SOC (State of Charge) shows how much usable charge remains in a battery at a specific moment, while SOH (State of Health) shows how much of the battery’s original performance or capacity remains as it ages. Both are normally expressed as percentages, but SOC changes every time the battery charges or discharges, while SOH changes gradually over the battery’s lifetime.
For example, a battery can be at 100% SOC but only 80% SOH. It is fully charged, but its maximum available capacity may have fallen to about 80% of what it could store when new.
In simple terms: SOC tells you how full the battery is now; SOH tells you how healthy the battery still is.
What Are SOH and SOC in a Battery?
The main difference between SOC and SOH is time scale: SOC changes continuously as a battery charges and discharges, while SOH changes gradually as the battery ages and loses capacity or performance.
| Metric | SOC | SOH |
|---|---|---|
| Full Name | State of Charge | State of Health |
| Meaning | Remaining charge right now | Battery condition compared with new |
| Unit | % | % |
| Changes | Minutes/hours during use | Months/years with aging |
| 100% Means | Battery is fully charged | Battery is near its original condition |
| Main Use | Runtime and charging decisions | Aging and replacement assessment |
| Simple Analogy | Fuel gauge | Health report |
How Do You Calculate Battery SOC and SOH?
SOC and SOH are both expressed as percentages, but they compare different battery conditions: SOC compares the charge remaining now with the battery’s available full capacity, while SOH compares its present maximum capacity or performance with its condition when new.
SOC Formula
A simplified State of Charge formula is:
SOC (%) = Remaining Charge ÷ Current Full Charge Capacity × 100
For example, if a battery currently has 60 Ah remaining and its present full available capacity is 100 Ah:
SOC = 60 ÷ 100 × 100 = 60%
SOC therefore answers: How much charge is available right now?
In real lithium battery systems, the BMS normally estimates SOC using current, voltage, temperature, battery models, operating history, and calibration rather than relying on one simple measurement.
SOH Formula
A common capacity-based State of Health formula is:
SOH (%) = Current Maximum Capacity ÷ Original Capacity × 100
For example, if a battery originally stored 10 kWh but can now store a maximum of 8.5 kWh:
SOH = 8.5 ÷ 10 × 100 = 85%
SOH therefore answers: How much of the battery’s original capability remains?
Capacity is one of the most common ways to express SOH, but it is not the only method. Depending on the BMS, manufacturer, and application, SOH may also consider internal resistance, power capability, cycle history, temperature exposure, and other degradation indicators.
SOC vs SOH Example: What Does 100% SOC and 80% SOH Mean?
Suppose a battery was rated at 10 kWh when new. After several years, its capacity-based SOH has fallen to 80%, so its present maximum capacity is approximately 8 kWh.
If the battery is then fully charged, it may still display 100% SOC — but that 100% now represents about 8 kWh, not the original 10 kWh.
Therefore, 100% SOC does not mean 100% battery health.
| Battery Condition | New Battery | Aged Battery |
|---|---|---|
| Original Capacity | 10 kWh | 10 kWh |
| SOH | 100% | 80% |
| Current Max Capacity | 10 kWh | ~8 kWh |
| SOC | 100% | 100% |
| Approx. Stored Energy at Full Charge | 10 kWh | ~8 kWh |
SOC vs SOH vs DoD vs SOE: What Is the Difference?
SOC and DoD usually describe opposite sides of the same charge window, while SOH describes long-term degradation. SOE focuses on available energy, and SOP focuses on the battery’s present power-delivery capability.
| Metric | Full Name | Main Question | Example |
|---|---|---|---|
| SOC | State of Charge | How much charge is left? | 70% SOC |
| SOH | State of Health | How healthy is the battery? | 90% SOH |
| DoD | Depth of Discharge | How much has been used? | 30% DoD |
| SOE | State of Energy | How much usable energy remains? | 7 kWh |
| SOP | State of Power | How much power can it provide now? | 5 kW |
How Does a BMS Estimate SOC and SOH?
| Method / Data | SOC | SOH |
|---|---|---|
| Coulomb Counting | ✓ | Indirect |
| Open-Circuit Voltage | ✓ | Limited |
| Current & Voltage Data | ✓ | ✓ |
| Temperature | ✓ | ✓ |
| Capacity Tracking | Limited | ✓ |
| Internal Resistance | Limited | ✓ |
| Cycle / Usage History | Limited | ✓ |
| Model-Based Algorithms | ✓ | ✓ |
Why SOC and SOH Matter in Solar Battery Storage
In solar battery and BESS applications, SOC determines how much energy is available for charging, discharging, backup and load shifting now, while SOH indicates whether the battery can still meet its required capacity, power and lifetime expectations over the long term.
For homeowners, SOC helps answer: “How much backup power do I have tonight?” SOH helps answer: “Is my battery still performing well after years of use?”
For installers, distributors, and energy storage project buyers, SOC and SOH help with:
- Battery sizing
- System commissioning
- Warranty evaluation
- Maintenance planning
- Inverter communication
- Load management
- Backup runtime calculation
- Long-term performance assessment
In a home energy storage system, SOC data helps the inverter and BMS decide when to charge from solar panels, when to discharge to home loads, and when to preserve backup reserve. SOH data helps evaluate whether the battery still has enough usable capacity for the customer’s needs.
Installers and distributors need battery systems that can communicate clearly with inverters, monitoring platforms, and energy management systems. Avepower provides an inverter compatibility list to help match battery communication protocols such as CAN and RS485 with different inverter brands.
Smart Energy Storage Starts with Accurate SOC & SOH Control
Avepower engineering team provides customized energy storage system design for residential, commercial, and industrial applications.
Why SOH Declines Over Time
All rechargeable batteries age. Even when a battery is not used, chemical aging slowly happens. When the battery is cycled daily, capacity fade can gradually increase.
The main causes of SOH decline include:
High Temperature
High operating or storage temperature can accelerate chemical reactions inside the battery. This may shorten battery life and reduce usable capacity.
Deep Discharge
Frequent deep discharge can put more stress on the battery. Many lithium battery systems define a safe discharge limit to protect the cells.
Overcharging or Over-Discharging
A reliable BMS helps prevent voltage from going too high or too low. Without proper protection, cell damage and safety risks may increase.
High Current Stress
Charging or discharging at very high current can increase heat and stress the cells. Proper current limits are important for long-term battery health.
Cell Imbalance
If cells inside a battery pack become imbalanced, some cells may reach voltage limits earlier than others. This can reduce usable capacity and affect system performance.
SOC, SOH, and BMS: How They Work Together
The BMS is the control center of a lithium battery system. It monitors the battery and helps keep it within safe operating limits.
A BMS typically monitors:
- Cell voltage
- Pack voltage
- Charge current
- Discharge current
- Temperature
- SOC
- SOH
- Protection status
- Communication with inverter or EMS
For modern solar batteries, BMS communication is critical. In many systems, the battery sends SOC, alarms, voltage limits, current limits, and operating status to the inverter. The inverter then adjusts charging and discharging behavior based on battery data.
Avepower’s all-in-one home battery energy storage system supports CAN, RS485, and RS232 communication, with smart BMS monitoring and multi-level safety protection. This type of integration is useful for residential solar storage, backup power, and installer-led energy storage projects.
What Is a Good SOC and SOH for a Battery?
There is no single “best” SOC or SOH value for every battery. SOC operating limits depend on chemistry, BMS settings and application, while SOH is evaluated against the battery’s original performance and the minimum capacity required for its intended use.
Common Misunderstandings About SOC and SOH
“100% SOC Means the Battery Is Healthy”
Not always. A degraded battery can still charge to 100% SOC, but its full capacity may be lower than when it was new. This is why SOH is important.
“Voltage Alone Gives Accurate SOC”
Voltage can help estimate SOC, but it is not always accurate, especially under load or during charging. Lithium battery voltage behavior depends on chemistry, current, temperature and rest time.
“Cycle Count Alone Defines SOH”
Cycle count is useful, but it does not tell the full story. A battery cycled gently in a moderate-temperature environment may age more slowly than a battery exposed to high temperature, deep discharge or high current.
“SOH Drops Evenly Over Time”
Battery aging is not always linear. Storage conditions, operating temperature, charging behavior and load profile can all affect SOH.
How to Maintain Better SOH
To help maintain battery SOH, follow these practical guidelines:
- Avoid long-term storage at 0% SOC.
- Avoid leaving lithium batteries fully charged for extended periods unless the system requires it.
- Keep the battery within the manufacturer’s recommended temperature range.
- Use a compatible inverter and charger.
- Avoid excessive charge and discharge current.
- Follow recommended maintenance and firmware guidance.
- Use batteries with a reliable BMS.
- Size the battery correctly so it is not deeply discharged every day.
For energy storage projects, battery quality, BMS design, thermal management and inverter communication are all important. Avepower is a battery energy storage system manufacturer focused on LiFePO4 battery systems for residential, light-commercial and OEM/ODM energy storage applications.
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Conclusion
SOC and SOH are both essential battery indicators, but they measure different things.
SOC tells you how much charge is available now.
SOH tells you how healthy the battery is compared with when it was new.
For lithium batteries, solar storage systems and backup power applications, both values matter. SOC helps manage daily energy use, while SOH helps evaluate long-term performance, safety and investment value. A reliable battery system should combine quality cells, a smart BMS, proper inverter communication and correct system sizing.
For residential and project-based energy storage, Avepower provides LiFePO4 battery solutions for solar self-consumption, backup power, load shifting and scalable energy storage applications.
FAQ
SOC means State of Charge. It shows how much usable charge remains in the battery at a specific moment, usually as a percentage from 0% to 100%.
SOH means State of Health. It indicates how the battery’s current condition or performance compares with when it was new.
SOC shows how full the battery is now, while SOH shows how much the battery has aged or degraded over time. SOC changes during charging and discharging; SOH normally changes gradually.
A capacity-based SOH of 80% generally means the battery can retain about 80% of its original maximum capacity under the specified measurement conditions. Actual SOH definitions may vary by manufacturer and application.
Yes. An aged battery can still reach 100% SOC, but its maximum capacity may be lower than when it was new.
A simplified formula is SOC = remaining charge ÷ current full capacity × 100. Real battery management systems normally estimate SOC using current, voltage, temperature, calibration and battery models.
A common capacity-based formula is SOH = current maximum capacity ÷ original capacity × 100. Some systems also consider internal resistance, power capability and operating history.
Usually, the percentage shown on a battery display represents SOC. However, SOC is an estimated operating value calculated by the battery management system rather than a directly measured physical quantity.
Not exactly. Capacity retention is a common way to calculate SOH, but battery health can also include internal resistance, power capability, cell condition and other aging indicators.
SOC helps the BMS manage available energy and charge/discharge limits, while SOH helps assess degradation, capacity retention, maintenance needs and long-term battery performance.
