CTE Research Identifies Best Practices for Extending Transit Bus Energy Storage System Life
Authored by Robert Kasse, Director of Client Services
Contact: robert@cte.tv
The Center for Transportation and the Environment (CTE) has completed a comprehensive research project examining the factors that influence the health and lifespan of energy storage systems (ESS) in transit buses. Drawing on operational data from more than 1,000 buses, the study provides transit agencies with practical strategies to extend ESS life, reduce maintenance costs, and improve fleet performance. Energy storage systems are among the most expensive components of transit bus powertrains, notably for diesel hybrids and others, making their longevity a critical factor in total cost of ownership. Understanding how operational practices affect ESS lifespans can help agencies maximize the value of their investments and potentially avoid costly mid-life replacements.
Analyzing Real-World Transit Bus Performance
CTE developed an extensive database of operational and ESS performance data from 1,012 transit buses operated by 26 transit agencies across the United States and Canada, covering 45 million operating miles and over 88 million kWh of ESS energy throughput.Using this dataset, CTE analyzed how factors including mileage, charging practices, state of charge (SOC) levels, and vehicle utilization affect ESS state of health (SOH), a measure of an energy storage system's present storage capacity relative to its original capacity. The research also established a method for transit agencies to independently calculate and track ESS health using available vehicle telematics data, reducing reliance on proprietary measurements from manufacturers. CTE's findings and methodology are detailed in a white paper, providing transit agencies with practical insights and tools to better understand, monitor, and manage ESS performance over time.
Key Findings and Best Practices
The study identified several important insights that challenge common assumptions about ESS life cycles:
- Vehicle utilization matters more than age. Unlike light-duty passenger vehicles, transit bus ESS degradation is primarily driven by mileage and cumulative energy throughput rather than time alone.
- ESS degradation slows over time. State of health typically declines more rapidly early in an ESS's life before stabilizing, meaning that estimates based upon initial degradation rates can underestimate ESS lifetimes.
- Avoiding low states of charge is critical. Storing an ESS below 30% state of charge can accelerate degradation, while keeping systems within intermediate charge ranges supports longevity.
- High-power charging is not inherently more damaging. Charging above 200 kW did not independently accelerate degradation in the study, largely because high-power energy storage systems operate within narrower usable charge ranges.
- Proper maintenance and storage practices support ESS health. Maintaining thermal management systems and avoiding prolonged unpowered storage in extreme temperatures can help protect ESS performance.
CTE’s findings also underscore the importance of selecting ESS capacities that are appropriate for specific route requirements and structuring ESS warranties around measurable energy storage capacity rather than complex state-of-health definitions.
Helping Transit Agencies Maximize Their Investments
Beyond identifying best practices, CTE's research provides transit agencies with a practical framework for monitoring ESS health, benchmarking bus fleet performance, and making more informed operational and procurement decisions. By applying these findings, agencies can better understand expected ESS lifespans, optimize vehicle utilization and charging strategies, and strengthen future procurement specifications and warranty requirements. CTE is ready to help transit agencies apply these findings to their own fleets through customized ESS health assessments and operational data analysis. By evaluating fleet-specific performance trends, CTE can help agencies identify opportunities to extend ESS life, improve operational efficiency, and make more informed maintenance and procurement decisions.
