How to define battery State-of-Health

1. What is SoH ?

When it comes to assessing the health of an EV battery, most people naturally turn to the State-of-Health (SoH). SoH is a percentage-based metric that compares a battery’s current performance with its performance when brand new. It is commonly used to express how much the battery has aged.

SoH can be calculated using different physical quantities: capacity, power, or energy. Depending on the reference metric, one speaks of capacity SoH (SoHC), power SoH (SoHR), or energy SoH (SoHE).

In practice, energy-based SoH is increasingly becoming the industry default.

When used this way, SoH essentially tells you how much of the battery’s initial usable energy remains, serving as a high-level proxy for how much energy the battery can still deliver efficiently.

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2. The European regulation doesn't rely on SoH, but on SoCE

However, the European regulatory framework (UN GTR 22, Euro 7) does not rely on SoH. Instead, it defines a strictly standardized metric: the State of Certified Energy (SoCE).

SoCE is defined as the ratio between the measured usable battery energy (UBE) and the certified nominal energy of the battery pack - both determined through the controlled charge–discharge test procedures specified in UN GTR 22. The certified nominal energy is established during type-approval, and subsequent SoCE measurements later in the battery’s life reveal what portion of that certified energy the battery can still provide under standardized, repeatable conditions. This makes SoCE a reproducible, regulation-grade indicator of energy-related degradation.

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3. Yet, SoH remains the commonly used term. So harmonizing its definition is essential

Despite the absence of a harmonized definition for SoH, the term remains the most widely used throughout the EV industry. Different OEMs compute SoH using distinct criteria (capacity retention, power capability, internal resistance, or proprietary model-based algorithms) making the notion inherently variable across brands.

This is why harmonizing SoH’s definition is essential, and organizations like VDE are already working toward it.

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4. In practice, SoH = SoCE

In fact, when market stakeholders refer to “battery SoH,” they are often implicitly describing the remaining usable energy of the battery, which conceptually aligns with SoCE, even if the regulatory terminology is not explicitly used.

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