Battery State of Health – What is It? Why is It Important?

Lithium-ion batteries are efficient, compact, and have a long lifespan – all factors that enable electric vehicles (EVs), which are powered by these batteries, to be a great substitute for their gasoline counterpart. Transportation is a large contributor to greenhouse gas emissions. By switching to EVs, and away from these highly polluting gasoline vehicles, a huge reduction in planet-warming emissions is possible.  

While these batteries are revolutionary, they don’t last forever. As years pass, and as the batteries charge and discharge, their storage capacity begins to slowly reduce. This is a normal process that is influenced by factors such as temperature, number of cycles, and depth of discharge. Knowing how much life is left in an EV battery is pretty critical for someone who is buying a used EV and for deciding if a battery should be reused, repurposed, or recycled when the EV is retired.  

Similar to gasoline vehicles, an EV is expected to last between 8-15 years, at which point the battery will likely have between 70-80% of its capacity left. While this reduced capacity is not ideal for use in an EV, it can continue to be utilized in a second-life application such as stationary storage. Stationary storage is a vital part of the renewable energy transition. The batteries help smooth the supply of renewable generation, such as solar, to meet energy demand. This is done by charging the batteries during the day, when the sun is out, and then using the energy in the evening.  

In order to determine if the battery is suited for a second-life application, the state of health must be assessed. The battery state of health is a measurement that indicates the level of degradation and remaining capacity of the battery. It is essentially the difference between the health of a new battery and the health of a used battery, and typically represented as a percentage of its initial capacity.  

The iPhone is a great example of how we interact with battery state of health in our daily life. Most of us have had the experience of noticing that over time our phone battery cannot hold a charge for as long. Apple got wind of this frustration and now includes a ‘Battery Health’ indicator in the settings. This communicates the battery’s percent maximum available capacity in comparison to when the phone was new.  

The importance of information sharing 

State of health has emerged as an important indicator throughout the lifespan of the battery. When the vehicle is on the road, EV owners want to know the reliability of their vehicle. When owners resell, battery state of health information can more accurately value the product and purchasers can have increased confidence in the EV’s worth, longevity, and range. When the EV is retired, knowing the battery’s state of health is essential for determining if the battery is viable for reuse and repurposing, or if it should be sent directly to recycling.  

Currently, EV battery state of health information is only sometimes accessible at these points along the battery life cycle, and that’s not good enough.  

While the battery is in the vehicle, the EV owner can view their available range on the dashboard or download an app on their smartphone which predicts the state of health for some EV models. In addition, auto manufacturers can access further information through the onboard diagnostic system, a proprietary technology that is only available at certified mechanics. When the EV is retired, things get especially tricky. If the vehicle can’t be turned on, or if the battery has already been removed, the automaker’s state of health estimation can only be accessed via a proprietary connector that is not available at a mechanic, or for purchase.  

So, how is state of health determined by repurposers?  

Repurposers typically receive batteries that have already been removed from the vehicle. They purchase these batteries with the hope that they have adequate capacity for a second-life application but there is uncertainty about their state of health. If there is no visible damage to the battery, then the state of health is tested. This is typically completed by monitoring a full charge and discharge – a process estimated to take about four hours.  

Several research labs and startups have identified this process inefficiency and are developing technology that can be used to estimate the state of health. Repurpose Energy, a battery repurposing start-up that evolved out of UC Davis, has reported that they developed technology that can estimate the state of health of the previous generation Nissan Leaf batteries within 15-20 minutes. This estimation is model-driven and done by collecting data points during the testing process. This technology must be designed differently for each pack type — even for the newer Nissan Leaf batteries – due to the lack of standardization.  

The California Advanced Clean Cars II regulation 

California has already identified the sharing of battery health information to the EV owner as essential. The Advanced Clean Cars II regulation, the most recent version of clean air rules for passenger vehicles, includes requirements for a standardized state of health indicator that is displayed on the dashboard and available via a standardized connector for EVs produced in 2026 and thereafter. This indicator will increase consumer awareness and trust in the product, but the regulation stops short of requiring access to the indicator after the battery is out of the vehicle.  

This shortcoming was also pointed out by the California Lithium-ion Battery Advisory Group, a group of stakeholders that was convened by Assembly Bill 2832. They identified this as a missed opportunity to advance the reuse and repurposing of batteries and make them more cost-competitive with new stationary storage batteries.  

Why should the state of health be available once the battery is removed?

When batteries are retired from use in an EV, they should be routed to the most efficient next use. These batteries are a huge asset in the renewable energy transition and making the process for their reuse and repurposing run smoothly can decrease unnecessary costs. Reuse and repurposing are also essential aspects in increasing battery sustainability. They prolong the lifespan of batteries already in circulation and potentially offset the need for new batteries to be manufactured.  

Currently, repurposers are spending time and money developing technology to determine information already accessible to automotive manufacturers. In addition, there are parties involved in this process, such as automotive dismantlers, that don’t have access to state of health information and are therefore unable to properly value the battery or determine the best next use.  

The battery end-of-life industry can greatly benefit by being able to easily access state of health information after the battery is removed from the vehicle. The battery management system within the EV holds this valuable information, and by providing access through a standardized connector, the second-life industry could increase the efficiency of repurposing and potentially the safety, reliability, and consumer trust of the repurposed products.  

Originally published in Union of Concerned Scientists.