The integration of repurposed electric vehicle (EV) batteries into residential energy storage systems presents a promising avenue for extending battery lifespans while reducing costs. As EV batteries degrade below automotive standards, they often retain sufficient capacity for less demanding applications, such as home energy storage. This approach aligns with circular economy principles, minimizing waste and maximizing resource efficiency. However, deploying second-life batteries in homes requires careful degradation assessment, economic evaluation, and mitigation of technical challenges.

Degradation assessment is critical for determining the suitability of repurposed EV batteries in residential systems. Automotive batteries are typically retired when their capacity drops to 70-80% of their original specification. While this level of degradation may no longer meet the high-power demands of EVs, it remains viable for home energy storage, where charge and discharge cycles are less aggressive. Key degradation mechanisms include capacity fade, increased internal resistance, and electrode material deterioration. These factors must be evaluated through standardized testing protocols, including capacity measurement, impedance spectroscopy, and cycle life analysis.

State of Health (SOH) estimation is particularly important for second-life batteries. Unlike new batteries, repurposed units exhibit non-uniform aging patterns due to their prior usage history. Advanced algorithms that combine historical data, real-time performance metrics, and machine learning can improve SOH accuracy. Additionally, thermal imaging and voltage profiling help identify weak cells or modules that could compromise system reliability. Without rigorous assessment, the risk of premature failure increases, potentially offsetting the economic benefits of using second-life batteries.

From a cost perspective, repurposed EV batteries offer significant advantages over new residential energy storage systems. The upfront price of second-life batteries can be 30-50% lower than equivalent new systems, depending on remaining capacity and cycle life. This cost reduction makes home energy storage more accessible to a broader range of consumers. Furthermore, the environmental benefits of reusing batteries contribute to sustainability goals, reducing the need for raw material extraction and manufacturing emissions associated with new battery production.

However, the total cost of ownership must account for additional expenses. Repurposing involves disassembly, testing, reconfiguration, and integration into new enclosures with updated battery management systems (BMS). These processes add to the overall system cost, though they remain lower than manufacturing new batteries. Maintenance costs may also be higher for second-life systems due to their inherent variability in aging. Despite these factors, the levelized cost of storage (LCOS) for repurposed batteries often proves competitive, especially in scenarios where daily cycling is moderate.

Technical challenges pose the most significant barriers to widespread adoption of second-life batteries in residential applications. One major issue is compatibility. EV batteries are designed for high-energy and high-power applications, whereas home energy storage prioritizes energy density and long-duration discharge. Retrofitting these batteries requires modifications to the BMS to accommodate different voltage and current profiles. Additionally, the physical form factor of EV battery packs may not align with residential space constraints, necessitating custom enclosures or modular designs.

Another challenge is the lack of standardization. Different EV manufacturers use varying cell chemistries, module configurations, and BMS architectures. This heterogeneity complicates the repurposing process, as each battery type demands unique handling and integration approaches. Standardizing interfaces and communication protocols could streamline deployment but remains an ongoing industry effort.

Safety is a paramount concern when deploying second-life batteries in homes. Aged batteries are more prone to thermal runaway due to increased internal resistance and material degradation. Robust thermal management systems are essential to mitigate these risks. Furthermore, the BMS must be recalibrated to account for the altered behavior of aged cells, ensuring accurate state estimation and fault detection. Fire suppression systems and fail-safe mechanisms should also be incorporated to address potential hazards.

Performance variability is another issue. Unlike new batteries, which exhibit predictable degradation, second-life batteries have unique usage histories that influence their future performance. Some cells may degrade faster than others, leading to imbalances that reduce overall system efficiency. Active cell balancing and adaptive charging algorithms can help manage these disparities, but they add complexity to the system design.

Despite these challenges, several real-world implementations demonstrate the feasibility of repurposed EV batteries in residential settings. Some homeowners have successfully integrated second-life batteries into solar energy systems, using them for load shifting, backup power, and peak shaving. These deployments highlight the potential for cost savings and sustainability benefits, provided that proper degradation assessment and system design are prioritized.

The future of second-life batteries in home energy systems depends on advancements in repurposing technologies and regulatory support. Improved sorting and testing methods will enhance the reliability of used batteries, while economies of scale could further reduce costs. Policymakers can incentivize adoption through subsidies or tax credits, similar to those available for new residential energy storage systems.

In summary, repurposed EV batteries offer a viable and cost-effective solution for residential energy storage, but their successful deployment hinges on rigorous degradation assessment, economic viability, and overcoming technical hurdles. As the EV market continues to grow, the availability of second-life batteries will increase, creating opportunities for homeowners to participate in sustainable energy practices. Addressing compatibility, safety, and performance challenges will be key to unlocking the full potential of this emerging market.