How high heat affects EVs and what you can do about it

Ideal working temperatures

Potential EV owners are usually concerned about the range that the vehicle offers and the time needed to achieve a full charge. For years, EV makers have also focused on achieving a 620-mile (1,000 km) range target by increasing the size of the battery packs.

However, whatever range an EV gets, what is often left out is that lithium-ion battery packs can generally only achieve their maximum range when operating within optimum temperatures.

Battery makers claim peak performances in temperature ranges from 50° F to 110° F (10^oC to 43^oC) but the optimum performance for most lithium-ion batteries is 59° F to 95° F (15^oC to 35^oC).

"This narrow optimum temperature range for lithium-ion batteries is why the miles traveled and operating hours in hot and/or cold weather are hard to predict", Brian Palmieri, President of the Power Ahead Group told Interesting Engineering in an email. Palmieri's company has a decade's experience in manufacturing lithium-ion batteries for EVs and energy storage.

Risks from higher temperatures

Range anxiety, the fear of being stranded somewhere with no charge left on the battery pack, is one of the the biggest concerns among potential EV buyers. While the construction of a large number of charging stations and inter-operability between them might ease this, the impact of heat on the battery itself is also worrying, especially as periods of extremely high heat are predicted to be more common as the planet warms.

"Battery packs are the single most expensive component in an EV, meaning if they fail and the owner has to cover the replacement cost it can represent up to half or more of an EV’s total value," said Karl Brauer, Analyst at iSee Cars, a US-based automotive search engine for used cars. "Older EVs without warranty coverage lose substantial value if the battery is shown to have reduced capacity."

Resale value may not be high on the priority list of new EV owners but increased ambient temperatures can accelerate chemical reactions inside a battery pack, leading to faster degradation of components.

"The self-discharge rate of lithium-ion batteries increases with temperature", added Palmieri. "This means that the battery loses its charge more quickly when not in use, resulting in the reduction of standby time."

Not only do higher temperatures impact the overall capacity of the battery, but they also increase the internal resistance, which in turn increases energy losses during charging and discharging cycles.

Under extremely high temperatures, the components of a lithium-ion battery can undergo rapid and uncontrolled chemical reactions, often referred to as thermal runaway. This causes the battery to release large amounts of heat, which can lead to a fire or even an explosion.

Countering the heat

EV makers do account for these risks and work with battery makers and other third-party vendors to improve the thermal management of battery packs in their cars. For example, a cooling fluid can be used to take the heat away from the battery and keep it near its optimum operating temperatures.

"Our partners come to us with design challenges, and we work together to deliver solutions specific to their needs versus a one-size-fits-all approach," said David Arney, Global Lab Director for Automotive Electrification at 3M.

The company's proprietary products for thermal management include Glass Bubbles for lighter weight of parts, and Boron Nitride Cooling Fillers for high thermal conductivity and heat removal. "Our bench-to-bench collaboration with major automakers allows us to tailor 3M products to deliver solutions specific to our partner’s needs," says Arney.

There isn't a lot of data available to EV manufacturers so far about the impact of heat waves on batteries but the general impact is well known. In his experience, Brauer has seen batteries lose 20 percent of their overall range when temperatures soar to 100 Fahrenheit (37^oC).

"A battery operating in a moderate climate might retain most of its capacity for 10 or more years, but a battery in a hot climate could suffer noticeable range and performance degradation in as little as five years," Brauer added.

Arney is confident that electrical battery technology will advance to be able to sustain peak performance in extreme temperatures, both hot and cold. He also suggests solid-state batteries could replace lithium-ion and achieve better performance. Power Ahead's Palmieri is betting on sodium-ion batteries, which have a much broader optimum operating range. Neither technology, however, is available in EVs today.

So, what can EV owners do now?

Steps to take

EV owners should ensure that their cars are not exposed to direct sunlight or extreme heat during heat waves. Owners should also aim to maintain a state of charge of between 20 and 80 percent for their vehicles, unless they are planning to take long trips or have extensive use planned in a short period of time, Arney told Interesting Engineering.

"When the ambient temperature is high, the charging station may struggle to dissipate heat generated during the charging process. This can result in reduced charging efficiency and slower charging times," pointed out Palmieri, when asked about the impact of heat on charging stations. He also recommended that fast charging, in particular, can be less efficient during periods of extremely high heat.

When charging your EV in a garage, heat dissipation and ventilation are also important. "Monitoring and controlling the temperature during the charging process can help prevent overheating and potential damage to both the batteries and the charging infrastructure," Palmieri added.

So, do garages need air-conditioners now? Arney does not suggest this, as it would contribute to overall energy use. Rather, he recommends using simpler alternatives like parking under a tree or in the shade during extreme weather conditions.

"Following your vehicle's guidelines to maintain your battery's state of charge and temperature within the recommended operational range is likely your best line of defence," he added.

Palmieri agreed both that air conditioning in a garage could provide a stable and controlled environment for charging an EV, and that this is not necessary. "Ultimately, the goal is to maintain a moderate and stable temperature during the charging process to optimize battery performance, charging efficiency, and safety," he concluded.