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.