EV charging primarily slows down to safeguard the battery from overheating, which could lessen its lifespan. This “tapered charging” sees high rates when the battery is low, reducing as it fills up.
To understand why EV charging might slow down, it’s important to first grasp how EV charging works in general.
Notably, charging can greatly decelerate when the battery is over 80% full to maintain battery health, potentially taking as long to reach full charge from 80% as it did to initially reach that 80%.
The taper is more pronounced with DC charging than it is with AC. And the AC taper occurs further down the charge cycle. The difference between AC and DC EV charging can play a significant role in the charging speed of your EV.
Now that you know the primary reason is a cautionary one, let’s look into the details of the DC fast charging curve. The key takeaways below are for you to keep in mind as you read.
At a Glance:
1️⃣ Battery degradation increases with faster charging rates, emphasizing the need for balance between quick charging convenience and long-term battery health.
2️⃣ Charging points reduce output when they detect the battery is nearly full, preventing overcharging and extending battery lifespan.
3️⃣ Modern EV batteries can last between 12 to 15 years, provided good charging practices are followed, such as slower charging rates and avoiding frequent fast charging.
DC Fast Charging Curve
The DC Fast Charging Curve represents how an EV’s charging speed changes during a fast charging session, typically peaking at a certain point and then slowing down as the battery fills up.
The type of EV charging connector used can influence the charging speed and might be a reason why charging is slow.
It is common for the charging rate to ramp up at the beginning and then taper off as the battery reaches its optimal capacity, usually around 80%.
There are a few key factors that contribute to this non-linear charging rate. First, your EV’s battery management system (BMS) is designed to protect the battery from damage, ensuring it has a longer lifespan.
As the battery becomes more charged, the BMS will reduce the charging rate to avoid overloading the battery cells and generating excessive heat.
Another reason behind the curve is that higher charging speeds at the beginning of the session help to fill the battery quickly during the first part of the charging process; this supplies significant driving range in a short amount of time.
But, as the battery fills up, the energy transfer efficiency decreases, and the charging speed must be reduced to maintain the battery’s health.
DC fast charging won’t always happen at a consistent rate. So awareness of the charging curve can help you optimize charging stops.
You are better off getting that last 80%-100% by plugging in at home overnight.
Factors Affecting EV Charging Speed
State of Charge
The charging speed of an EV is affected by its current State of Charge (SoC), which refers to the percentage of how full the battery is.
At lower SoCs, batteries usually charge faster; however, as the battery reaches around 80% full, the charging speed decreases to protect the battery from overcharging.
Battery Capacity and Chemistry
Another factor that influences EV charging speed is the battery capacity and chemistry. Larger batteries take longer to charge, and the chemistry of the battery impacts the charging performance.
For example, lithium-ion batteries are commonly used in EVs due to their high energy density, but they have certain limitations that lead to slower charging speeds, such as heat generation during the charging process.
Charging speed is also affected by the type of charging station being used. Different EV charging levels have varying charging speeds, which could explain why your EV charging might slow down.
Level 1 charging stations provide the slowest charging speed at about 2-5 miles of range per hour.
Level 2 charging stations, which are usually found in public areas or installed in homes, provide approximately 10-60 miles of range per hour.
The fastest charging is DC Fast Charging, which can provide 60-100 miles of range in just 20-30 minutes.
Battery temperature plays a crucial role in EV charging speed. Knowing when to charge your EV battery can help optimize charging speed and prevent it from slowing down.
Cold batteries charge slower than warm batteries, and high temperatures can also slow down the charging process as the battery management system regulates the battery temperature to ensure its safety.
Finally, the charging infrastructure available can affect the charging speed of an EV. Infrastructure factors such as the availability of high-power stations or having multiple chargers at a location can impact how fast your EV charges.
Additionally, network connectivity and charging station maintenance play a role in the overall charging experience.
Battery Management Systems and Safety Measures
A BMS plays a crucial role in maintaining the efficiency and longevity of an electric vehicle’s battery pack. Deciding whether or not to charge your EV to 100% can impact the lifespan.
They monitor and control various aspects of the battery, including temperature and charging rate, to ensure optimal performance and safety.
As the battery reaches a higher SoC, its temperature tends to rise. To avoid potential damages, the BMS reduces the charging speed, keeping the temperature within safe limits.
Monitoring and managing the temperature is essential to protect the battery from thermal runaway, a chain reaction that generates excessive heat and can potentially lead to spontaneous ignition.
The BMS also regulates the charging rate, especially towards the end of the charging cycle.
When the battery is almost full, the charging process slows down significantly, which helps prevent overcharging and extend the battery’s lifespan.
Additionally, this gradual decrease in charging rate allows for better energy management and ensures that the battery remains inside the safe operating zone.
It’s important to mention that the management systems in EVs incorporate numerous safety measures, such as protective circuitry and balance charge modules.
These components work in unison with the BMS to ensure that the charging process remains safe and reliable.
Impact of Charging Rate on Battery Lifespan
This study by findings.org found that battery degradation increases as the charging rate rises, with a 10% degradation at 1.2C and 23% degradation at 1.5C after 300 cycles, compared to just 7% degradation at 1C.
This information is critical when considering the impact of fast charging on EV batteries. Car makers are well aware of battery degradation due to higher charging rates.
Some, like Kia, even advise customers to use DC fast chargers sparingly. It is essential to strike a balance between the convenience of quick charging and the long-term health of the EV battery.
It’s worth noting that charging points will start reducing output when they detect the battery is almost full, preventing overcharging and ensuring the battery lasts longer. This precaution is crucial in maintaining the optimal lifespan of the battery.
Furthermore, the U.S. Department of Energy predicts that modern EV batteries should last between 12 and 15 years, well past their warranty period.
This longevity is achievable as long as we establish good charging practices, like slower charging rates and avoiding frequent fast charging.
Striking a balance between convenience and battery health can help maximize the life of an electric car’s most essential component.
Having explored the topic of why EV charging slows down, it’s worth highlighting a few conclusions from the discussion.
1️⃣ EV charging slows down to safeguard the battery from overheating, a process known as “tapered charging,” which sees high rates when the battery is low, reducing as it fills up.
2️⃣ This deceleration is most pronounced when the battery is over 80% full, potentially taking as long to reach full charge from this point as it did to reach 80% initially.
3️⃣ The tapering effect is more noticeable with DC fast charging than AC, and it serves to maintain battery health and lifespan.