Debate over DC fast charging points to fleet needs, expectations

Quimby Mug Bayou Florida Headshot
Updated Jun 24, 2020

Much like internal combustion engines, battery-electric motors have to battle heat to maximize performance.

“Heat is always the enemy,” said Greg Less, technical director for the University of Michigan’s battery lab.

Like conventional engines, electric vehicles (EVs) have to dispel heat that’s generated in the battery during charging and discharging. Faster current demands, particularly those experienced during DC fast charging, crank up the heat, which can lead to faster battery degradation and, in some cases, potentially dangerous battery damage if proper safeguards are not in place.

“Fast charging definitely puts a strain on current-generation batteries,” Less said. “There are research projects at the University of Michigan that have shown that it can be done safely as well, but that’s not currently available to the broader fleet.”

The University of California-Riverside reported that fast charging led to increased battery degradation and damage. (Photo: University of California-Riverside)The University of California-Riverside reported that fast charging led to increased battery degradation and damage. (Photo: University of California-Riverside)

The University of California, Riverside went so far as to advise against using current DC fast charging methods. That advice has raised concerns among EV business leaders.

UCR husband-and-wife professors Mihri and Cengiz Ozkan led a team that used industrywide DC fast charging to repeatedly charge Panasonic NCR 18650B lithium-ion cylindrical cells found in Tesla cars. The results?

“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 wrote in a UCR press release.

Ozkan told CCJ that some batteries used in the experiment actually cracked and exploded during fast charging. The problem, she said, was mitigated by using an algorithm that UCR developed, which was reported to lower high temperatures created by current demands. Their findings were reported earlier this year in Energy Storage Journal.

Tesla, which has promoted a 30-minute fast charge for its electric Semi, did not respond to interview requests. The California-based automaker currently is under federal investigation for noncollision-related battery fires that have been reported in some of its electric cars.

However, a former leading engineer at Tesla’s sister company, SpaceX, challenged UCR’s study.

“Tesla and other manufacturers have thermal systems for the battery cells that ensure the battery cells stay at optimum temperatures,” said Porter Harris, founder and chief executive officer for Power.Global, who previously led battery development at SpaceX. “There is no way the OEMs would be charging cells over 50 degrees Celsius, as this causes serious damage, including lithium plating. I’ve never seen a cell split like this, so I guarantee they were running it well out of the acceptable operating points and maybe even bypassing safety features in the cell, such as the current interrupt device (CID).”

EVgo, which bills itself as the nation’s largest public EV fast-charging network with more than 800 EV fast-charging stations in 34 states, also questioned UCR’s study.

“EVgo welcomes research into battery management that will prolong capacity and performance of the battery,” the California-based company wrote in a statement to CCJ. “In all cases, fast charging is controlled by the vehicle’s battery management system, not charging infrastructure. Also, it is important to note that no EV manufacturer uses the profile described as ‘Industry Fast Charging Technique.’ Instead, all use proprietary schemes that include active thermal management, among other techniques, to prolong battery life.”

Know when to say when

Fast charging can breathe new life to drained batteries quickly, but if a charging system is not equipped properly to handle high currents, it can lead to trouble.

“When you fast-charge a conventional lithium-ion battery, you are creating a concentration gradient within the electrolyte (all the lithium ions rush to one side of the cell) and cause what is essentially a pileup,” Less said. “It quickly becomes very difficult to get the ions into available intercalation sites in the anode. This is seen in the battery as an increase in electrical resistance and the generation of heat. This both reduces the apparent capacity of the cell and over time reduces the actual capacity of your cell. Heat is always the enemy.”

To help fight against heat, battery-cooling systems may rely on air, glycol or even refrigerant to bring down temperatures. Charging control systems also play a vital role. As current flow rises, they carefully measure critical battery indicators, such as resistance and temperature, and can step in to ensure safe charging.

Motiv founder and CTO Jim Castelaz said if fleets opt for fast charging, they need to be prepared for higher upfront costs and utility bills. Unlike most businesses, the more electricity that’s consumed, the higher the rates. Power demands during peak use times also can raise rates, he said.Motiv founder and CTO Jim Castelaz said if fleets opt for fast charging, they need to be prepared for higher upfront costs and utility bills. Unlike most businesses, the more electricity that’s consumed, the higher the rates. Power demands during peak use times also can raise rates, he said.

Jim Castelaz, founder and chief technology officer for California-based Motiv, said automated systems can keep EVs healthy. Motiv, a Ford-approved eQVM (electric Qualified Vehicle Modifier) builder, provides all-electric chassis for medium-duty commercial trucks and buses.

“At least in Motiv’s case and every other EV – I certainly know, because I drive a Tesla – the batteries get thermally managed,” Castelaz said. “DC fast charging – your rate, the speed at which you can charge the battery – is usually limited by thermals.

“What you do is you don’t charge the battery any faster than what prevents the temperature from rising,” he said. “What you see in this (UCR) study is that they’re charging this battery really fast, the temperature’s shooting up, and then the battery’s degrading very quickly. I agree with them (UCR) that we don’t want to do that. But I think you can DC fast-charge at a given rate of charging, so long as you can remove heat fast enough to support that given rate of charging.”

Growing access to chargers, faster charge times, zero emissions, lower vehicle prices and lower maintenance requirements are becoming appealing to more buyers, including deep-pocketed commercial fleets. But it’s the feature of fast charging on the Monroney sticker that should be examined carefully, especially for fleets pushing to maximize time and money.

“If the cells and the battery were engineered to accept a 10-minute charge, then it is likely OK, but certainly not the best practice for keeping your battery in top condition for as long as possible,” Less said. “Think of it like redlining your ICE (internal combustion engine). You can do it once in a while without causing any serious damage to the engine, but you wouldn’t want to do it all of the time.

“Fast charging can not only lead to excess heat which can damage the battery components, it also can, in extreme cases, lead to lithium plating on the anode,” he continued. “Lithium plating represents irreversible loss of capacity in the cell and can possibly lead to a hard short in the cell. As part of a larger module or pack, losing one cell will cause a more rapid decrease in the capacity of the other cells in the unit.”

Castelaz advised fleets to consider their use cases carefully when considering EVs and the charging infrastructure needed to support them. Slow charging when the vehicle is parked for at least six hours or more is preferred, because it doesn’t require larger investments in charging infrastructure.

“If a fleet really needs DC fast charging every day instead of just occasionally, then there are a lot of issues around the batteries and the grid that need to be worked out, and maybe there are alternative-use profiles,” Castelaz said. “Maybe they need more vehicles so that they can park some and slowly charge some at a time. Or maybe they need battery backup. Either way, whether it’s more vehicles or battery backup, they probably need more energy so that they can lessen the impact on both the batteries and the grid.”