SOC: State of Charge
State of Charge (SOC) is the “fuel gauge” of a battery. It represents the ratio of the remaining capacity to the total capacity after a period of use or long-term storage. Usually expressed as a percentage, SOC ranges from 0 to 1 (or 0% to 100%), where 0 indicates a fully discharged battery and 1 indicates a full charge.
SOC is a critical parameter in Battery Management Systems (BMS), but it cannot be measured directly. Instead, it must be estimated using variables like terminal voltage, charge/discharge current, and internal resistance. These estimates are complicated by factors such as battery aging, ambient temperature, and vehicle driving conditions. Consequently, achieving accurate SOC estimation remains a top priority in the development of electric vehicles (EVs).
In the EV sector, precise SOC data is vital for:
- Maximizing battery utilization.
- Preventing overcharging and over-discharging.
- Extending battery cycle life.
- Ensuring overall vehicle safety and reliability.
Beyond EVs, SOC is a universal metric used in stationary energy storage and portable electronics to track remaining power.
SOH: State of Health
State of Health (SOH) measures a battery’s degree of aging or degradation. It is a key metric used by the BMS to evaluate performance over the long term.
SOH is defined as the ratio of the battery’s current maximum capacity to its original nominal capacity. Over time, physical and chemical changes—such as the loss of active material and increased internal resistance—cause the battery’s capacity to shrink. By comparing current capacity to “factory new” capacity, the SOH provides a snapshot of the battery’s condition.
Accurate SOH assessment is crucial for systems requiring long-term reliability. It helps users understand remaining service life, predict when a replacement is needed, and optimize maintenance strategies. Furthermore, SOH data provides essential feedback for manufacturers to improve battery design and durability. Common estimation methods include:
- Capacity and Internal Resistance testing.
- Voltage Curve and Incremental Capacity Analysis (ICA).
- Differential Voltage Analysis (DVA).
SOE: State of Energy
State of Energy (SOE) represents the remaining energy available in a battery or storage system. While SOC focuses on “charge” (Coulombs/Ah), SOE focuses on “work” (Watt-hours). It accounts for external factors like efficiency, temperature, and aging that impact the actual energy a battery can deliver.
In applications like EVs and power grids, SOE is arguably more practical than SOC. For instance, an EV uses SOE to provide a more accurate driving range estimate, helping drivers avoid being stranded. In grid storage, SOE allows operators to schedule charging and discharging more efficiently to improve economic returns.
Estimating SOE is more complex than SOC because it requires sophisticated algorithms and models to account for the dynamic energy efficiency of the battery under different loads.
DOD: Depth of Discharge
Depth of Discharge (DOD) indicates the percentage of the battery’s rated capacity that has been discharged. It is effectively the inverse of SOC.
DOD has a direct impact on battery longevity. Generally, the deeper the discharge, the shorter the battery’s cycle life. Every deep discharge cycle causes incremental stress to the battery’s internal structure and chemistry; over time, this damage accumulates, leading to performance fade.
To extend battery life, users are typically advised to avoid high DOD (deep cycling) whenever possible. In EVs and energy storage systems, the BMS monitors DOD in real-time to adjust charge/discharge limits, protecting the battery from premature failure and ensuring it operates within its “sweet spot” for longevity.
