Matthew Lumsden, CEO of Connected Energy, shares findings from a decade of designing and operating second-life stationary battery storage.
Late last year, the UK government published its Battery Strategy, setting out its vision for developing a globally competitive battery supply chain.
While the potential for second-life batteries is not well recognised by the strategy, a decade of research and development confirms that they offer a sustainable, low risk and readily available alternative to new batteries for the fast-growing energy storage market.
While exhibiting similar performance capabilities to new batteries, they do not depend on the import of critical minerals. Comprehensive tests show that an EV battery with around 75% capacity or more could be economically repurposed as stationary storage, extending the battery’s useful life by up to 100%.
Safety considerations
Although safety incidents at battery energy storage sites are rare, safety is paramount, and a significant consideration industrywide. Connected Energy’s approach is to only work with OEMs who will collaborate closely with us, sharing data to ensure the highest product safety and performance.
At present, most second-life battery stock considered by Connected Energy for stationary storage comes from fleet vehicles such as vans via automotive original equipment manufacturers (OEMs).
There are several practical reasons for this, not least because EV manufacturers are enthusiastically engaged in exploring new avenues to reuse spent batteries, which regulations require to be collected and disposed of. This means large volumes of batteries are increasingly available.
Fleet vehicles typically have excellent traceability with good service history, which provides a baseline of high-quality data. Most fleet vehicles also have predictable daily duty cycles and are charged steadily overnight which makes them relatively homogeneous in terms of use and degradation.
This has made fleet vehicle batteries ideal as a basis on which to develop the necessary technologies for a stationary storage solution using second-life batteries.
Contrary to early industry expectations, battery degradation has not been as significant as anticipated. Our test data revealed that under normal operating conditions, most second-life batteries offer 80–85% efficiency, with theoretical lithium always at the high end of 90% – not dissimilar to what is experienced in a car.
Similarly, we have now gathered more data on second-life battery performance than has existed before, covering a wide range of duty cycles. This is being used to continually improve prediction capabilities and ensure robust safety processes.
Stationary storage
In Connected Energy’s second-life stationary storage solution, battery packs are controlled in pairs. Containerised systems consist of between 24 and 100 packs, depending on the minimum system capacity. A control system manages each pair, allowing higher capacity packs to be called upon more frequently so that packs reach the same state of health over time. The system also provides greater levels of dynamism and flexibility, optimising how the batteries are used and monetised.
Nottingham City Council’s 600kW second-life stationary storage at their EV fleet depot, for example, consists of 48 packs. Installed to help transform the site’s energy use, the system stores excess electricity from three on-site solar arrays which is then used later to charge their EV fleet and reduce electricity use during peak times.
The site also aims to participate in grid services by trading stored electricity and through vehicle-to-grid services via the 40 bi-directional EV chargers.
Ongoing operation
Once in operation each pack is monitored and controlled remotely 24/7. As the peak power requirements of an EV are much higher than what is required of stationary storage, the packs are charged and discharged at a rate of about a third to a quarter than in peak EV use, making for gentler operation and lower degradation. Power can also be rebalanced across the system to minimise the batteries being subjected to a prolonged high state of charge thereby increasing efficiency overall.
Data from the packs, including operating temperature, charge, efficiency and exception alerts, are analysed to assess the system’s health and machine learning is used to identify anomalies, trends and relationships between the variables.
The results inform real-time operation to enhance performance as well as identifying preventative operation and maintenance strategies to optimise the system. The data is also used to update the models and assumptions to improve future systems and business cases.
Finally, the OEM’s original battery management system remains in use for continued safety.
The future
The potential for using second-life batteries in stationary storage is hugely exciting with the next five years set to see a significant increase in the volume of batteries reaching the end of their first life, along with the benefits of a more supportive policy environment.
