS is therefore the EV part that has the biggest role in reducing battery degradation.

Let's look at some studies that show how much damage vehicles go through due to fast charging. Four 2012 Nissan Leaf electric vehicles were driven in Phoenix, Arizona by the Idaho National Laboratory. Two vehicles were charged using DC fast charging, while the other two were charged using AC level 2 charging, with the following results:

Battery degradation occurs regardless of the charging method, but it is increased in vehicles that are fast-charged; the difference is about 5 percent.

In a separate experiment from the one above, two Nissan Leaf battery packs were tested in laboratory conditions by the Idaho National Laboratory. One was DC fast-charged, while the other only got AC charging. This test's goal was to see what happens to the whole pack as opposed to each cell like in the previous experiment.

This shows a strong correlation between battery pack degradation and temperature, suggesting that fast charging isn't as significant a factor for battery degradation.

In another study, Geotab, a fleet management company, collected battery health data from 6,000 EVs and concluded that fast charging increased the rate at which a battery degrades. This study, just like many others, showed that fast charging increased the rate at which the lithium-ion battery in your vehicle degrades and highlighted the BMS' vital role in keeping degradation as low as possible.

Your EV's battery pack is bound to lose capacity as time passes. That said, the rate at which this degradation occurs depends on several factors, and fast charging is certainly one factor that can speed this up.

Another thing to note is that using fast charging in moderation will not reduce the range of your battery extensively, and you can use it on long road trips to reduce the time it takes to juice up your vehicle.

Nischay is an Electronics and Communication engineering graduate with a knack for simplifying everyday technology. He has been making tech easy to understand since 2020, working with publications like Candid.Technology, Technobyte, Digibaum, and Inkxpert.In addition, Nischay loves automotive technology and has been working as an Engineer with Stellantis for the last two years. He has adept knowledge about the features that makes today's cars safer and easier to drive.