Battery – The Basics

Due to their high energy density and satisfactory power density, modern Battery Electric Buses (BEBs) utilize lithium-ion batteries. These batteries store electrical energy in the form of chemical energy and convert it into electricity when needed. BEB batteries are integrated into the bus structure, typically located at the top, bottom, or rear, depending on the BEB Original Equipment Manufacturer (OEM). Compared to diesel buses, BEBs are more fuel-efficient due to their electric motors, which incorporate regenerative braking and consume less energy than diesel engines. The amount of energy remaining in a battery pack is referred to as the State of Charge (SOC). Similar to a fuel gauge in conventional vehicles, the SOC indicates the remaining energy to the driver.

In theory, the SOC can range from 0% to 100% capacity, but in practice, there are minimum and maximum limits set to safeguard the battery’s long-term lifespan.

In the next image, the lower limit of the SOC (represented by the color red) should be set above 0% to minimize battery performance degradation. An additional lower level of SOC (represented by the color yellow) is also reserved as a backup for emergencies and unforeseen delays. The total margin for these limits can range from 15% to 35%, depending on the specific requirements.

With the implementation of the minimum and maximum limits, the usable energy (represented by the color green) accounts for approximately 70% of the battery’s theoretical capacity at the start of its useful first life.

Click to expand. Batterie’s type of energies at the beginning of their useful life. PEM Motion (2023)

End-of-Life Batteries

Battery capacity diminishes over time and usage. As batteries undergo charging and discharging cycles, their internal physical and chemical structures deteriorate. Batteries naturally experience aging over time, irrespective of usage, but the degradation rate amplifies with increased use. Because bus batteries are a relatively new and rapidly evolving technology, there is some uncertainty regarding their service life, although certain bus manufacturers provide 12-year warranties for their batteries. Typically, the end of a battery’s useful life is considered when it retains less than 80% of its original capacity, although this threshold can be as low as 60%.

The next image, shows the SOC limits for a battery near the end of its lifespan. Battery degradation is also accelerated by keeping batteries at a high SOC. BEB battery life can be extended by: cycling the battery to lower SOCs when possible, avoiding high SOCs when the BEB is not in use, and/or lowering the upper SOC limit. For example, it is much better for the life of the battery to cycle a BEB battery from a SOC of 40% to 70% than to charge it frequently from a SOC of 60% to 90%.

Click to expand. Batterie’s type of energies at the end of their useful life (End-of-Life). PEM Motion (2023)

When a battery reaches the end of its useful life in BEB applications, it typically retains 60% to 80% of its original capacity, presenting opportunities for second-life applications before recycling. It is crucial to acknowledge that once a lithium-ion battery has degraded to the extent that it is no longer viable for any application, it can be recycled.

Conclusion

Understanding the lifespan of the batteries in an E-bus is crucial for knowing how to use them properly, identifying the factors that affect their longevity, and determining how to dispose of them at the end of their useful life, whether through recycling or finding alternative uses.

Later, you will explore the topic in more depth and examine the concept of giving batteries a “second life.”

References

• Aamodt, A., Cory, K., & Coney, K. (2021) Electrifying Transit: A Guidebook for Implementing Battery Electric Buses. National Renewable Energy Laboratory. Retrieved from: Link