Battery State of Charge and Battery State of Health

Battery State of Charge (SoC) is the percentage of remaining energy in a battery, like a fuel gauge, while Battery State of Health (SoH) measures how much capacity and performance the battery retains compared to when it was new. Understanding SoC and SoH is critical for optimizing battery performance, preventing failures, and extending lifespan across devices from smartphones to electric vehicles.

This article explains the key differences between SoC and SoH, how to measure them, and best practices to maintain your battery for longer life and reliable performance.

Part 1. Understanding battery state of charge (SoC)

1 Battery State of Charge (SoC)

The Battery State of Charge (SoC) is the ratio of the current charge in the battery to its maximum possible charge. It is like a fuel gauge for batteries. SoC indicates how much charge remains in the battery and is usually displayed as a percentage. For example, 100% means the battery holds a full charge, and 0% is empty. SoC can also be shown using voltage levels; a higher voltage indicates a higher SoC.

2 Methods of Measuring SoC

1. Coulomb Counting

Coulomb counting is a common way to measure SoC. This method counts the electric charge in and out of the battery. It works by measuring the current flow and keeping track of the time. This gives an accurate measure of the charge used and the charge left.

Advantages:

• It is accurate when the initial SoC is known.
• Useful for tracking change over time.

Limitations:

• Errors can build up over time if not corrected.
• An accurate initial SoC is needed to be effective.
• Sensitive to measurement errors and noise.

2. Voltage Method

The voltage method measures the battery’s voltage to estimate the SoC. Different voltage levels correspond to different SoC values. This method is simple and easy to use.

Factors affecting accuracy:

• Battery type: Different batteries have different voltage characteristics.
• Temperature: Temperature changes can affect voltage readings.
• Load: The current load on the battery can change the voltage.

Advantages:

• Simple and quick.
• No need for complex equipment.

Limitations:

• It is less accurate than other methods.
• It is affected by many external factors like temperature and load.

3. Impedance Spectroscopy

Impedance spectroscopy measures the battery’s internal resistance to estimate SoC. This method sends a small alternating current (AC) through the battery and measures the voltage response. It helps to understand the battery’s internal properties.

Benefits:

• Provides detailed information about the battery’s condition.
• It can be very accurate when done correctly.

Drawbacks:

• Requires specialized equipment.
• More complex and time-consuming.
• It is not commonly used for everyday applications due to its complexity.

3 Importance of SoC Monitoring

Role of SoC in Preventing Overcharging and Deep Discharging

• Monitoring SoC helps avoid overcharging, which can damage the battery.
• It also prevents deep discharging, which can shorten the battery life.
• Keeping SoC in the correct range makes the battery last longer.

Impact on Battery Performance and Lifespan

• Correct SoC ensures the battery works well.
• It makes the battery reliable.
• It helps the battery to perform at its best.

Applications Where Accurate SoC Monitoring is Critical

• Electric Vehicles: They need precise SoC to avoid running out of power.
• Renewable Energy Storage: Accurate SoC helps use solar and wind energy efficiently.
• Portable Devices: Phones and laptops need good SoC to keep running throughout the day.

4 How to Measure Battery State of Charge?

• Step 1: Choose measurement method – Use voltage testing for quick estimates or Coulomb counting for precision measurements
• Step 2: Calibrate equipment – Ensure multimeters or battery testers are properly calibrated
• Step 3: Monitor under load – Measure voltage during discharge cycles for real-world usage data

5 Key Takeaways about Battery SoC

• Definition: Battery SoC (State of Charge) means how much charge remains, like a fuel gauge.
• Measurement: Common methods include voltage check, coulomb counting, and impedance spectroscopy.
• Optimal Range: Keeping SoC between 20%–80% prevents stress and extends battery lifespan.
• Applications: EVs, solar storage, and consumer electronics all require accurate SoC tracking.

Part 2. Understanding battery state of health (SoH)

1 Battery State of Health

Battery State of Health (SoH) tells how good a battery is. It shows how much life the battery has left. SoH compares the battery’s current condition to when it was new. If a battery has 80% SoH, it can only hold 80% of the charge it could when it was new. This helps you know if the battery needs replacing soon.

2 Methods of Assessing SOH

1. Capacity Testing

Process of Capacity Testing:

• Fully charge the battery.
• Discharge it while measuring how much energy it gives out.
• Compare this to the battery’s original capacity.

Interpretation of Capacity Test Results:

• If a battery gives out much less energy than it used to, its SoH could be better.
• For example, if a 1000mAh battery only gives 800mAh, it has 80% of its original capacity.

2. Internal Resistance Measurement

How Internal Resistance Reflects the Health of the Battery:

• Low resistance means the battery can deliver power efficiently.
• High resistance means the battery struggles to deliver power.
• As batteries age, their internal resistance goes up.

Techniques for Measuring Internal Resistance:

• Use a specialized meter to measure voltage and current.
• Calculate resistance using Ohm’s Law (Resistance = Voltage / Current).
• Compare the result to the battery’s specs.

3. Self-discharge Rate

Explanation of Self-discharge Rate as an SOH Indicator:

• Self-discharge is how fast a battery loses charge when not in use.
• A high self-discharge rate means the battery is not healthy.
• Healthy batteries keep their charge for a long time.

Methods to Measure and Interpret Self-discharge Rates:

• Fully charge the battery and let it sit for a week.
• Measure how much charge is left.
• If the battery loses a lot of charge quickly, its SoH is poor.

3 Factors Affecting SoH

Age of the Battery

• Batteries degrade over time, even without use.
• Older batteries hold less charge and perform worse.

Number of Charge-Discharge Cycles

• Each charge and discharge cycle wears the battery down.
• More cycles lead to a shorter battery lifespan.

Operating Temperature and Environmental Conditions

• Extreme heat or cold can damage batteries.
• Stable, moderate temperatures are best for battery health.

Storage Conditions and Maintenance Practices

• Proper storage and regular maintenance extend battery life.
• Keep batteries clean, dry, and at optimal charge levels.

4 How to Check Battery Health?

• Perform full discharge test to measure actual capacity against original specifications
• Use professional diagnostic tools like battery analyzers for internal resistance measurement
• Track self-discharge rates over 48 hours to identify abnormal energy loss

5 Key Takeaways about Battery SoH

• Definition: Battery SoH (State of Health) shows how much of the original capacity remains.
• Indicators: Tested via capacity, internal resistance, and self-discharge rate.
• When to Replace: Most lithium batteries should be replaced when SoH drops below 70–80%. For car batteries, replacement is recommended once SoH is under 60% or frequent starting issues occur.
• Meaning: A higher SoH means longer run time, stronger performance, and fewer failures.

Part 3. Battery SoC vs SoH: Key differences explained

The key difference between State of Charge (SoC) and State of Health (SoH) is that SoC measures how much energy is left right now, while SoH measures the long-term condition of the battery. SoC is like a fuel gauge, and SoH is like a health check-up.

Part 4. Relationship between SoC and SoH

1 How SoC Influences SoH

Impact of Overcharging and Deep Discharging:

• Overcharging a battery makes it too hot and damages it over time. Deep discharging stresses the battery, making it wear out faster.

Optimal SoC Range and Battery Life:

• Keeping the battery charged between 20% and 80% helps it last longer. It reduces strain on the battery and keeps it working well for a longer time.

2 How SoH Affects SoC Accuracy

Battery Aging and SoC Accuracy:

• As batteries age, they cannot hold as much charge. This affects how accurately we can tell how much charge is left (SoC). Older batteries might show wrong readings or lose charge quickly.

Adjusting SoC Estimates for Aging Batteries:

• Innovative systems adjust how they measure SoC as batteries age. They use battery resistance and past use to give better readings, even as the battery ages.

3 Summary of SoC and SoH Relationship

• SoC affects SoH: Frequent deep discharges reduce health.
• SoH affects SoC accuracy: Aging batteries make SoC readings less reliable.
• Balanced charging (20–80%) improves both SoC stability and SoH longevity.

Part 5. Real-world applications of SoC and SoH management

1 Electric Vehicles (EVs)

Maintaining 20-80% SoC range extends EV battery lifespan by up to 60%. Regular SoH checks every 3,000 miles prevent unexpected capacity drops.

2 Solar Energy Storage

Deep cycle batteries in solar systems require weekly SoC monitoring to balance energy input/output. Annual SoH testing ensures 90%+ capacity retention for 5+ years.

3 Medical Equipment

Critical devices like portable oxygen concentrators need daily SoC checks and monthly SoH verification to maintain uninterrupted operation.

Part 6. Best practices for maintaining SoC and SoH

Regular Monitoring

Regularly check your battery’s SoC and SoH. Use tools like battery management systems (BMS) to get accurate readings. This helps you keep track of the battery’s condition and make necessary adjustments.

Optimal Charging Practices

Avoid overcharging your battery. Once it reaches 80% charge, try to stop charging. Similarly, don’t let the battery go below 20%. These practices prevent stress on the battery and extend its life.

Temperature Management

Keep your battery at a stable temperature. Avoid exposing it to extreme heat or cold. If possible, use temperature control systems. This helps maintain both SoC and SoH.

Proper Storage

Store your battery in a cool, dry place. If you don’t use it for a while, keep it at about 50% charge. This prevents degradation and keeps the battery healthy.

Regular Maintenance

Perform regular maintenance on your battery. Clean the terminals and check for any signs of wear or damage. Replace the battery if it shows a significant decline in SoH. Regular maintenance ensures the battery remains in good condition and performs well.

Part 7. FAQs about battery SoC and SoH