Battery SOC vs SOH: What’s the Difference?

A battery can show 100% charge and provide less runtime than it did when it was new. This is because the charge shown on the display and the battery's overall condition are two different things, represented by State of Charge (SOC) and State of Health (SOH).

Battery SOC indicates how much usable charge is available at a given time, similar to a fuel gauge. Battery SOH, on the other hand, describes how the battery's condition and performance have changed compared with when it was new.

For example, a battery that has retained only 80% of its original capacity can still reach 100% SOC when fully charged. The 100% reading means the battery is fully charged relative to its current available capacity, not that it still has the same capacity it had when new.

For lithium-ion batteries, SOC and SOH are not measured directly by a single sensor. A battery management system (BMS) estimates them using data such as voltage, current, and temperature together with battery models and algorithms.

What Is Battery SOC (State of Charge)?

State of Charge (SOC) represents the amount of charge currently available in a battery relative to its available full-charge capacity. In practical use, SOC indicates how much charge remains before the battery needs to be recharged.

A simplified expression is:

SOC (%) = Remaining Available Capacity ÷ Full Available Capacity × 100

For example, an 80% SOC means that approximately 80% of the battery's currently available charge capacity remains under the relevant operating conditions.

SOC is therefore a short-term operating state. It changes continuously as the battery charges and discharges. A battery at 80% SOC today could still be at 80% SOC after many years of use, even though its actual energy capacity may have declined because of aging.

Researchers describe SOC as an internal battery state that cannot be measured directly with a conventional sensor. Instead, a BMS estimates it using measurable variables such as current and voltage and mathematical models or estimation algorithms.

SOC vs Depth of Discharge (DoD)

SOC describes how much charge remains in the battery, while Depth of Discharge (DoD) describes how much of the battery's capacity has been used. DoD (%) = 100 − SOC (%). For example, a battery at 80% SOC has undergone 20% DoD from a fully charged state.

What Is Battery SOH (State of Health)?

State of Health (SOH) describes the condition of a battery compared with its reference or new condition. SOH generally declines gradually as the battery ages and degrades. A lower SOH can mean less usable capacity, reduced power capability, or other changes in battery performance, depending on how SOH is defined.

For lithium-ion batteries, SOH is commonly associated with capacity loss and/or increases in internal resistance. One widely used capacity-based definition is:

SOH (%) = Current Full Capacity ÷ Initial Capacity × 100

For example, if a new battery had a measured capacity of 2,000 Wh and its current maximum usable capacity were 1,800 Wh, its capacity-based SOH would be approximately 90%.

However, SOH is not a single universally defined physical quantity. Research literature uses several health indicators, including:

• Capacity: how much charge or energy the battery can store
• Internal resistance: how much the battery's resistance has increased with aging
• Power capability: how effectively the battery can deliver power
• Energy capability: how much usable energy remains

A 2024 review in Energy Conversion and Economics emphasizes that SOH can be defined using capacity, resistance or energy depending on whether the assessment is being made at the cell, module or battery-pack level.

Key Differences Between SOC and SOH

The main difference between SOC and SOH is that SOC describes a battery's current charge level, while SOH describes how its condition has changed relative to its new or reference state. SOC changes continuously during charging and discharging, whereas SOH generally changes gradually as the battery ages.

The table below summarizes the key differences:

Why Both SOC and SOH Are Critical?

SOC and SOH provide complementary information about a battery. SOC shows how much charge is currently available, while SOH indicates how the battery's condition and capability have changed as it ages.

A battery can display 100% SOC but provide less runtime than it did when new if its usable capacity has declined. Likewise, two batteries at the same SOC can have different amounts of usable energy if their available capacities differ.

SOC alone does not show how much the battery has degraded, while SOH alone does not show how much charge is currently available. Together, they help distinguish current charge level from long-term battery degradation.

How to Monitor Battery SOC and SOH?

Modern battery-management systems monitor battery condition using measurements such as:

• Cell and pack voltage
• Charge and discharge current
• Battery temperature
• Charging and discharging history
• Capacity estimates
• Internal resistance or impedance
• Cell-to-cell differences

The BMS combines these measurements with algorithms to estimate internal states that cannot be measured directly.

A major challenge is that lithium-ion batteries are nonlinear, time-varying electrochemical systems. Consequently, the relationship between voltage, current, temperature, SOC, and SOH changes with operating conditions and aging.

How Is Battery SOC Estimated?

One of the most established methods for estimating SOC is coulomb counting, which tracks current flowing into and out of the battery over time. Because sensor errors can accumulate, BMSs often combine it with voltage measurements and battery models.

Kalman-filter-based methods, including EKF and UKF, are also widely studied for online SOC estimation. A 2026 Nature Reviews Clean Technology article identifies Kalman filtering as a prominent SOC estimation approach, while a 2022 Journal of Energy Storage review highlights EKF, AEKF, UKF, and AUKF among extensively studied methods.

The key point is that the SOC shown on a battery display is an estimate, not a direct measurement.

How Is Battery SOH Assessed?

SOH is more difficult to determine because aging happens slowly and is affected by operating history. Researchers generally use three broad approaches:

1. Direct or experimental methods

   The battery can be fully charged and discharged under controlled conditions to measure its actual capacity. Internal resistance can also be measured using controlled electrical tests.

   These approaches can provide highly informative measurements, but a complete capacity test can take significant time and is not practical every day in a consumer product.

2. Model-based estimation

   A BMS can use an electrical or electrochemical battery model to infer degradation from variables such as voltage, current, and temperature.

3. Data-driven estimation

   Machine-learning models can identify relationships between historical battery measurements and degradation indicators. These approaches can potentially estimate SOH without repeatedly performing full capacity tests.

A 2024 review covering cell-, module-, and pack-level SOH estimation classifies the major approaches into direct measurement, model-based, data-driven, and hybrid model-data methods.

What Affects SOC and SOH Accuracy?

SOC and SOH values are estimates rather than direct measurements, so their accuracy can vary with battery conditions, sensor data, and the estimation method used by the battery management system (BMS).

Several factors can affect these estimates:

• Temperature: Battery voltage, internal resistance, and available capacity change with temperature. Very high or low temperatures can therefore make SOC and SOH estimation more difficult.
• Current and voltage measurement errors: SOC estimation relies heavily on current and voltage data. Small sensor errors can accumulate over time, particularly when methods such as coulomb counting are used.
• Battery aging: As a battery ages, its capacity and internal resistance change. Models based on a newer battery may become less accurate if they do not account for these changes.
• Calibration and battery models: A BMS uses battery models and algorithms to interpret measured data. Differences between the model and the battery's actual condition can affect the estimated SOC or SOH.
• Cell-to-cell variation: Individual cells in a battery pack may age or behave differently. Differences in cell voltage, capacity, temperature, or resistance can make pack-level SOC and SOH estimation more complex.
• Operating conditions: Charging rate, discharge rate, recent usage, rest periods, and load changes can all influence the battery measurements used to estimate SOC and SOH.

For this reason, the SOC shown on a battery display should be understood as an estimate of the battery's current charge level rather than an exact direct measurement. SOH can be even more difficult to assess because battery degradation develops gradually and may be evaluated using capacity, internal resistance, or other health indicators.

How Does a Battery Management System Monitor SOC and SOH?

A Battery Management System (BMS) collects battery data such as voltage, current, and temperature. SOC and SOH are not measured directly in the same way as these parameters. Instead, the BMS uses this data and estimation methods to determine the battery's current charge level and longer-term condition.

SOC is updated as the battery charges and discharges, helping the BMS track how much charge is available. SOH changes more gradually and can be assessed using indicators such as available capacity and internal resistance. The exact method varies depending on the battery chemistry, BMS design, and how SOH is defined.

The BMS uses SOC and SOH information to manage charging and discharging, balance individual cells, and detect operating conditions that could affect battery safety or performance. Because cells within a battery pack can age at different rates, cell-level monitoring helps the BMS identify differences between cells and keep the pack operating within its designed limits.

For users, SOC is usually the more visible of the two values because it shows the battery's current charge level. BLUETTI portable power stations display SOC on the built-in screen so users can check the remaining charge during operation. Some compatible battery systems may also provide SOH information to help users understand longer-term changes in battery condition.

How to Maintain Battery State of Health

Key ways to maintain battery State of Health (SOH) include temperature management, appropriate charging and discharging practices, and proper battery storage.

Temperature Management

Extreme temperatures can affect battery aging and performance. High temperatures accelerate chemical reactions inside the battery, which can speed up capacity loss and degradation. Low temperatures slow lithium-ion movement, which can reduce charging efficiency and available performance. Follow the manufacturer's recommended temperature range to help maintain battery health.

Charging and Discharging Practices

High charging and discharging rates can generate more heat inside the battery, which may accelerate degradation over time. For example, repeated fast charging or operation under heavy loads can increase thermal stress on the battery. When possible, follow the manufacturer's recommended charging and operating limits to help extend battery life.

Battery Storage Practices

Keep the battery in a cool, dry place away from direct sunlight and heat sources. High SOC and high temperatures can accelerate calendar aging, so avoid storing a battery fully charged or nearly empty for long periods. For long-term lithium battery storage, follow the manufacturer's recommended SOC and temperature range.

Final Thoughts

SOC and SOH answer two different aspects about a battery. SOC tells you how much charge is currently available, while SOH describes how the battery's capability has changed as it ages. An 80% SOC battery is not necessarily an old battery, and a battery with 80% SOH means it has degraded 20% of its capacity. SOC describes the battery's current charge level; SOH describes its long-term condition.

Modern BMS technology combines voltage, current, and temperature measurements with mathematical models, filtering techniques, and increasingly data-driven methods to estimate these internal states. Research continues to improve the accuracy of SOC and SOH estimation under changing temperatures, aging conditions, and real-world workloads.

For users, the most practical ways to support battery longevity are to manage temperature, follow recommended charging and discharging limits, and use appropriate storage conditions.

FAQs About Battery SOC and SOH

What Does 80% SOC Mean?

80% SOC means the battery has approximately 80% of its available full-charge capacity remaining. In other words, about 20% has been discharged. SOC reflects the battery's current charge level, not its overall health.

What Is a Good Battery SOH?

A good battery SOH is generally one that remains close to its original condition and still meets the performance required for its intended use. For lithium-ion batteries, around 80% of original capacity is commonly used as an end-of-life reference when SOH is evaluated based on capacity, although the actual threshold varies by battery and application.

How Is Battery SOC Calculated?

Battery SOC is expressed as a percentage of the battery's available full-charge capacity:

SOC (%) = Remaining Available Capacity ÷ Full Available Capacity × 100

For example, 800 Wh remaining out of 1,000 Wh of available full-charge capacity equals 80% SOC. In practice, a BMS estimates SOC rather than measuring it directly, using data such as current, voltage, and temperature. Methods such as coulomb counting may also be combined with battery models to improve the estimate.

What Is the Difference Between Battery SOC and Voltage?

Battery SOC indicates how much charge is available, while battery voltage is the electrical potential measured across the battery terminals. Voltage can help estimate SOC, but the two are not directly interchangeable. For common voltage ranges at different charge levels, see our battery voltage chart.

The same voltage can correspond to different SOC levels depending on battery chemistry, temperature, load, and whether the battery is charging, discharging, or resting. This is vital for batteries with relatively flat voltage curves, such as LiFePO₄ batteries, where small voltage changes can represent a wider range of SOC. For this reason, a BMS uses voltage together with current, temperature, and other data to estimate SOC.

When Should a Battery Be Replaced Based on SOH?

For lithium-ion batteries, 80% of the original capacity is often used as an end-of-life reference, but it is not an automatic replacement point. A battery at this level can still work normally, but with less usable capacity than when it was new.

Whether it needs replacing depends on the battery and how it is being used. If the reduced capacity or power no longer meets your needs, check the manufacturer's recommended replacement criteria.