How uneven temperatures in battery packs affect performance?

Uneven temperatures within a battery pack can negatively affect its performance, longevity, and efficiency. Having all the cells at almost the same operating temperature is necessary for proper charging and discharging of the battery pack in an electric vehicle (EV) or energy storage system. Some of the most prominent effects of uneven cell temperatures are reduced capacity, faster degradation, and safety concerns.

Figure 1 shows how the charge capacity of some rechargeable batteries changes with temperature when current is fed into them and then discharged.

Figure 1. Temperature vs. charge capacity of different battery chemistry (Image: IEEE Access)

Depending on the energy and temperature, a battery will respond differently. Therefore, the battery works well when the temperature is within the range given. If you don’t, the battery could get damaged, leading to thermal runaway.

Figure 2 shows how temperature affects the operating range of a battery.

Figure 2. Battery operating range during charging and discharging process (Image: Elsevier)

As shown in Figure 2, when the temperature reaches below 15° C, the battery charging process becomes sluggish. This is bothersome, particularly in cold regions and during the winter season. On the other hand, when the temperature reaches more than 35° C, the battery life is reduced due to faster battery degradation.

One of the most direct and immediate effects of uneven temperatures in a battery pack is reduced capacity. Lithium-ion batteries, commonly used in EVs and many consumer electronics, have a temperature-dependent capacity. Lower temperatures reduce capacity, while higher temperatures can improve it slightly. However, it’s important to ensure the higher temperature is still within the operating range.

Temperature imbalances can cause uneven aging and degradation within a battery pack. Lithium-ion batteries degrade over time, and temperature plays a crucial role in this process. Cells that operate at higher temperatures tend to degrade more quickly than those at lower temperatures.

Safety is another concern due to uneven cell temperatures, particularly in high-energy applications like EVs. Lithium-ion batteries can become thermally unstable when exposed to notable variations in cell temperatures. Thermal runaway is a self-perpetuating process that can result from overheating and cause fires or explosions.

High-temperature variations in battery cells can reduce their ability to accept a fast charge. This limitation is mainly due to the risk of overheating and thermal runaway. As a result, when some cells in a pack are significantly warmer than others, the entire pack’s ability to accept fast charging is limited.

Case study
Scientists at Xi’an Jiaotong University’s School of Energy and Power Engineering have noticed some interesting things about how cells are linked in parallel. Different temperatures in battery packs have been seen to cause particular patterns of uneven aging and discharge.

In the beginning stages of the discharging process, wherever the cell temperature is high, a larger current runs. The period before 75% of the depth of discharge (DOD) constitutes the early stages of the discharging process. Upon reaching the turning point of the battery pack output voltage, the discharging current for the heated cells gradually drops. At this same point, the cooler cells experience an increase in discharging current.

As the discharging process reaches 90% DOD, the discharging current behaves like it did at the start. In other words, the discharging current flow through the cell reduces when the temperature is low and increases when the temperature is high.

When it comes to battery capacity, the rate of capacity loss goes up as the temperature difference between the cells goes up. The capacity loss rate of the battery pack also goes up as the working temperature of the cell goes up.

Summary
Cell temperature imbalances in high-energy systems like electric vehicles can pose problems such as reduced battery capacity, battery degradation, thermal runaway, limited fast charging capability, and battery aging. Operating the cell and the battery pack within a given temperature range is necessary. For the same purpose, a dedicated battery management system can be used.

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