Commercial fast-charging stations subject electric car batteries to high temperatures and high resistance that can cause them to crack, leak, and lose their storage capacity, according to researchers at the University of California, Riverside (UCR) in a new open-access study published in the journal Energy Storage. To remedy this, the researchers have developed a method for charging at lower temperatures with less risk of catastrophic damage and loss of storage capacity.

In order to make EVs more competitive with combustion engine vehicles, development of an effective fast charging technique is inevitable. However, improper employment of fast charging can damage the battery and bring safety hazards. Herein, industry based along with our proposed internal resistance (IR) based fast charging techniques were performed on commercial Panasonic NCR 18650B cylindrical batteries. To further investigate the fast charging impact and electrode degradation mechanisms, electrochemical analysis and material characterization techniques including EIS (electrochemical impedance spectroscopy), GITT (galvanostatic intermittent titration technique), SEM (scanning electron microscopy), and XRD (X-ray diffraction) were implemented.

—Sebastian et al.

Mihri Ozkan, a professor of electrical and computer engineering and Cengiz Ozkan, a professor of mechanical engineering in the Marlan and Rosemary Bourns College of Engineering, led a group that charged one set of discharged Panasonic NCR 18650B cylindrical lithium-ion batteries, found in Tesla cars, using the same industry fast-charging method as fast chargers found along freeways.

An electric car battery before and after industry standard fast charging. (Ozkan Lab/UCR)

They also charged a set using a new fast-charging algorithm based on the battery’s internal resistance, which interferes with the flow of electrons. The internal resistance of a battery fluctuates according to temperature, charge state, battery age, and other factors. High internal resistance can cause problems during charging.

The UC Riverside Battery Team charging method is an adaptive system that learns from the battery by checking the battery’s internal resistance during charging. It rests when internal resistance kicks in to eliminate loss of charge capacity.

For the first 13 charging cycles, the battery storage capacities for both charging techniques remained similar. After that, however, the industry fast-charging technique caused capacity to fade much faster—after 40 charging cycles the batteries kept only 60% of their storage capacity. Batteries charged using the internal resistance charging method retained more than 80% capacity after the 40th cycle.

Difference in charge capacity from industry vs. internal resistance charged electric vehicle batteries. (Sebastian et. al.)

At 80% capacity, rechargeable lithium-ion batteries have reached the end of their use life for most purposes. Batteries charged using the industry fast-charging method reached this point after 25 charging cycles, while internal resistance method batteries were good for 36 cycles.

Worse, after 60 charging cycles, the industry method battery cases cracked, exposing the electrodes and electrolyte to air and increasing the risk of fire or explosion. High temperatures of 60 degrees Celsius/140 degrees Fahrenheit accelerated both the damage and risk.

Capacity loss, internal chemical and mechanical damage, and the high heat for each battery are major safety concerns, especially considering there are 7,104 lithium-ion batteries in a Tesla Model S and 4,416 in a Tesla Model 3.

—Mihri Ozkan

Internal resistance charging resulted in much lower temperatures and no damage.

The researchers have applied for a patent on the adaptive internal resistance fast-charging algorithm that could be licensed by battery and car manufacturers. In the meantime, the UC Riverside Battery Team recommends minimizing the use of commercial fast chargers, recharging before the battery is completely drained, and preventing overcharging.

Resources

• Sandeep S. Sebastian Bo Dong Taner Zerrin Pedro A. Pena Amirali S. Akhavi Yige Li Cengiz S. Ozkan Mihrimah Ozkan (2020) “Adaptive fast charging methodology for commercial Li‐ion batteries based on the internal resistance spectrum,” Energy Storage doi: 10.1002/est2.141