Heat is the enemy of electric-car battery packs. And while high-performance driving might heat up the packs momentarily, few things you can do to the pack come close to the potential wear and tear inflicted by the kind of charging you’d need to use on a highway trip: DC fast charging.
Using what they term “the same industry fast-charging method as fast chargers found along freeways,” researchers at the University of California Riverside looked at the damage to one kind of EV cell—the Panasonic NCR 18650B, similar to those used in the Tesla Model S and X—and proposed a new algorithm primarily focused around the internal resistance of individual cells.
The group paper, published in Energy Storage, proposes what it calls a new method for keeping temperatures down, with lower chances of losing cells or their capacity prematurely.
According to the researchers, batteries charged with the industry standard charging method degraded to 80 percent of their original capacity after just 25 charging cycles, while those charged with the research team’s internal resistance method lasted 36 charging cycles before getting to that level of degradation—effectively increasing the pack’s life and the cells’ longevity.
To put it in terms of resistance, the paper explains that batteries cycled under existing fast-charging standards showed a 78% increase in internal resistance after 120 cycles, while the internal-resistance-based fast-charging brought just a 29% increase over that many cycles.
Keeping the cells safe
They further noted that with the conventional fast-charge strategies, the temperatures accelerated and the battery cases of the cells cracked after 60 charging cycles—subjecting them to a greater risk of fire—while the cells using the internal resistance method showed no physical damage and kept to lower temperatures.
Using its algorithm, the team claims better safety performance and a longer battery lifespan. “Our alternative adaptive fast charging algorithm reduced capacity fade and eliminated fractures and changes in composition in the commercial battery cells,” researcher Cengiz Ozkan said.
Internal resistance-based fast charging vs. standard method - UC Riverside research
Simply put, the researchers’ adaptive algorithm checks internal resistance during charging and adapts the rate. Heat and cell degradation are all related to that one factor. As the authors explain, “this work could pave the way for the optimization of fast charging techniques to secure the lifespan and safety of various types of lithium-ion batteries.
The researchers compared it with a constant-current-constant-voltage (CCCV) industry fast charging technique, which means that the charger applies a constant current until the battery hits a predefined voltage; the voltage then stays constant while the current is gradually reduced until it gets to a full charge. In later versions on some vehicles this strategy was given more steps to increase battery longevity.
A step in the right direction, but likely not the huge leap it sounds to be
The asterisk to this is that most electric-vehicle makers no longer employ that method at face value; instead they use a highly variable method based on a wide range of details.
Tesla, for instance, bases the rate on ambient temperature, pack temperature, cell and pack health, and how the pack is used, and its age, among other factors.
Tesla’s DC fast charging—Supercharging—does not follow a specific charge curve, but the company notes that the battery charge rate will decrease “when its condition changes with usage and age.” It says that the peak rate “will vary with the size and age of battery pack, state of charge, and ambient temperature conditions.”
Tesla Supercharger site in Rocklin, California, before expansion [photo: George Parrott]
To help preserve the condition of the cells, it also will decrease the peak charging rate slightly after “a large number of high-rate charging sessions, such as those at Superchargers.”
Tesla vehicles are capable of monitoring pack health down to the cell level, and Tesla now provides suggestions through the interface screen on how to keep pack health at its best through preset charge limits and alternating AC and DC charging.
Likewise, Audi has told Green Car Reports that to fine-tune the charge curve of any fast-charging session, it monitors the temperature of individual modules within its E-Tron pack, as well as the internal resistance on a cell level.
Such an algorithm might prove more useful where such sophisticated monitoring doesn’t already exist—in budget-conscious electric vehicles for emerging markets, for instance.
The researchers have applied for a patent on the “adaptive internal resistance fast-charging algorithm,” and might possibly seek to license it to automakers.