[Complete Battery Guide](37) Electric Cars Now Compete on Charging Speed... But What Is C-Rate?

Editor's NoteWe are now in the battery era. Batteries are found in almost everything, from mobile phones and laptops to electric vehicles. [Complete Battery Mastery] is a section created to easily convey the basics of batteries, their ecosystem, company information, industry trends, and outlook to general readers, students, and investors interested in the battery industry and related companies. We visit you every Saturday.

Chinese battery cell company CATL announced during the Beijing Auto Show held last April that it has developed an electric vehicle battery called 'Sensing Plus (shenxing plus)' capable of driving 370 miles (about 600 km) after just 10 minutes of charging. This corresponds to a charging speed of 1 km of driving distance per second.

CATL emphasized that 'Sensing Plus' is the world's first lithium iron phosphate (LFP) battery to support 4C high-speed charging and can drive up to 620 miles (about 1000 km) on a single charge. Although this result is based on China's relatively lenient certification standard (CLTC·China light-duty vehicle test cycle) compared to Europe or the United States, foreign media have evaluated it as a significant technological advancement.

New product launch scene of Chinese battery company CATL. Image source=CATL

Domestic battery cell companies are also racing to secure charging technology. This is based on the judgment that charging time is a key factor in accelerating the popularization of electric vehicles. Until now, the driving range, which has been a high demand among electric vehicle consumers, has been somewhat resolved by increasing energy density, and prices have recently dropped significantly. The next issue electric vehicles need to solve is charging time.

Battery companies expect that if the charging speed of electric vehicles is reduced to a level similar to the refueling time of conventional internal combustion engine vehicles, consumer complaints about charging time will be somewhat alleviated. The refueling time for internal combustion engine vehicles is around 5 minutes.

Similar to Refueling Time... "300 km Driving Range with 5 Minutes Charging"

Domestic battery companies are setting a short-term goal to reduce electric vehicle charging time to within 9 to 10 minutes.

Samsung SDI plans to develop a battery capable of driving 600 km with 9 minutes of charging by 2026. In this case, a 5-minute charge would secure a driving range of 300 km. Since more than 99% of drivers drive less than 300 km per day on average, it is expected that this charging speed will not cause significant inconvenience in electric vehicles.

SK On is preparing for mass production of the SF Plus (+) battery. This battery has reduced the charging time from 10% to 80% from the existing 18 minutes to 15 minutes. SK On also plans to release a battery capable of driving 600 km with 10 minutes of charging and 300 km with 5 minutes of charging by 2030. The company announced that it has secured patented technology that can reduce fast charging time to 7 minutes.

The SF+ battery exhibited by SK On at InterBattery 2024. Photo by Kang Hee-jong

LG Energy Solution plans to produce batteries that can charge up to 80% within 20 to 30 minutes for the mainstream electric vehicle market and batteries that can reduce charging time to 10 to 20 minutes for the premium market.

Although each company applies various technologies to shorten charging speed, a common factor is silicon anode material. In lithium-ion batteries, charging involves lithium ions moving from the cathode to the anode. Silicon anode material theoretically has 10 times the energy capacity compared to conventional anode materials. Applying silicon anode material increases the lithium-ion storage capacity, allowing faster charging.

However, silicon anode material experiences severe swelling during charge and discharge, making it difficult to add large amounts to the anode. How to solve this is a challenge. Currently, about 5% silicon anode material is mixed with graphite, and research and development (R&D) to increase this ratio is underway. LG Energy Solution was the first in Korea to mass-produce silicon anode material in 2019 and applied it to Porsche's electric vehicle (EV) 'Taycan'.

Efforts are also being made to shorten the lithium-ion migration path during charging and reduce resistance within the material to increase lithium-ion migration speed. SK On explains that it reduced lithium-ion migration distance and increased migration speed by arranging high-capacity silicon and low-resistance graphite in its unique dual-layer structure. Samsung SDI states that it applied materials that can shorten lithium-ion migration paths within the electrode and uniformly distributed binders to enable high-speed charging.

Samsung SDI introduced its 9-minute ultra-fast charging and 20-year long-life battery technology at the EVS37 event held last April. Photo by Kang Hee-jong

Reducing charging time without affecting energy density is also crucial. It is generally known that rapid charging reduces energy density. Achieving a balance between energy density and high-speed charging while shortening charging time is a key technological capability.

Even if a battery supports high-speed charging, charging time can vary depending on the type of electric vehicle and charging infrastructure. Hyundai Motor's electric vehicles apply an 800V charging system capable of charging at 350kW. Slow chargers are installed in residential areas such as apartments, while fast chargers are installed at highway charging stations.

How to Know Charging Time

One factor affecting electric vehicle charging time is battery capacity. Just as refueling time increases with larger fuel tank capacity in internal combustion engine vehicles, charging time increases with larger battery capacity.

Battery capacity is usually expressed in milliampere-hours (mAh) for smartphones, but in kilowatt-hours (kWh) for electric vehicles. Both are units representing battery capacity but are used slightly differently.

Ampere (A) is the basic unit representing the amount of current. 1A means the amount of electrons (1 Coulomb = 6.25×10^18 electrons) flowing per second. Ampere-hour (Ah) is the amount of electricity when 1A current flows for 1 hour. Ampere is named after the French physicist Andr? Amp?re.

Voltage (V) is electrical potential energy. Electricity is often compared to water falling from a waterfall; voltage corresponds to the height of the waterfall, and current corresponds to the width of the waterfall.

Watt (W) is the basic unit of power. 1W is the amount of power produced or consumed in 1 second, and 1Wh is the amount produced or consumed in 1 hour. Watt is proportional to current and voltage. The wider and higher the waterfall, the more water falls.

Electric energy (Wh) is calculated by multiplying current (Ah) by voltage (V). In electrical terms, power (P) = voltage (V) × current (I). For example, a smartphone battery with a capacity of 5000 mAh and voltage of 3.85V has an electric energy of 19.25 Wh.

1Ah = The amount of electricity when 1A current flows for 1 hour.
1Wh = The amount of power produced or consumed in 1 hour.
Power (P) = Voltage (V) × Current (I)

Charging time can be roughly estimated by dividing the battery capacity by the power supplied by the charger.

Assuming the previously mentioned smartphone battery with 5000 mAh capacity and 3.85V voltage is charged with a 45W fast charger, dividing 19.25 Wh by 45 W results in approximately 0.42 hours (about 25 minutes) of charging time.

However, this is a simple calculation, and actual charging may take longer. Electric energy and charging time are not perfectly proportional; the charging rate slows down after 80% charge to ensure safety until reaching 100% full charge, as controlled by device manufacturers.

This also applies to electric vehicles. For example, charging an electric vehicle with a 70 kWh battery using a 7 kW slow charger takes about 10 hours. Using a 100 kW fast charger takes about 0.7 hours (42 minutes). This is also a simple calculation, and actual times vary depending on battery type and vehicle manufacturer design.

Hyundai Ioniq 5 Catalog. Image source=Hyundai Motor Company website

Recently, electric vehicle manufacturers have introduced technology to increase battery system voltage to 800V to speed up charging. Most electric vehicle battery systems, such as Tesla's, are configured at 400V, and charging systems are designed accordingly. In contrast, some automakers like Hyundai and Porsche have increased battery system voltage to 800V and introduced charging systems with output up to 350 kW.

The latest Ioniq 5 is equipped with an 84 kWh battery. Charging it with a 350 kW fast charger would take approximately 0.24 hours (about 14 minutes) by simple calculation. Actual charging time is slightly longer. The vehicle catalog states that using a 350 kW fast charger, charging from 10% to 80% can be completed within 18 minutes.

C-Rate Explained

The term C-rate often appears when discussing battery charging speed. C-rate stands for Current-rate and refers to the battery's charge and discharge rate. The unit uses 'C,' which stands for Capacity.

C-rate is calculated by dividing the charge/discharge current (A) by the battery capacity (Ah), with the standard value being 1C. For a battery with 1000 mAh capacity, a C-rate of 1C means charging with 1000 mA current for 1 hour.

Knowing the C-rate allows estimation of charging time. If a battery is rated for 2C charging, it means it can be fully charged in 0.5 hours (30 minutes). Earlier, CATL stated that the C-rate of Sensing Plus is 4C, meaning it can be fully charged in 0.25 hours (15 minutes). A 10C battery can be fully charged in 0.1 hours (6 minutes).