By David Warren
How battery-electric propulsion works, and why demand continues to grow
Public transit manufacturers are intensifying focus on zero-emission transportation that runs cleaner, greener, and leaner for growing cities. Battery-electric transit has shot into pop culture, and North American OEMs are finding themselves in a moment to shine. With this comes an opportunity to educate and enlighten the general public about battery-electric power. As a relatively new propulsion option, battery-electric power (and its charging infrastructure) is not widely understood, leaving room for assumption and ambiguity when it comes to “How it works.”
New Flyer is actively participating in and leading the conversation, while investing in the advancement of battery-electric and autonomous bus technologies in North America. In September, New Flyer of America held a ground-breaking event for its $25 million expansion and renovation project in Anniston, AL, and also announced the creation of a Vehicle Innovation Center (VIC) as part of the project, which opened in October. The VIC has a mandate to:
• Explore and advance bus and coach technology through sustainable research and development, fresh innovation, progressive manufacturing, and bold thinking;
• Foster dialogue through discussion, education, and training on the latest zero-emission and autonomous driving vehicle technologies;
• Engage learning through current and interactive exhibits, simulation, hands-on experiences, and observations;
• Generate energy and commitment to clean air quality, safety, and economic benefits for people, communities, and business; and
• Harness the positive influence of collaboration, environmental stewardship, and social change to advance mobility solutions.
As manufacturers continue to invest in the research and development of battery-electric power, we must consider how advances in batteries and the benefits of battery-electric bus (BEB) technology are relayed to the end-user. If we, as an industry, intend to build excitement and demand for zero-emissions electric propulsion, we must ensure basic physics and chemistry principles are truly and responsibly understood by various stakeholders.
The most important fundamentals of battery-electric buses include energy storage, battery cell technology, and the integration of battery packs into the BEB. Although there are close similarities between hybrid buses and electric motors and batteries, BEB customers must have a clear understanding of today’s basic battery chemistry and battery integration when considering an all-electric transit bus system.
Chris Stoddart, vice president of engineering and customer service, starts with basics to demystify an anode is made of one type of chemistry, and a cathode made of another type of chemistry. The electrolyte provides the flow of electrical charge between the cathode and anode. When a load, such as an electric motor, is connected to the battery terminals, a closed circuit is created and a chemical reaction occurs.
The next important consideration is the re-charge capability of lithium-ion batteries; from in-motion created regenerative (“regen”) power, or power transferred from an off-board charger such as a depot plug-in charger or from an on-route rapid charger. Both regen and off-board charging transfers electricity to the battery cells to create a reverse chemical reaction that restores the charge.
The entire battery cycle is repeated multiple times throughout the life of a BEB, with stages consisting of:
1) Chemical energy stored in the lithium-ion battery cells,
2) To energy released from the battery as electricity,
3) Converted to kinetic energy through the motor for propulsion
4) Recovering electricity from the motor (turned generator)
5) Causing a reverse chemical reaction to recharge the battery cells,
6) Supplemented by off-board plug-in or overhead charging to create a reverse chemical reaction to restore the battery cell to a full state-of-charge.
Re-chargeable batteries eventually experience capacity fade. This is why battery engineers must select the ideal chemistry and manufacturing quality provisions to achieve a long-life and reliable performance in transit BEBs. New Flyer batteries are designed and tested over 5,000 deep depth-of-discharge cycles to achieve an expected service life of 12 years or more.
At New Flyer, battery engineers selected Nickel Manganese Cobalt (NMC) for the lithium-ion battery cell chemistry. New Flyer works with two Michigan based battery companies, XALT Energy, and A123 Systems, based on many successful years of industry experience with NMC batteries for heavy vehicle applications, operating in the harshest environmental conditions throughout the globe.
Stoddart shares the key role of New Flyer system integration engineers; having ultimate responsibility for the battery integration comprised of safety monitoring systems, power distribution units, thermal management in cold and hot weather, and battery packaging to optimize the weight distribution over the bus axles and maximize the passenger-carrying capability.
The most difficult challenge for batteries in transit BEB applications is specific energy, measured in Wh/kg. Diesel has a near 60:1 advantage over the highest technology battery cells and is the sole reason BEBs have range limitations in transit where gross axle load ratings have finite limits.
Stoddart notes a major difference in the engineering mindset needed to design a transit BEB carrying massive batteries. He starts with an electric car, where engineers methodically balance the load between the four corners to create both high-performance handling and tight cornering. For transit buses, this mindset is not ideal, a concept that has not been recognized by all BEB manufacturers. A transit BEB is similar to a vocational truck such as a Peterbilt, where four load-carrying tires are attached to the rear axle, with only two steer tires on the front axle. Spreading the heavy battery load evenly between the two axles is fundamentally flawed on a six-wheel vehicle, and undoubtedly, will cause overloading of the front axle, at a load well below the passenger load-carrying capability of an equivalent diesel-powered bus. New Flyer, on the other hand, methodically biases the battery loads to the rear axle, where more of the load can be carried by four traction tires.
Stoddart concludes that U.S.-based battery manufacturers, complemented by New Flyer’s battery integration expertise, offer transit agencies unsurpassed life-cycle value from cradle-to-grave. Battery cell technology will continue to evolve, as noted by an annual 12 percent increase in specific energy capability over the past five years.
As the cell technology improves, we’ll be able to offer BEB customers even further range between recharging.
David Warren serves as director of sustainable transportation for New Flyer of America. Visit www.newflyer.com for more information