Lithium-ion batteries in electric vehicles (EVs) have an optimal temperature range between 15°C and 35°C. Outside of this range, the battery's performance during charging and discharging can be affected. The main requirements for rechargeable batteries are energy, power, lifespan, duration, reliability, safety, and cost—all of which are influenced by operating temperature.
This article reviews the impact of both high and low temperatures on lithium-ion batteries. It also looks at EV drivers’ habits and how local driving conditions, besides temperature, can either increase or decrease the effect of environmental temperatures.
High temperatures can break down the chemical components in the battery, speeding up the degradation of lithium ions. To prevent or slow this degradation, the battery's power rating can be lowered, or an active cooling system can be used to keep temperatures from rising too quickly.
Although low temperatures can also damage the battery, primarily through lithium plating (discussed below), the main issue is reduced performance due to slower electrochemical reactions, which lowers the battery’s power and energy capacity (Figure 1).
Figure 1. The ideal battery performance curve relative to temperature, showing a decline in performance when operating outside the optimal 15-35°C range. The green line represents battery discharge, and the yellow line represents charging (Image: NREL).
A key function of the battery management system (BMS) in EVs is thermal management, which involves:
- Keeping the battery within the optimal temperature range for best power and energy performance
- Minimizing temperature differences between battery cells
- Preventing the battery from exceeding its maximum and minimum limits to ensure safe and reliable operation
- Maximizing the long-term energy capacity of the cells
Operating EV batteries at excessively high temperatures can pose significant safety risks. On the other hand, low temperatures significantly affect performance. One of the most obvious effects is that using battery energy to maintain its temperature reduces driving range. Below 0°C, lithium-ion battery capacity drops significantly, further reducing range. The lithium plating that occurs during low-temperature operation happens when metallic lithium forms around the anode during charging.
A typical lithium-ion battery in an EV consists of a lithium metal oxide cathode and a layered graphite anode. At low temperatures, the diffusion of lithium ions into the cathode slows down, leading to lithium plating, which can permanently reduce battery capacity. In extreme cases, dendrites can form and cause internal short circuits, increasing safety risks.
Three critical low-temperature ranges have been identified:
- Below 10°C, lithium deposition and permanent degradation may occur if the battery is fast-charged. Slower charging can help address this.
- Below 0°C, lithium plating becomes a bigger concern, especially if charging occurs at a rate higher than 1C.
- Below -20°C, both battery performance and charging capability drop further, and prolonged exposure to such temperatures can damage the battery.
Actual results may vary The temperature-related performance mentioned above represents averages.
Although the general curve described in Figure 1 applies to all EVs, the actual performance of a given EV model can vary significantly depending on driving habits.
In a study of real-world driving conditions and trips, the top-performing 10% of vehicles (the 90th percentile) had 32% better range than the overall average, and twice the range of the lowest-performing 10% of vehicles (the 10th percentile) (Figure 2).
Figure 2. Real-world EV battery performance is highly dependent on how the vehicle is driven, with the 90th percentile range being double that of the 10th percentile (Image: Alaska Center for Energy and Power).
The curves in Figure 2 are based on actual performance measured during real-world trips. In these trips, vehicles encountered various conditions, such as different terrains, driving habits, acceleration and deceleration rates, maximum speeds, vehicle condition at the start of the trip (e.g., parked in a warm garage during cold weather), and trip length.
For different EV models, the curve might be flatter or sharper within the optimal temperature range depending on the efficiency of the thermal management system. Heat pump-based systems tend to be more efficient, using less energy, and better at maintaining the required operating temperature. Vehicles equipped with heat pumps show flatter curves, with less impact from high or low temperatures.
This article was reposted from the WeChat public account: qicheyanjiuyuanauto