ZEEbus, a UK-based innovator in electric vehicle repowering, is transforming public transport by converting diesel buses into zero-emission electric vehicles. Their latest project involves retrofitting an Alexander Dennis E400 double-decker bus with a bespoke electric drivetrain powered by eight lithium-ion battery packs. To ensure the safety and reliability of these battery systems, ZEEbus has integrated Metis Engineering’s Cell Guard sensors into each battery pack.
Cell Guard is a next-generation CAN-based device designed to enhance battery safety by monitoring critical environmental parameters. It detects volatile organic compounds (VOCs), absolute pressure, air temperature, humidity, dew point, and, optionally, hydrogen levels and shock loads up to ±24G. This comprehensive monitoring allows for early detection of potential issues such as thermal runaway, enabling proactive measures to prevent catastrophic failures .
Battery Safety Sensors
In the context of the ZEEBUS project, the integration of Cell Guard provides multiple layers of safety and valuable work in data analytics. Each of the eight battery packs is equipped with a Cell Guard sensor, ensuring real-time monitoring of internal conditions. The sensors’ ability to detect VOCs and pressure changes offers early warnings of cell venting, a precursor to thermal runaway. Additionally, the optional accelerometer feature provides crucial crash detection capabilities, allowing the system to shut down safely in the event of a collision.
How the Sensors Work
Each of the eight battery packs in the repowered ZEEBUS is fitted with a Metis Engineering Cell Guard sensor, strategically installed inside the enclosure to monitor internal conditions. The sensors are connected together in a daisy-chain configuration over the CAN Bus, with each unit assigned a unique CAN address corresponding to its specific battery pack. This setup allows precise identification of any issues within the system.
If a Cell Guard sensor detects a combination of volatile organic compounds (VOCs) and a sudden pressure spike, key indicators of cell venting, it can immediately signal which pack is affected. The system can then be configured to isolate that specific pack by cutting the electrical load to it, allowing the affected cell to cool down. This early intervention can stall the onset of thermal runaway or significantly mitigate its effects, enhancing overall vehicle safety and reducing the risk of escalation.
A ZEEbus spokesperson highlighted the importance of Cell Guard in their safety system:
“The Metis Cell Guard is a key part of our bespoke battery pack safety system, not only providing early warning of issues but also working as a backup for critical temperature and pressure data. Additionally, the G sensor provides crucial crash detection which allows the pack to shut down safely in a controlled manner if the worst happens on the road. By logging the rich data from the Cell Guard unit in use we can enhance our system modeling to improve accuracy and help us design the next generation systems.”
The data collected by Cell Guard sensors not only enhances immediate safety measures but also contributes to the long-term development of ZEEbus’s electric vehicle systems. By analyzing the logged data, engineers can refine system models, improve predictive maintenance schedules, and optimise future battery pack configurations for performance and safety.
Upcycling Not Scrapping
The Alexander Dennis E400 double-decker, known for its robust vehicles and widespread use in urban passenger transport, serves as an ideal platform for ZEEbus’s electric conversion. By integrating advanced battery monitoring systems like Cell Guard, ZEEbus ensures that the repowered buses meet high safety standards and operational efficiency, contributing to the broader goal of sustainable public transportation.
Jules Tipler, Business Development Lead at Metis Engineering, commented:
“While the risk of thermal runaway in electric vehicles remains very small, the reality is that as the number of EVs on our roads increases, so too does the likelihood of incidents occurring. At Metis Engineering, our goal is to stay ahead of that risk by providing early warning systems that give operators the best experience and chance to respond proactively. Cell Guard adds a critical layer of safety by detecting the earliest signs of thermal events, well before they become hazardous, giving drivers valuable time to pull over and evacuate passengers safely. It’s about protecting lives and growing confidence in the future of electric transport.
The team here at Metis Engineering are delighted to support innovative companies such as ZEEbus that can re-power existing diesel buses, keeping them on the road and carrying passengers. Not only does this reduce the need to build new vehicles, but it also takes polluting diesel engines out of our towns and cities, accelerating the transition to cleaner, smarter public transport.”
The Future Starts Here
ZEEbus’s use of Metis Engineering’s Cell Guard sensors exemplifies the integration of advanced safety technology in the evolution of electric public transport. This collaboration not only enhances the safety and reliability of repowered electric buses but also provides valuable data to inform future developments in electric vehicle technology.
Companies like ZEEbus play a vital role in accelerating the transition to a cleaner, more sustainable transport network. By repowering existing diesel vehicles with electric drivetrains, making them full EV, they extend the useful life of buses already on the road, reducing manufacturing emissions and conserving resources. This approach not only avoids the carbon footprint associated with building entirely new vehicles, but also removes polluting diesel engines from our towns and cities. Repowered electric vehicles help cut urban air pollution, lower greenhouse gas emissions, and support more efficient use of public funding, all while delivering the benefits of electrification at a faster and more scalable pace.
