Automakers, and researchers, are pursuing many avenues to extend the lifetime of batteries for electric vehicles (EVs). Central to this endeavor will be developing an improved understanding of the mechanisms of battery degradation – complex and coupled electrochemical, thermal and mechanical processes that lead to a loss of performance over time, reducing the battery’s ability to store charge and deliver power. This understanding is a prerequisite for developing strategies to prevent the most damaging degradation mechanisms from occurring, and slow others.

One technique being pursued by automakers and battery developers to extend lifetime is ‘derating’, where a battery is operated more conservatively in certain situations to reduce degradation. An example of derating is to slow the rate of charging when the battery nears its maximum storage capacity, which limits the potential damage to the cells.

Dr Billy Wu, a senior lecturer in electrochemical engineering at Imperial College London, asserts that in battery engineering everything has a trade-off, but with a deeper understanding of physics the optimum can be approached more closely.

“You can’t have your cake and eat it. It may be convenient to charge a battery really quickly, but the penalty is that you’ll degrade it faster. So, the question is where do you sit on that trade off curve? How much do you compromise performance for longevity? We’ve looked at how you can achieve maximum performance without pushing a battery too far, preventing degradation.”

A Faraday Institution Industry Fellowship between Imperial College London and WAE, an Oxfordshire-based technology and engineering services business, has investigated the effectiveness of various derating approaches. The collaboration completed a critical review of derating methods and produced a valuable go-to resource for a wide range of organisations (automakers, battery developers, grid energy storage system operators, and developers of consumer electronics) seeking to exploit the considerable potential of the technique.

Derating is appealing because it can be implemented at minimal cost, applied via software updates to existing EV fleets, and does not influence system reliability or generate additional safety issues. However, batteries need to be treated differently depending on the battery chemistry and variables such as temperature and age. A balance needs to be struck in order not to overly derate, (for example, significantly reducing the ability of the vehicle to accelerate or lengthening the time it takes to charge the battery) which would negatively impact customer satisfaction. However. considerable uncertainties remain around optimal derating approaches, and this is complicated by the diversity of influencing factors.

The study concluded that the process of designing any control strategy requires a balance between ‘hard’ derate limits that maximise performance within safety constraints, and ‘soft’ limits that promote enhanced lifetime. The partnership’s suggested approach is to use emerging techniques, such as machine learning-based diagnostic and prognostic methods and improved sensors and telematics, to develop a dynamic hybrid derating framework where, with battery aging, operating limits would be gradually narrowed to circumvent fast degradation. Physics-based modelling and sensitivity analysis would be used as a guide to determine how and when to derate operating parameters. This could ultimately lead to the development of battery digital twins, which fuse real-time data with physically relevant models, towards making optimal asset specific decisions, as outlined in a recent perspectives paper in Joule.

 

Tim Engstrom, Technical Lead for Elysia, WAE’s recently launched battery intelligence arm, and one of the paper’s authors comments: “We have highlighted the significant potential for extending battery lifetime by employing smart, health-adaptive control strategies to control a battery as it ages to maximise its lifetime. Understanding batteries better is key to achieving a sustainable net-zero world – this means understanding the sensitivity of battery usage to long term performance, reliability and safety characteristics.

“EVs are expensive and part of the reason for this is because the batteries are over-engineered; they’re bigger than they need to be. By treating batteries better, we can make them last a lot longer, which completely transforms the business case.”

Co-author, Dr Billy Wu, continues: “Batteries degrade over their lifetimes, just like people. As we age, we may be less physically fit than when we were younger and could endure more strenuous activity without damaging our bodies. We’ve highlighted ways we can extend battery lifetime in relation to how we use them as they age.

“For your average car buyer, one of the most important factors is how fast your car depreciates; battery life comes into this, and increased battery life is a game changer for companies like fleet operators looking to switch to EVs. Improving battery life by say 50% would have a huge operational and financial impact on these businesses.”

Research is ongoing at Imperial to demonstrate the effectiveness of derating techniques, including on e-bikes.

“Lots of techniques work in the lab but we need to see how they work in reality – for example, we don’t yet know how to determine the operating limits in each aging stage,” adds Dr Wu. “No unified derating method exists for all battery types – the approach needs to be adapted for each battery chemistry, design, application and acceptable perceptible impact on the customer.”

 

About Industry Fellowships

The Faraday Institution Industry Fellowship scheme aims to strengthen ties between battery researchers based in industry and academia, seeding longer-term collaboration and providing career development opportunities.

The WAE/Imperial fellowship contributed to the lead author of the Journal of Power Sources paper, Dr Haijun Ruan, securing his first academic position as Assistant Professor at Coventry University.

The Fellowship also helped Dr Billy Wu win an Impact Acceleration Account award at Imperial on battery digital twins.

Case study published August 2023.