There’s an optimal temperature range for Li-ion batteries in electric vehicles (EVs). It lies between about 15° and 35° C. Outside of this range, performance suffers when charging and discharging the batteries. The major requirements for rechargeable batteries are energy, power, lifetime, duration, reliability, safety, and cost — and they can all be affected by operating temperatures.
This FAQ reviews the impact of high and low-temperature operation on Li-ion batteries. It looks at how the driving habits of EV owners and local driving conditions other than temperature can increase or decrease the influence of ambient temperature.
High temperatures tend to degrade Li-ions more quickly by breaking down the chemical constituents in the cells. To prevent or slow the degradation, the power rating of the batteries can be reduced, or an active cooling system can be used to keep the temperature from rising too quickly.
While low temperatures can also degrade batteries, primarily by Li plating (discussed more below), the primary effect is to reduce performance due to sluggish electrochemistry that reduces the power and energy capabilities of the cells (Figure 1).
A primary function of the battery management system in an EV is thermal management, which:
- Keeps the cells in the optimal temperature range for the best power and energy performance
- Minimizes cell-to-cell temperature differences
- Prevents the cells from going outside maximum and minimum limits to ensure safe and reliable operation
- Maximizes the long-term energy capacity of the cells
Figure 1. Idealized battery performance curve versus temperature, showing the drop in performance for operation outside the optimal range of 15 to 35° C. The green curve is for battery discharging, and the yellow is for charging (Image: NREL).
Running EV batteries too hot can have serious safety considerations. But running them too cold has severe performance impacts. Most obviously, using the energy in the battery to keep it warm reduces driving range. Below 0° C, the capacity of Li-ion batteries is significantly reduced, further decreasing the driving range. Lithium plating caused by cold temperature operation results from the formation of metallic lithium around the anode during charging.
Typical Li-ion batteries in EVs have a Li-metal-oxide cathode and a layered graphite anode. Cold temperatures cause slower diffusion of Li ions into the cathode during charging, resulting in Li plating that can permanently reduce battery capacity. In extreme cases, dendrites can form and cause internal shorts that increase safety risks.
Three important cold temperature ranges have been identified, including:
- Under 10° C, Li plating and permanent degradation can occur if the battery is fast-charged. Slower charging can help address this concern.
- Under 0° C, Li plating is even more of a concern, particularly if the battery is charged faster than a 1C rate.
- Under -20° C, battery performance and its ability to accept a charge are further reduced. Prolonged exposure to these temperatures can destroy the battery.
Actual results may vary
The temperature-related performance described above represents averages. Although the general arc of the performance curve described in Figure 1 applies to all EVs, actual performance for a given model of EV can vary significantly based on how the vehicle is driven.
In one study using actual driving conditions and real trips, 10% of vehicles with the best performance, the 90th percentile, enjoyed 32% more range than the overall average and twice as much range as the lowest 10%, the 10th percentile, of vehicles (Figure 2).
Figure 2. Actual EV battery pack performance strongly depends on how the vehicle is driven, with the 90th percentile realizing twice the range of the 10th percentile (Image: Alaska Center for Energy and Power).
The curves in Figure 2 are based on measured performance during actual trips. During these trips, the vehicles were exposed to a range of conditions like varying terrain, driver habits, acceleration and deceleration rates, maximum speed, condition of the vehicle at the beginning of the trip (for example, was it in a garage on a cold day and therefore already warm?), and the length of the trip.
For different models of EVs, the curves can be flatter or more peaked in the optimal temperature range based on the efficiency of the thermal management system. Thermal management systems based on heat pumps tend to be more efficient, use less energy, and they can also be better at maintaining the desired operating temperature. Vehicles using heat pumps had flatter curves exhibiting reduced impact from high or low temperatures.
Summary
Operating temperature is a significant factor in EV battery performance and can impact the system’s energy, power, lifetime, duration, reliability, safety, and cost. The effects of operation at too low or too high temperatures differ, but both can be damaging and limit vehicle performance. As the impact of temperature is not a given, it can be strongly impacted by driver habits and driving conditions other than temperature.
References
Climate Control Load Reduction Strategies for Electric Drive Vehicles in Warm Weather, SAE International
Cold Weather Issues for Electric Vehicles (EVs) in Alaska, Alaska Center for Energy and Power
Energy and Power Evolution Over the Lifetime of a Battery, ACS Energy Letters
How does temperature effect EV performance?, GreeneMotion
Tools for Designing Thermal Management of Batteries in Electric Drive Vehicles, NREL
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Filed Under: Batteries, FAQs, Featured, Thermal Management