Level 3 charging uses DC electricity to quickly recharge an EV battery - up to 350 kW can be delivered at the peak rate. It is supposed to be the answer to long range anxiety by offering the freedom to“fill up” while you wait for coffee or run to the bathroom.
However, conventional wisdom also holds that it will lead to faster battery degradation, rendering your EV less valuable.
The truth of the matter remains an outstanding question and a big source of uncertainty in the EV community: laboratory experiments show that fast charging damages individual battery cells, but these tests can’t really be applied to real-world battery packs with thermal and voltage management. And, it is hard to do large scale, controlled experiments in cars on the road. We will dive into both sides of the conversation.
A simple answer?
If you’re looking for a simple answer about whether or not DC fast charging is OK, think about it like a milkshake or Thanksgiving feast. Fine once in a while, but not for daily consumption. It will have a marginal effect on your battery’s health and longevity over the course of many years, but not enough that you should stress about it or let it prevent you from using your car. And, if regular DC fast charging is a necessary part of EV ownership for you, that is also totally fine. Consider:
- Leasing your vehicle to avoid any long-term effects from frequent fast charging, or
- Buying one with an LFP battery chemistry, which is much more resistant to the heat generated by fast charging and has slower battery degradation.
If you're looking for a more in-depth analysis, read on!
The Science of Battery Degradation
Let’s start with the basics: large EV batteries are made up of many cells, each of which contains a positive and negative node, as well as an electrolyte solution that lithium ions flow through. In a battery pack in an EV, many cells are linked together and hooked up to a computer and management system that helps keep the cells balanced in charge and temperature.
When a battery cell is charged, ions move from the positive electrode to the negative electrode, storing energy in the process. When discharged, the ions move back in the opposite direction, releasing the stored energy.
Studies identify two main forms of battery degradation that are often related: capacity fade and power fade. Capacity fade is a decrease in the amount of energy a battery can store and use, usually due to the loss of some lithium ions. These lithium ions can be lost to secondary, waste reactions within the battery. Power fade is a decrease in the amount of power, or the rate of energy, that the battery provides. It usually happens when a build up of unwanted materials, such as metallic lithium plating, blocks the free flow of lithium ions.
How does this degradation happen? The process of charging a battery is a physical process, in addition to a chemical one. Charging causes lithium ions to physically nestle into the negative electrode, like cars filling in a parking lot or passengers filling up a plane.
However, it takes a certain amount of force, or voltage, to squeeze the ions into the node. This force can cause small cracks or damage to the battery material.
As the battery charges or discharges, some of the energy is lost as heat due to the physical resistance of the material and the chemical reactions taking place within the cells. This waste energy takes the form of heat. If the battery gets too hot, chemical reactions happen more rapidly, which can cause the battery to degrade more quickly. Similarly, if a battery is charged using a high voltage, it may also get hot, accelerating the same reactions.
What laboratory science says
Scientists have been studying what causes lithium ion batteries to degrade in laboratories and with computer models for nearly forty years, and high voltage charge (or discharge) is generally listed as one of the factors that leads to premature degradation in individual battery cells.
The more energy that is used to charge a battery, the greater the chance that small cracks or fissures will occur in the electrode material as the ions rush in. This is just like with parking cars: the more forcefully the cars enter, the more likely there is to be structural damage - cars may bump into each other, or damage the pavement. Once there are bumps in the road and pavement out of place, the more likely there are to be future collisions or damage.
Similarly, cracks in the battery become places where secondary, wasteful reactions occur.
Studies such as Vetter et al., 2005, Pelletier et al 2017, and others use experiments to show that individual battery cells are negatively affected by higher heat and higher voltages. However, those battery cells are not quite the same thing as a battery pack in a car. Once the cells are nestled into a high voltage battery pack, they are coupled with sophisticated computers and software to prevent the battery from getting too hot or receiving a dangerous amount of voltage. It’s been hard to study the effects of high voltage on battery packs, since most people don’t want to use their everyday cars in the highly regulated ways that a science experiment demands.
A Real world study
Let’s compare the laboratory science performed on battery cells with results seen in real world battery packs.
A highly touted Idaho National Laboratory study (inl.gov) prepared for the Department of Energy in 2013 showed that charging on a level 3 charger exclusively will lead to greater battery capacity degradation than charging with a level 2 charger. According to the study, “There were small differences in the baseline energy capacity measured among the test vehicles. The DCFC (level 3) group shows distinct differences in capacity loss compared to the ACL2 (level 2) group, noticeable beginning with the 30,000 mile test results.” It went on to say, “while the DC fast charged vehicles did lose more capacity than the control vehicles, that difference is small relative to the total capacity loss.” As a reminder, EV batteries naturally lose capacity with use.
With regard to power fade, the study noted that, “the minor differences in maximum speed achieved by each vehicle, about one MPH, and measured independently from the vehicle speedometer, is likely due to differences in tire wear related to when the tires were replaced rather than any changes in vehicle performance capability over the course of the study.”
What does this mean? Batteries that were fast charged only degraded slightly faster with regard to capacity, but did not see any noticeable difference in power output.
It’s important to understand the reason for the study. It was not to prove or disprove any theory, or promote one type of vehicle over another. The study was conducted so that BEV owners and manufacturers will be able to make informed decisions about what affects the longevity of the vehicle. This can help EV owners and buyers make the best decisions on how to care for the vehicle, and help manufacturers determine and develop the electric vehicle supply equipment that they bring to market.
What did we learn?
Given the results of that study, where are we 10 years later? Nissan states that a 100 kW fast charger will fill its 62 kWh Nissan Leaf battery to 80% in 40 minutes, while the Hyundai Ioniq 5 boasts the ability to charge from 10% to 80% of capacity in 18 minutes. Electrify America, which has free charging agreements with Hyundai and most other manufacturers, has almost exclusively 150 kW DC fast chargers. This leads us to believe that fast charging is here to stay and the speed at which EVs can be recharged will only increase to meet the demands of new EV drivers.
Yet all manufacturers still recommend level 2 charging over the level 3. They recognize that level 3 charging does degrade the battery faster yet most have also cited the ability to fast charge as selling points. As a buyer of an EV this should tell you two things:
- Level 3 fast charging is available and will continue to be available, decreasing the ‘range anxiety’ issue and increasing the ‘freedom to go.’
- For the good of your investment, don’t rely on it. Fast charging, like the cheeseburger you get at a rest stop, should be for road-trips or special occasions.
For everyday driving, level 2 or even the 120V garage plug may suffice while maintaining the value of your investment. In many markets, level 3 charging is also comparable in cost to regular gas, so that alone may give you pause about relying on it.
A final note: if you live in an apartment or are a rideshare driver and must rely exclusively on fast charging, consider getting a car with an LFP battery. This battery chemistry holds up much better to the high heat and voltage that comes with level 3 charging.
If you are smart about how and when you charge your EV, you don’t need to worry too much about capacity degradation. If you charge your battery on level 1 or 2 daily and level 3 when necessary, the small decrease in battery life should never affect you. Just like an ICE with over 100,000 miles, the interior, and exterior appearance and wear will likely contribute more to any depreciation, than the capacity of the battery. So be smart, use the fast charge only when needed, follow the manufacturer’s recommendations and you’ll enjoy thousands of miles of emission free driving.