SOH: How Battery Degradation Is Measured
State of Health quantifies how degraded a battery is relative to its original condition. It is defined in terms of capacity fade and evaluated through resistance, capacity, self-discharge, cycle count, and age.
State of Health (SOH) is a measure of a battery’s present condition relative to its ideal condition when new. It is the primary metric for tracking long-term degradation.
Definition
SOH is defined as:
SOH = (present condition of battery / ideal condition) × 100In terms of capacity specifically:
SOH = (current maximum capacity / capacity of new battery) × 100%A new battery has an SOH of 100%. A battery at 80% SOH can only store 80% of its original charge.
Evaluation Parameters
SOH is not a single measurement. It is inferred from several observable indicators:
- Increase in cell resistance / impedance: as a battery ages, internal resistance rises, reducing the power it can deliver
- Decrease in capacity: the battery holds less charge per cycle
- Self-discharge rate: a degraded battery loses charge faster when idle
- Number of charge-discharge cycles: each cycle causes incremental chemical wear
- Age of battery: calendar ageing occurs even without use
Capacity Fade
SOH is often defined and modelled specifically through capacity fade. Capacity is the total amount of electric charge a battery can deliver at a specific voltage, from fully charged to fully discharged, measured in Ampere-hours (Ah).
Capacity fade models are mathematical equations that predict remaining battery life by fitting measured Ah capacity data across hundreds of cycles to a curve. The curve shows how quickly capacity is dropping and extrapolates to end-of-life.
The Arrhenius equation is used to model the temperature dependence of the chemical reactions driving capacity fade. Higher temperatures accelerate degradation in a predictable way.
The Knee Point
The capacity fade curve is not linear. Batteries typically show a slow, gradual decline for most of their life, followed by a sudden acceleration in capacity loss. The point where the curve bends sharply is called the knee point. Once a battery crosses its knee point, degradation accelerates rapidly. SOH monitoring aims to detect the approach of this point before it is reached.