Keeping Electric Vehicle Battery Systems Cool
A look at technologies used to keep EV traction batteries and power electronics cool and comfortable – and what happens when they are not.
For optimum power output and longevity, the lithium-ion traction battery used in an electric vehicle (EV) must be maintained between 15 °C (59 °F) and 35 °C (95 °F). At low temperatures, the electrochemical reactions necessary to produce electricity are sluggish, limiting the amount of power available. This becomes increasingly apparent as ambient temperatures drop below freezing.
The ALTDRIVE battery thermal management system from MEDATech Engineering Service. Image courtesy of MEDAtech
At high temperatures, the electrochemical reactions take place at a much higher rate, and if the temperature of the battery cells rises too high, the result can be degradation or even catastrophic thermal runaway. To prevent damage to the cells and promote long life, the pack temperature should be kept below 35 °C, even though the battery can provide the most energy at around 45 °C (113 °F). The cooling is done by a battery thermal management system (BTMS).
Cooling the Battery Pack
A variety of methods have been employed to keep an EV traction battery pack within acceptable temperature limits. One of the early EVs of the modern era was the Nissan LEAF. This vehicle used air to cool its battery pack. This was acceptable in temperate climates, but the Leaf suffered premature battery failures in places with high summer temperatures, such as Phoenix.
Similarly, in extremely cold climates, when the temperature drops below zero, the LEAF’s range is reduced. Although Nissan provided the vehicle with an onboard battery heater, all EVs suffer some range reduction at low temperatures, and the lack of an effective thermal management system on the LEAF did limit its usability.
Most EVs use liquid cooling to keep their traction battery pack within the desired temperature range. Typically, a liquid coolant, similar to the antifreeze used in a conventional internal combustion engine (ICE), is circulated through passages surrounding the cells and modules in the traction battery. The coolant is pumped through a radiator heat exchanger that rejects excess heat to the air, cooling the liquid so that it can be returned to the pack. The coolant can also be diverted to the vehicle interior to provide cabin heat on chilly days.
An Audi EV with a liquid cooling system. Image used courtesy of Audi
In EVs with really large traction battery packs—like electric buses, delivery trucks, and industrial equipment—a heat pump powered by the high-voltage traction battery can be used to provide heating or cooling inputs to the battery’s liquid cooling system. This can either be a standalone heat pump or, in the case of some buses, it can use the same air conditioning system that keeps the passengers cool to also cool the traction battery pack.
Electric trucks, buses, and delivery vans often use heat pumps to cool traction batteries. Image used courtesy of Wikimedia Commons (By Peter J. Dunn)
The heat pump solution works well because it can provide either cooling or heating, depending upon the needs dictated by the ambient temperature conditions.
Heat While Charging
Batteries don’t just produce heat when they are discharged. During charging, huge amounts of energy are pumped into the battery, raising its temperature. The charging cable can also increase in temperature and may require liquid cooling. This is especially true for fast-charging of the large-scale, mega-watt-size battery packs that are planned for use in commercial vehicles.
Cooling the Motor and Power Electronics
Because the traction battery is sensitive to damage from temperatures that are too high or too low and is subject to dangerous thermal runaway if its limits are not observed, traction battery cooling gets most of the attention.
Other EV components also must be kept with certain thermal limits. The rare-earth permanent magnets in an electric motor, for example, must be kept below their Curie Temperature—the point at which they lose their magnetic field. This can be as low as 80 °C (176 °F), so liquid cooling of the traction motors in EVs is not uncommon.
The power electronics that control the flow of energy from the traction battery to the electric motor or motors are also affected by high temperatures. Previously, silicon-based electronics were limited to around 150 °C (302 °F), requiring significant cooling capacity. The adoption of silicon carbide-based electronics, however, with operating temperatures as high as 600 °C (1112°F), has reduced the need for aggressive cooling strategies. However, cooling the electronics for controls like the battery management system (BMS) must be considered.
When Battery Cooling Goes Wrong
Although battery cooling systems are robust, they do suffer from a variety of challenges, including leaks, corrosion, and aging of the components.
For example, recently, electric truck maker Nikola Motors voluntarily recalled 209 Class 8 battery-electric trucks after one of the vehicles caught fire at the company’s Phoenix headquarters. The cause of the fire was determined to be a coolant leak inside a single battery pack. One component provided by an outside supplier is thought to be the primary source of the coolant leak.
Thus far, only one pack and another from an engineering evaluation vehicle had thermal events, though Nikola has produced more than 3,100 electric trucks to date. However, this event does point out the types of challenges that engineers dealing with battery cooling must face.