Many electric vehicle drivers marvel about what controls how fast their EVs charge with rapid chargers in different conditions. Others, are curious about what causes the car charging to slow down when the vehicle gets old or what happens when you charge it over 70 – 80 percent. In this article, Awadhesh Jha, VP Fortum Charge & Drive attempts to answer a few of these questions that cross several EV enthusiasts' mind but often go unanswered.

An EV controls how much power it takes from DC rapid charging. Therefore, a combination of factors such the vehicle, its electronics, and battery determine how quickly the EV charges. In particular, there are two main factors that have a bearing on EV charging: first, how full the battery is (voltage/state of charge) and second, what's its temperature like (i.e. how cold or hot).

EV slows down charging when it reaches 70 – 80 percent state-of-charge 

To understand how this happens with rapid charging, we must understand how the battery is built. The battery cells in your EV consist of a positive and negative electrode, and an electrolyte that allows the lithium ions to move between the electrodes in one direction or another, depending on whether the car is being charged. When charging the battery, positive lithium ions are transferred to the negative electrode until it is full.
Charging power decreases when you have reached a certain SoC level in the battery cells. This is typically around 70-80 percent.
When it starts to get full, there is a narrower amount of space for the lithium ions in the negative electrode. The electrode needs more time to free up space for the lithium ions that are flowing into it. Therefore, charging power decreases when you have reached a certain SOC level in the battery cells. This is typically around 70-80 percent.

Effect of temperature on battery charging 

Batteries operate over a wide temperature range. While both charging and discharging speeds are affected by different temperature conditions, the charging process is more delicate than discharging and special care must be taken. Extreme cold and high heat reduce charge acceptance, so the battery must be brought to a moderate temperature before charging.

Effects of cold temperature 

When it's cold, lithium ions move slower in the electrolyte. Electrochemical processes are sensitive to temperature. Using the same charging power, the cell voltage will be higher than at more ideal temperatures. This can destroy the cells, and consequently charging power may be reduced. The car communicates with the charger and automatically ensures that the current decreases. One supplier has estimated that a normal size battery pack on an EV can expect 30 minutes charging time from 0 – 80 percent at 10°C. At O°C, charging time will increase to 45 minutes, and at -10°C, charging time may reach 90 minutes. At -20°C, it will take about 90 minutes with battery heating; without battery heating it will take about 3 hours.

Effect of Heat 

Heat is the worst enemy of batteries. Lithium-ion performs well at elevated temperatures but prolonged exposure to heat reduces longevity. Charging and discharging at elevated temperatures is subject to gas generation that might cause a cylindrical cell to vent and a pouch cell to swell. Most of the time high temperature effects are attributed to the high internal temperature of LIBs during operation. The high internal temperature is caused by heat generation inside the LIBs, which happens at high current state, including operations with fast charging rate and fast discharging rate. The high temperature effects will also lead to the performance degradation of the batteries, including loss of capacity and power. Generally, the loss of lithium and the reduction of active materials under high temperatures will result in the loss of the capacity, while the increase of internal resistance is responsible for the loss of power. When it's hot, lithium ions move slower in the electrolyte.

How to get the best charging in summer 

The highest power is obtained when the car is moderately hot and has little power in the battery.
• Do not use a fast charger immediately after driving for a longer distance. Allow the battery to cool before plugging in at a fast-charging station. In this case, charging will be more efficient after a cooling time or at a standard charger (AC).
• Do not charge more than 70 percent at a fast charger if you can reach the next charger within the range it provides. The more power you require in your battery, the slower the charge goes. The charging becomes slow after 70 percent SoC, it would be good to allow others to charge.

Effect of life of battery on charging speed 

The health of a battery is determined by examining three fundamental attributes:
• Capacity of the battery - The ability of the battery to store energy
• Internal resistance of the battery - The ability to deliver current; and
• Self-discharge ability of the battery
An old Li-ion battery is likely to take more time to charge than a new Li-ion battery for the same power. With age, Li-ion battery loses its charge transfer capability. This is caused by the formation of passive materials on the electrodes, which inhibits the flow of free electrons which diminishes the porosity on the electrodes, decreases the surface area, lowers the lower ionic conductivity and raises migration resistance.
This can also be understood by having an analogy of lifting a weight with a rubber band. The new battery has less slack than to the aged pack and can accept charge longer before going into saturation. The charge current is constant till the cell reaches its voltage limit, after that the battery saturates and the current drops until the battery can no longer accept further charge and the fast charge terminates. This explains the longer charge time of an older Li-ion with less capacity. I liked the analogy of this phenomena with a young athlete running a sprint with little or no slow-down towards the end, while the old athlete gets out of breath and begins walking, prolonging the time to reach the goal.