The charging schedule that protects electric bus battery life
Electric bus battery life depends on the charging schedule a transit agency builds around its own routes, a plan for when and how fast each bus charges rather than plugging in whenever convenient. The National Renewable Energy Laboratory says cycling a battery from 40% up to 70% of full charge, called state of charge, protects battery health better than cycling it from 60% up to 90%, because both extremes wear a lithium battery down faster. A federal survey of 25 United States transit agencies, published by the Federal Transit Administration in August 2023, found 48% already used a charging schedule built to manage the battery this way. The other 52% said they had not run their buses long enough, or did not have enough, to need one.
What happens when a fleet skips it
Foothill Transit, a Los Angeles area agency, told federal researchers its buses reach a battery life expectancy of 6 years before degrading and depleting faster, part of why it aims to extend its 12 year bus life expectancy to 14 years. County Connection, in Concord, California, now keeps its bus batteries above 60% state of charge because it recognized that same degradation. Long Beach Transit found the opposite problem. Its 10 bus pilot fleet developed battery and degradation problems traced to conservative use that never let the batteries discharge below an optimal charge level, keeping them too full rather than empty.
Real time scheduling beats a fixed plan, and costs less
2 academic studies built that same mid range discipline into automated charging models, and found it also saves money. Researchers at Utah State University, using real Utah Transit Authority data, simulated 3 charging plans for a Salt Lake City bus fleet. A simple threshold rule, charge whenever the battery drops below a set point, cost 698.10 dollars a day. A plan set once daily cost 315.14 dollars, cheapest of the 3, but left buses under minimum required charge in 45 of 150 runs. A feedback based schedule, adjusting through the day, cost 384.49 dollars but violated that same requirement in just 1 of 150 runs. Against a method that ignores real time electricity prices, the feedback based schedule cut cost by as much as 52%.
The open loop plan is the cheapest of the 3 but left buses under their minimum required charge in 45 of 150 simulated runs, against 1 of 150 for the smart feedback strategy, so it is not the recommended strategy despite costing less.
Show the numbers
| Typical threshold | 698.10 |
| Open loop plan | 315.14 |
| Smart feedback | 384.49 |
A Brussels model builds the same limit into its own math
Researchers at KU Leuven and the Luxembourg Institute of Socio Economic Research modeled a real Brussels bus line, using real STIB MIVB and electricity price data. The model sets hard limits on how far each bus battery charge level can swing, the same mid range discipline as the federal guidebook. Price optimization alone cost 859.58 euros a day for the line, 28 buses and 232 daily trips. Adding peak demand management and selling bus battery power back to the grid brought that to 813.61 euros. Adding solar panels and battery storage at the depot cut it to 359.90 euros, under half the starting cost. The model declines to sell power from the bus batteries back to the grid in its base case, a choice visible only by reading the full model, because its own degradation cost calculation outweighs the electricity selling price.
These figures cover 1 Brussels bus line, 28 buses and 232 trips on a single representative day, and are specific to that line own solar and storage setup, not a savings figure other agencies should expect.
Show the numbers
| Price only | 859.58 |
| Adds peak, grid sales | 813.61 |
| Adds solar, storage | 359.90 |
An agency already running this software
King County Metro, in Seattle, Washington, was running charge scheduling software across 40 battery electric buses by March 2023, and won the CIO 100 Award, a national technology recognition award, for the project. Michelle Allison, the agency General Manager, spoke about it when the award was announced.
The CIO 100 Award recognizes the collaborative approach Metro and KCIT has in place as we continue our transition to a zero-emission future.
Michelle Allison, King County Metro General Manager, speaking about the agency award for its charge scheduling software. Source 5.