The automotive market share for electric vehicles (EVs) is growing and so is the number of retired lithium-ion batteries (LiBs) looking for new homes. Repurposed EV LiBs are proving to be a sustainable, cost-effective energy option for domestic battery energy storage systems (BESSs). Being an added-value product, they are also providing original equipment manufacturers (OEMs) in the battery market and battery recycling businesses with an extra revenue stream.

Finding a second life

LiBs power applications and devices from smartphones to ferries. But it is from the EV market that most second-life LiBs are sourced, because of the capacity and recycling challenges.

Manufacturers of EVs suggest replacing LiBs when their capacity reaches 70% to 80% with a resting self-discharge rate of about 5% over a 24-hour period. EV LiBs have a typical lifespan of about 1,500 to 2,000 charge/discharge cycles, or 15 to 20 years, depending on usage patterns.

But there is still practical usage in an LiB that can have dozens or hundreds of kWh remaining. When repurposed, used LiBs may have enough capacity for double the amount of charge/discharge cycles, depending on the application. They reach the end of their second life usually when capacity is reduced to about 30%, after which the energy storage capacity has degraded to a point where the battery reaches its actual end of life.

These batteries contain materials that are potentially dangerous, but also valuable. Recycling is a difficult process that shreds the battery materials, then requires the materials to be separated chemically. It is less expensive to mine new battery materials than to recycle them.

As a matter of business, in addition to maximizing the useful lifespan of electronics, repurposing EV batteries is a practical decision. Domestically, old EV batteries are most often repurposed and reused in a stationary BESS. They are also used for home solar and wind energy storage systems, at EV charging stations, in residential behind-the-meter (BTM) backup systems, and for voltage control during peak shaving to support public electricity grids.

In addition, without a BESS, homeowners are less likely to incorporate solar into their energy mix. Reusing LIBs provides a less expensive option for homeowners considering adding renewables.

There are other easy-to-see benefits. Research by scientists at the University of Oxford and Cambridge University found that second-life LiBs were a suitable solution to providing energy in rural communities. Their study looked at rural schools in Kenya, although the solutions are practical for any community or building needing to stabilize their off-grid energy sources.

[See also: Old EV batteries live on in the grid]

Vehicle-to-grid (V2G), sometimes called vehicle-to-home (V2H), technology is a bidirectional gateway for EVs to communicate with the public power grid and battery manufacturers and is creating new revenue opportunities for both parties. For instance, with EVs mostly parked and capable of being on charge for long periods, there is an opportunity for vehicle owners to store energy on relatively inexpensive used batteries and then sell it back to the grid.

Considerations for repurposing LiBs

The first step in assessing whether a battery can be repurposed is to examine the pack and review data from the battery management system (BMS). UL 1974 is a manufacturing process standard and the BMS data is assessed against it to review the pack's historical state of health (SoH). It offers parameters for critical questions like: Why was it withdrawn from service? What was its handling history? How often was it charged and discharged? What extreme temperatures was it subjected to? How many errors were thrown during the battery's first life?

The next step is the disassembly of the pack into its component modules (the batteries themselves), which can be an expensive, labor-intensive process. Third, to estimate a module’s SoH, the state of charge (SoC) must be measured. SoC describes the residual capacity of a battery. There are numerous complex methods to measure this parameter, including capacity and internal impedance tests, like the coulomb counting, voltage and Kalman filter methods.

Finally, the batteries may be refurbished or repurposed in a reassembly process. Refurbishing involves replacing damaged parts or modules in a battery pack and using them as second-hand parts. Repurposing involves adapting battery packs for a new application, like stationary storage. The advantage of refurbishing is that battery packs have a performance warranty whereas repurposed packs have only a capacity warranty but are cheaper to adapt for reuse.

[Discover more about LiB technology and battery suppliers on GlobalSpec]

Are there safety risks in second-life LiBs?

Although very rare, the thermal runaway and flammability concerns for LiBs are well-founded. Per a 2023 U.K. study, the potential safety hazards for used LiBs are on par with new ones. The main argument against the safe domestic use of second-life LiBs is that the point where a battery has deteriorated to the extent it is dangerous cannot easily be predicted because there are no proven, global standards for testing second lifers.

The possible effects of unexpected deterioration include thermal runaway, internal short circuits and joule heating. Other safety issues are damage from dropping or puncturing batteries, operating in temperatures over 130° F or under 32° F when charging, human error and cell failures resulting in chemical or combustion reactions. Europe is ahead of the U.S. in sourcing and redeploying second-life batteries. The main reason for this, according to Energy Storage News, is that in the U.S. a BESS must have been built in a UL 1974-certified facility.

Summary

Used LiBs are a viable option for domestic use. And battery manufacturers are working on new ways to optimize the repurposing process and reduce costs. For example, ReJoule has developed a device for new EV batteries that will monitor their use and create battery profiles. Those profiles will inform each battery’s suitability for second-life applications.

However, understand that even using an LiB twice is still less desirable than other battery technologies or energy storage types. Lithium-sodium batteries are considered more easily recycled and have comparable or better energy performance. According to Bloomberg NEF, non-battery technologies such as compressed air and thermal energy storage may soon be viable competition to batteries for energy storage with sodium-ion batteries giving LiBs a run for their money.