SoH (State of Health)

With the rise of electromobility, there is increasing discussion around the actual lifespan of batteries in electric vehicles. One of the key indicators of battery wear is the so-called State of Health (SoH). This parameter is usually expressed as a percentage and indicates how well the battery has retained its original capacity and performance compared to when it was new.

The SoH value allows both users and manufacturers of electric vehicles to monitor battery condition and make decisions about continued use or replacement. It is also a critical factor when buying a used electric vehicle. The SoH value directly affects the vehicle’s range, charging time, and overall lifespan.

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Definition of SoH

State of Health (SoH) is a parameter expressed as a percentage that describes the wear level of a battery (or its cell). SoH is typically defined as the ratio of the battery’s current capacity to its original rated capacity. If the current condition matches that of a new battery, the SoH = 100%.

 

If a battery has SoH = 80%, it means that its maximum capacity has decreased to 80% of its original rated capacity. In practice, this means that even if the battery is fully charged (SoC = 100%), its usable capacity is only 80% of what it originally was.

It’s important not to confuse SoH with SoC (State of Charge). While SoH describes the long-term wear and tear of a battery, SoC indicates the current charge level – that is, how much energy is stored in the battery at a given moment.

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Concrete example of SoH calculation:

• a new battery has a rated capacity of 10 Ah
• due to wear, the capacity decreases to 8 Ah
• this means the current capacity is 80% of the original → SoH = 80%
• in other words: even at full charge (SoC = 100%), the battery can store a maximum of 8 Ah instead of the original 10 Ah

 

 

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Why does the battery SoH decrease over time?

Repeated charging and discharging, varying temperature conditions, material aging, and regular use all contribute to battery degradation. This primarily shows up as capacity loss, increased internal resistance, and other performance-impacting changes.

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Factors influencing battery degradation:

Battery degradation is influenced by a range of external and internal factors, especially:

• Number of charge and discharge cycles – more cycles mean more wear
• Operating temperatures – extreme heat or cold negatively impacts lifespan
• User behavior – charging/discharging habits, usage frequency, storing battery at high/low SoC for extended periods

In electric vehicles (EVs), additional specific factors influence SoH decline:

• Driving style – frequent hard acceleration and braking strain the battery
• Climate conditions – extreme temperatures or high humidity can accelerate degradation
• Charging infrastructure and type – frequent fast charging may cause more wear

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Measuring and evaluating SoH

The method of determining the SoH value depends on the type and manufacturer of the vehicle. SoH is usually measured using a Battery Management System (BMS), which analyzes various battery parameters such as:

• Capacity – current vs. rated
• Internal resistance or impedance
• Voltage at various states of charge
• Self-discharge – the battery’s ability to retain charge
• Number of charge/discharge cycles
• Battery age
• Operating temperature

Each of these parameters can be weighted and combined differently to provide an overall picture of battery health. The specific algorithm used to calculate SoH is often a proprietary secret of the manufacturer.

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SoH threshold and its significance

The SoH threshold refers to the minimum value at which a battery is still considered suitable for a given application:

• Electric vehicles: The threshold is typically around 70–80%. Below this, the battery is considered unfit as it can no longer reliably deliver the required performance.
• Critical applications: In medical devices, for example, the minimum SoH might be 90% or higher due to the need for maximum stability and reliability.

If the SoH drops below the set threshold, the battery is typically either recycled or repurposed for less demanding uses (e.g., household energy storage).