Brushing up on batteries

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Battery power is one of the critical ingredients to making hybrid trucks – much less all-electric vehicles – not only operate properly but deliver expected fuel savings as well.

Thus it’s important that fleets understand how the battery systems powering hybrid trucks are changing – and how maintaining such systems is changing, too, especially as new technology like lithium titanate oxide is introduced into the hybrid vehicle world.

To that end, researchers at Penn State University's Thomas D. Larson Pennsylvania Transportation Institute, funded through a grant from the Mineta National Transit Research Consortium, studied the use of lithium titanate oxide battery chemistry in transit buses.

Timothy Cleary, assisted by research associates Harshad Kunte and Jim Kreibick, crafted a dense 100-plus page report on the characteristics of those batteries dubbed Electrical and Thermal Modeling of a Large-Format Lithium Titanate Oxide Battery System.

While it’s exceeding dense – the mathematical formulations alone made my head spin – Cleary’s year of research highlighted some important points; especially that electrical energy storage, rather than fuel combustion, is now becoming a key component in transit fleet operation and maintenance as more them deploy hybrid buses, both in the U.S. and overseas.

Yet Cleary’s team discovered something else: other than proprietary data/models, there is little technical information or research on electrical and thermal modeling where lithium titanate oxide is concerned – and that could be a problem, especially when it comes figuring out where the “danger levels” for such batteries in terms of temperature might lie.

“The research conducted in this project supports an effort to better understand the thermal and electrical performance of advanced batteries used in today’s public buses,” Cleary noted in the report.

“Understanding these responses and comparing them in real time with validated models can help to better predict battery failures and provide quicker response in thermal events,” he added. “This work also supports the ability for manufacturers to provide validated safety systems in these battery systems.

Cleary stressed, too, that a “means of evaluating these systems to ensure their safety” should be required by the Federal Transit Authority (FTA) – those are the folks who regulate transit bus operations in the U.S. – and they should be performed by an independent agency.

Here are a few other interesting tidbits from this report:

  • In terms of nomenclature, state-of-charge or “SOC” is the percentage of battery capacity remaining relative to the same fully charged battery, while depth of discharge or “DOD” is similar but indicates the percentage of capacity that has been removed from a battery, again relative to a fully charged battery.
  • Thus SOC is analogous to the term “glass half-full,” while DOD is “glass half-empty.”
  • The all-important battery management system or “BMS” governing battery operation relies heavily on the sensors within the battery pack. Without those sensors and the data they provide, passenger safety and battery performance are sacrificed.
  • Such sensors typically include cell voltage, pack and/or string current, and a select distribution of temperature readings.
  • Using that data, The BMS “brain” then makes key decisions, adjusting how the battery pack is managed and if it will stay connected to the vehicle’s loads – often meaning whether said vehicle stays in operation or not.

Information to keep in mind as more hybrid systems – especially for transit buses – are expected to hit the road in the near future.

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