A Closer Look at SoC in Theory and Practice
State of Charge (SoC) is one of the most used metrics in battery engineering, but also one of the most inconsistently defined. SoC is often described as the percentage of charge remaining in a battery, but what that percentage refers to depends heavily on how the cell is tested, used and constrained.
At its core, SoC is not an absolute number. It is a ratio, and the definition of that ratio depends entirely on what you use as the denominator.
The Thermodynamic Perspective
In theory, SoC can be defined thermodynamically as the amount of extractable capacity remaining in a cell under equilibrium conditions, within a chosen allowed voltage window of use.
The maximum thermodynamic capacity is typically measured at a very low C-rate such as C/20 to minimise voltage losses and reflect the true electrochemical energy stored. However, this equilibrium SoC is strongly influenced by temperature, internal pressure, chemical kinetics, and voltage hysteresis. For chemistries such as LFP and silicon, there is no unique equilibrium due to the direction-dependent (charge / discharge) voltage response.
As a result, equilibrium SoC is not only difficult to measure in most systems, but also varies significantly between labs, protocols, and cell chemistries.
SoC Depends on Capacity, but Capacity is Contextual
SoC is usually expressed as a percentage of remaining capacity relative to some reference value. But capacity itself is not fixed: it varies depending on how the cell is subsequently used. Below are five common ways that capacity is defined, each resulting in a different interpretation of SoC.
Each is expressed below as an estimated SoC percentage relative to the stated capacity (definition 2), which is what most engineers default to when reading cell data sheets.
1. Thermodynamic capacity (approx. -5 to 100% of stated capacity)
Measured at very low rates (e.g. C/20) with long rest periods to reach near-equilibrium voltage.
Represents the maximum reversible lithium inventory. Essential for accurate open-circuit voltage characterisation and model development.
Highly sensitive to test conditions, making lab-to-lab comparisons difficult.
2. Stated (name-plate) capacity (0 to 100 per cent)
Defined by the manufacturer, typically at 0.5 C or 1 C between set voltage limits at 25 °C. The standard charge and upper voltage limit defining 100% SoC and the standard discharge and lower voltage limit defining 0% SOC.
This stated or nameplate value is used as the baseline for SoC. Balances test time and comparability, but may not reflect true in-use performance.
3. Practical capacity (approx. 0 to 90 per cent)
Measured under realistic load profiles such as drive or duty cycles.
Accounts for polarisation, thermal effects and early voltage cut-off under load.
Closer to real-world energy output, especially for mobility applications.
4. Application (design-limited) capacity (approx. 5 to 85 per cent)
Restricted by system designers to improve thermal management, cycle life and power availability.
Voltage limits and SoC windows are narrowed to avoid lithium plating and degradation.
Defined through the BMS and tailored to each use case.
5. Usable capacity in operation (approx. 10 to 75 per cent)
The energy actually accessible during system use, after applying all real-time constraints: thermal limits, current caps, SoH adjustments and cell balancing.
Declines with ageing and reflects what end-users experience.
Used for range prediction, warranty modelling and system performance evaluation.
Why SoC Is Not Universal
State of Charge is not a fixed or directly comparable metric because it depends on how capacity is defined — and capacity itself varies with context. Several physical and system-level factors cause this divergence:
- Kinetics and polarisation: At higher discharge rates, overpotential and internal resistance losses increase, causing earlier voltage cut-off and reduced accessible energy.
- Thermal constraints: Higher currents generate more heat, and thermal losses rise in SoC extremes due to higher cell resistance. To avoid overheating, systems restrict SoC limits and reduce usable capacity.
- Ageing sensitivity: Cycling near full or empty accelerates degradation mechanisms like lithium plating, positive electrode material decomposition, and copper dissolution. Designers limit SoC windows to extend cell life.
- System integration: Powertrains require voltage headroom for transients or cold starts. This forces additional SoC reserve to be kept unused.
- Imbalance and ageing: As cells age and diverge in performance, the BMS narrows the usable SoC range to protect the weakest cells and maintain safety.
These factors mean that SoC is inherently relative and application-specific. For example, a drone manufacturer might limit a high-power NMC cell to 20 to 80 percent SoC to manage heat and lifespan, while a lab validation test might cycle the same cell from 0 to 100 percent at C/20 to extract full electrochemical data. Both cases report SoC, but they are referencing different underlying capacities — and are not directly comparable.
Clear documentation of how SoC is defined and measured is essential when comparing datasets, validating models or evaluating system performance. Without it, SoC can be misleading or misinterpreted.
Implications for Engineers
Understanding and clearly defining SoC is essential for effective battery development. Without knowing which capacity definition underpins a reported SoC value, engineers risk drawing incorrect conclusions about performance, lifetime, or system behaviour.
Why this matters:
- Avoid misalignment across teams: Confusion often arises when test engineers, modellers, and system designers use SoC differently. A stated 80 percent SoC could mean very different things in the lab, in simulation, or in the field.
- Improve model accuracy and validation: Matching test data to simulations requires the simulation initial conditions to match the real-world cell state. A mismatch can lead to false validation failures or incorrect parameter tuning.
- Design better, safer systems: Application-level SoC limits directly impact thermal performance, power availability and degradation rates. Clear definitions help justify design decisions and explain trade-offs.
- Support clear communication with customers and partners: SoC misunderstandings can cause issues during cell evaluation, procurement or integration. Being explicit helps build trust and technical credibility.
At About:Energy, we define state of charge (SoC) consistently and explicitly across all our datasets, models, and tools. Whether calibrating an electrochemical model or building a digital twin for a full pack, we ensure SoC reflects the right context — whether thermodynamic, stated, practical, or usable. By parameterising our models using the most fundamental (chemical) definitions of SoC, we enable accurate prediction of remaining capacity under any condition. This allows for critical ‘what if’ scenarios. For example, a BMS might report 30% SoC at equilibrium, but a model can reveal only 10% is usable under race conditions. In this way, models are essential for bridging the gap between theoretical capacity and real-world performance.
For engineers making decisions that impact performance, safety, and cost, clarity around SoC is not a detail — it is foundational.
About:Energy
A data company that offers software to enhance battery performance throughout its lifetime. Its platform, “The Voltt”, reduces time and costs in developing new electric products for various industries. Utilizing advanced battery testing methods, this platform facilitates cell selection, benchmarking, and system design. Catering to anyone aiming to improve battery performance metrics, it efficiently trims down cell selection times from months to minutes through its comprehensive cell library. Users can leverage this resource to benchmark their emerging cell chemistries, maintaining a competitive edge by evaluating rival performance. Providing cloud-based tools and downloadable models, “The Voltt” aids in optimizing system designs by offering insights into battery pack behaviour
See also: State of Charge
Could be even more than -5%.