Dr Nam Truong, Co-Founder and CEO at STABL Energy GmbH, a Germany-based company, which provides renewable energy storage solutions for EV batteries, discusses the complexities of the 2nd life battery concept and how new approaches must be adopted to offer ‘comparable performance to 1st-life storage manufacturers’.
Lithium batteries are subject to a complex degradation process that gradually reduces their usable capacity. In the automotive industry, end-of-life is defined as 70% to 80% of the remaining capacity.
The 2nd life concept is the reuse of these discarded batteries in another application to extend their life. The idea is great in terms of sustainability and cost-effectiveness. The concept of repurposing batteries from very demanding mobility applications to less demanding grid applications is rather obvious, which is why many players, including utilities and car manufacturers, are and have been active in this field. Real-world implementation is full of technical challenges, and the vast majority of implemented projects are pilots. Only a few companies continue their efforts and show serious intentions to commercialize 2nd-life battery storage at scale, while the majority drops the topic.
The root cause of the technical challenges is the aging of batteries. Although battery aging is the dominant research topic in academia and the industry, it is surprisingly poorly understood and managed.
Every single battery cell in a high-voltage battery pack of several hundred batteries determines the available capacity and performance. As in a chain, the weakest link determines the strength of the whole chain. A single cell of lower capacity will limit the available capacity of the rest of the batteries in the pack, and any cell failure will cause the entire pack to fail.
Battery cells must therefore be quasi-identical throughout their lifetime. To ensure this, production variation and any environmental variation such as humidity or temperature must be minimized throughout the manufacturing process, including the assembly of battery modules and packs and throughout their service life.
Small differences, such as temperature gradients during transport or in the packs, or resistor variations at the contactors, lead to different rates of aging.
These effects are self-reinforcing: the risk of premature degradation of individual battery cells, affecting the entire pack, is greatly exacerbated by even small initial variations.
Combine this with the complexity of battery cell production, and you can see why it is industry standard to use only battery cells from the same production batch together or why obvious advancements such as liquid cooling to minimize temperature differences in battery packs are winning innovation awards.
The challenge of controlling battery aging is even greater in 2nd life, where batteries from different vehicles with different histories are combined. The use of full packs of several hundred cells means that any weakness in the pack design is accepted and may even be exacerbated if liquid cooling of the packs is omitted.
As for the concept of breaking down packs into smaller modules (with a dozen cells per module), these modules need to be tested and the most similar ones matched before using them together. The go-to approach of 2nd-life manufacturers with a battery module approach is AI-enhanced rapid battery testing to reduce the check-up cost while achieving ”sufficient“ accuracy for proper matching.
Scientific papers have identified processes in battery cells (Attia et al. have identified six dominant processes) that are difficult to measure or track individually, let alone all together. There is insufficient knowledge and data on battery aging after the first life of the battery to accurately predict future battery aging in the second life. To exclusively rely on AI appears to us like a risky gamble with high chances of future liabilities to customers.
This leaves 2nd-life battery manufacturers with two design options for battery storage: First, the 48 volts architecture, which is limited to smaller applications such as home storage or mobile applications for construction sites. Larger storage systems using this concept have far inferior performance compared to the state-of-the-art design using high-voltage battery packs.
Second, the high-voltage design, which is the dominant architecture for stationary battery storage (for good reasons), carries a high risk of premature performance degradation due to individual battery cells with accelerated aging.
The 2nd-life battery industry must adopt new approaches to offer comparable performance to the market-leading 1st-life storage manufacturers, while at the same time reliably handling the occasional rapid aging events. Otherwise, it will remain a niche market, serving only the brave first movers and having to impose prohibitively strict quality requirements on the used batteries.
At STABL, we firmly believe that smarter battery control (like ours), offering the highest efficiency with the lowest risk of battery degradation, is necessary and the catalyst for the success of the 2nd-life battery industry.
To find out more about STABL Energy GmbH, please go to stabl.com
