It is generally defined as the ratio of the current capacity to the initial capacity. SoH is generally defined with capacities (SOHc). However, SoH can also be defined in terms of resistance increase (SOHr) or it can be based on the available energy compared to the initial energy (SOHe). It is also seen that SoH is sometimes scaled between 0 and 1, where 1 is a new cell and 0 is when the cell reaches the End of Life criteria (e.g. 80% remaining capacity).
SoH is a challenging KPI. Firstly, there is often no attention paid to the right framing of the SoH. Since the value is a result based on the division of two values, we need to ensure that both values are actually comparable. That means, the available capacity needs to be determined under the same conditions as the initial capacity. Concretely that means, that i.e. voltage or State of charge limits need to be the same. Otherwise, the available and initial capacity represent different states and are not comparable.
Secondly, the available capacity, energy or resistance needs to be determined during the everyday operation, which follows dynamic and uncontrolled patterns. Additionally, temperature and the operation history influence the available capacity, energy and resistance but their influence must be compensated to determine the actual available state. Therefore, sophisticated models and algorithms are needed to ensure accurate results even with noisy and uncontrolled field data.