How to Test Lithium-Ion Battery Capacity Correctly: Charge & Discharge Curve Guide

A lithium-ion battery capacity test may show a result that is much lower than the rated capacity. This can raise questions about the battery itself, the test method, or the test conditions.

Battery capacity cannot be evaluated from the final mAh reading alone. Starting voltage, charging conditions, discharge current, cut-off voltage, temperature, and test equipment can all affect the result.

This guide explains how to test and measure lithium-ion battery capacity correctly. It also helps you read charge and discharge curves, identify abnormal test results, and understand why measured capacity may differ from the rated value.

How to Test Lithium-Ion Battery Capacity Step by Step

A reliable lithium-ion battery capacity test depends on both the procedure and the equipment used.

The tester should be able to control the required charge and discharge conditions and accurately record voltage, current, capacity, energy, and time.

For consistent and reliable results, the complete test should follow a controlled procedure. 

Step 1: Check the Test Equipment

Use a suitable battery tester or battery analyzer that can control both charging and discharging. Common examples include battery cyclers from Neware, Maccor, Arbin, and Chroma. A programmable electronic load can also be used for discharge testing.

Make sure the equipment supports the required voltage and current range and can accurately record voltage, current, capacity, energy, and time.

Before testing, check:

• voltage measurement accuracy
• current measurement accuracy
• charge and discharge current range
• cut-off voltage settings
• charge termination settings
• sampling interval
• calibration status
• cables, connectors, and test contacts

Different testers may use different measurement methods, sampling intervals, or termination logic. Small differences in measured Ah, mAh, or Wh can therefore occur between test systems.

Cable and contact resistance also matter, especially at higher currents. Excessive resistance creates additional voltage drop and may cause the battery to reach the cut-off voltage earlier.

Step 2: Set the Test Conditions

Set the test parameters according to the battery specification before charging or discharging begins.

Typical parameters include:

• charge current
• full-charge voltage
• CV termination current
• rest time
• discharge current
• discharge cut-off voltage
• test temperature

Use the same conditions when comparing repeated tests or different battery samples.

For example, a result obtained at 0.2C and 25°C should not be directly compared with a result obtained at 1C and a much lower temperature.

Step 3: Fully Charge the Battery

Charge the battery according to the manufacturer’s specified CC/CV charging procedure.

For a standard 3.7V lithium-ion or LiPo cell, the full-charge voltage is commonly around 4.2V per cell.

During the constant-current stage, the battery voltage rises gradually. Once the full-charge voltage is reached, charging enters the constant-voltage stage. The voltage remains nearly constant while the charge current decreases.

Charging should continue until the current reaches the specified termination level.

Depending on the cell specification and test method, a termination current around 0.02C to 0.01C may be used. The exact value should follow the cell datasheet or defined test procedure.

For a 6200mAh battery:

• 0.02C ≈ 124mA
• 0.01C ≈ 62mA

Reaching 4.2V alone does not necessarily mean the charging process is complete.

Step 4: Allow the Battery to Rest

After charging, allow the battery to rest before starting the discharge test.

A practical test may use a rest period of around 15 to 30 minutes at room temperature, such as 25°C ± 2°C. This allows polarization to decrease and the open-circuit voltage to stabilize.

Some battery specifications or test standards may require a different rest period. The specified procedure should always take priority.

Step 5: Confirm the Starting Voltage

Record the battery open-circuit voltage before discharge begins.

For a conventional 3.7V lithium-ion battery, the starting voltage should be consistent with the fully charged condition defined by the cell specification.

However, voltage alone should not be used to determine the exact state of charge. Temperature, cell chemistry, aging, rest time, and previous load conditions can all affect open-circuit voltage.

The starting voltage should still be recorded because it provides important context when reviewing a capacity result.

Step 6: Discharge at a Defined Current

Discharge the battery using a controlled constant-current load.

Common characterization rates include:

• 0.2C
• 0.5C
• 1C

The correct discharge current should follow the manufacturer’s specification.

For a 6200mAh battery:

• 0.2C ≈ 1.24A
• 0.5C ≈ 3.10A
• 1C ≈ 6.20A

Higher current normally causes greater voltage sag and polarization. The battery may therefore reach the cut-off voltage sooner than it would at a lower discharge rate.

For higher-current testing, also check voltage drop across cables, connectors, fixtures, welds, and protection circuits.

Capacity results should always be reported together with the discharge current.

Step 7: Discharge to the Specified Cut-Off Voltage

Continue the discharge until the specified cut-off voltage is reached.

Depending on the cell design, the test cut-off may be approximately:

• 2.5V
• 2.75V
• 3.0V

The correct value depends on the cell specification, pack design, and protection settings.

A higher cut-off voltage ends the test earlier and usually reduces the measured usable capacity.

For battery packs, the BMS or PCM can also terminate the discharge before the tester reaches its programmed cut-off voltage.

Possible causes include:

• cell-level undervoltage protection
• pack undervoltage protection
• overcurrent protection
• temperature protection
• cell imbalance

If a discharge test ends unexpectedly early, review both the tester settings and the protection circuit behavior.

Step 8: Record the Complete Test Data

A useful battery capacity test report should contain more than the final mAh value.

The complete voltage-capacity or voltage-time curve can help identify incomplete charging, abnormal voltage sag, early cut-off, unstable connections, high resistance, or BMS/PCM intervention.

Step 9: Review the Result Against the Test Conditions

Compare the measured discharge capacity with the rated value under the specified test conditions. The charging method, discharge current, cut-off voltage, and temperature should match the manufacturer’s requirements.

If the result is lower than expected, check the troubleshooting section below.

Typical Test Parameters for a 3.7V Lithium Battery

The exact test conditions should always follow the cell datasheet and pack protection settings.

These values are general references only. The manufacturer’s specified conditions should take priority.

How to Calculate Battery Capacity

After the battery is fully charged, discharge it at a defined constant current until the specified cut-off voltage is reached.

For a constant-current discharge:

Capacity (Ah) = Discharge Current (A) × Discharge Time (h)

For example, if a battery is discharged at 0.5A for 2 hours:

0.5A × 2h = 1.0Ah

Therefore:

1.0Ah = 1000mAh

This result is valid only for the defined test conditions.

In real battery tests, the discharge current may not remain perfectly constant. In that case, capacity is determined by integrating current over time rather than using a single current value.

Battery Capacity Calculator

A simple calculator can be placed here to let users enter:

• discharge current
• discharge time

and calculate:

• Ah
• mAh

This can improve usability and keep the article practical for engineers and customers reviewing test data.

How to Read Lithium-Ion Charge and Discharge Curves

Charge and discharge curves help explain what happened during the test.

They provide more information than the final capacity value alone and can help reveal whether charging was completed, whether voltage sag was excessive, and whether the discharge ended normally.

Charge Curve

A typical lithium-ion battery is charged using the CC/CV method. Before starting the discharge test, make sure the battery has reached the specified full-charge voltage and completed the CV stage at the required termination current. 

A complete charge curve should therefore show both the CC and CV stages.

If charging stops immediately after the battery first reaches 4.2V, the battery may not have completed the intended charging cycle.

Discharge Curve

A normal lithium-ion discharge curve usually has three main regions.

Initial voltage drop

When the load is applied, the voltage drops from the open-circuit level. The size of this drop depends on discharge current, internal resistance, wiring resistance, and temperature.

Main discharge region

Most usable capacity is delivered in this region. The voltage gradually decreases as the battery discharges.

End-of-discharge knee

Near the end of discharge, voltage begins to fall more rapidly. The test should stop at the specified cut-off voltage or when the protection circuit disconnects the load.

Why the Curve Matters

The curve helps determine whether the result is consistent with the expected test conditions.

An abnormal starting point, unusually large voltage sag, early cut-off, or a sudden protection event can explain why the measured capacity is lower than expected.

How Discharge Rate Affects Measured Battery Capacity

Battery capacity is affected by the discharge current used during the test.

At higher currents, internal voltage losses increase.

The battery may reach the system cut-off voltage sooner, even though additional electrochemical capacity could still be available at a lower discharge rate.

This is why capacity results should always be reported together with the discharge current.

Why Does a Lithium-Ion Battery Capacity Test Show Lower mAh Than Rated?

A low measured capacity can result from incomplete charging, excessive discharge current, early cut-off, low temperature, BMS limits, connection resistance, tester settings, battery aging, or differences between the actual test and the manufacturer’s rated-capacity conditions.

The complete charge and discharge data should be reviewed before concluding that the battery itself is under capacity.

One of the first items to check is the starting condition of the discharge test.

If a conventional 3.7V lithium-ion battery begins discharging from around 3.7–3.8V rather than after a completed full-charge process, the test may represent only part of the available capacity.

Lithium-ion batteries are commonly shipped at a partial state of charge, typically around 30% to 60% SOC. If the battery is tested soon after delivery without being fully charged first, the measured discharge capacity may be much lower than its rated capacity. 

Real Test Case: Why a 6200mAh Battery Measured Only 2269mAh

A customer tested an LP654968LT-1S2P 3.7V 6200mAh battery using an EBX battery tester.

The discharge test reported only approximately 2269mAh.

At first glance, the result appeared to be far below the 6200mAh rated capacity.

However, the discharge curve showed an important detail.

Key finding: The 2269mAh result did not prove that the 6200mAh battery was under capacity because the discharge test did not begin after a complete full-charge cycle. 

The test began at approximately 3.7–3.75V, rather than after the battery had completed a full CC/CV charging process near its specified full-charge condition.

The recorded 2269mAh therefore represented the capacity delivered from the actual starting condition down to the test cut-off voltage. It did not represent a complete full-charge-to-cut-off capacity test.

This case shows why the discharge starting condition and the complete test curve should be reviewed before comparing a measured capacity directly with the rated value.

Charge Capacity vs. Discharge Capacity: Why 5292mAh Was Not the Battery’s Total Capacity

The same test case also revealed another common misunderstanding.

The EBX tester displayed approximately 5292mAh during charging. This value was initially interpreted as evidence that the 6200mAh battery had only 5292mAh of capacity.

However, the battery began charging at approximately 3.5V. It already contained some stored energy before charging started.

The 5292mAh value therefore represented only the charge supplied during that charging session.

It should not be treated as the battery’s total rated capacity.

For battery verification, discharge capacity measured under controlled and defined conditions is generally more useful for comparison with the rated capacity.

Why Battery Voltage Alone Cannot Determine Capacity

Battery voltage is useful, but it cannot accurately determine remaining capacity by itself.

Two batteries at the same terminal voltage may still have different:

• state of charge
• internal resistance
• usable capacity
• temperature
• aging condition
• previous load history

Voltage should therefore be treated as one diagnostic reference rather than a direct measurement of capacity.

A controlled charge-discharge test provides a more reliable basis for capacity verification.

Does 3.5V Mean the Battery Is Empty?

For a standard 3.7V lithium-ion or LiPo battery, an open-circuit voltage of 3.5V does not necessarily indicate a fully discharged state. Voltage can provide a general indication of SOC, but the remaining capacity depends on the cell’s discharge characteristics and test conditions. 

Illustrative Open-Circuit Voltage vs. SOC Reference for a Typical 3.7V LiPo Cell

These values are illustrative only and should not be used as a universal SOC calibration table.

Actual voltage-SOC relationships vary with cell chemistry, design, temperature, aging, rest time, and whether the voltage is measured at rest or under load.

Common Lithium Battery Capacity Testing Mistakes

Several test errors can produce misleading capacity results.

Starting the Discharge Test Without a Full Charge

The test may measure only part of the available capacity.

Using Charge mAh as the Total Battery Capacity

The charger records only the charge supplied during that charging session. If the battery already contains stored energy, this value does not represent total rated capacity.

Ignoring the Starting Voltage

Without the starting condition, it is difficult to determine whether the battery began the discharge test from the intended state.

Using the Wrong Cut-Off Voltage

A higher cut-off voltage ends the test earlier and reduces measured usable capacity.

Comparing Different Discharge Rates

Capacity measured at 0.2C should not be directly compared with a result measured at 1C or 2C without considering the different load conditions.

Ignoring Temperature

Low temperature increases internal resistance and can reduce both operating voltage and available capacity.

Ignoring the Test Equipment

Incorrect calibration, unsuitable tester settings, cable resistance, unstable contacts, or different termination logic can also affect the result.

Ignoring BMS or PCM Intervention

For battery packs, the protection circuit may stop the discharge before the tester reaches its programmed cut-off voltage.

Battery Capacity Test Report Checklist

FAQ

What should a fully charged 3.7V lithium battery measure?

A conventional 3.7V lithium-ion or lithium-polymer cell commonly uses a full-charge voltage of approximately 4.2V per cell. The exact value should follow the cell specification.

How do you test the capacity of a lithium-ion battery?

Fully charge the battery according to the specified CC/CV procedure, allow it to rest if required, and discharge it at a defined constant current to the specified cut-off voltage. Record the discharged Ah or mAh together with the test conditions.

Is 3.7V fully charged for a lithium-ion battery?

Normally no. For a conventional 3.7V lithium-ion or LiPo cell, the nominal voltage is 3.7V, while the typical full-charge voltage is approximately 4.2V per cell.

Why does my lithium battery show less capacity than its rated capacity?

Possible causes include incomplete charging, high discharge current, an early cut-off voltage, low temperature, battery aging, BMS limits, tester accuracy, contact resistance, or test conditions that differ from the manufacturer’s specification.

Can charge capacity be used as battery capacity?

Not directly. If the battery already contains stored energy when charging begins, the tester records only the additional charge supplied during that charging session. A controlled discharge test is more useful for comparison with rated capacity.

Why does discharge capacity change with C-rate?

Higher discharge current causes greater voltage drop, polarization, and heat. The battery may reach the cut-off voltage earlier, which can reduce the usable capacity measured under that load.

Can different battery testers give different capacity results?

Yes. Differences in calibration, measurement accuracy, sampling interval, voltage sensing, termination logic, cable resistance, and contact resistance can cause variations between test systems.

Conclusion

A lithium-ion battery capacity result should always be reviewed together with the conditions used to produce it.

The most useful information includes the charging procedure, starting voltage, termination current, discharge rate, cut-off voltage, temperature, tester setup, and complete charge-discharge curve.

When a capacity result is unexpectedly low, reviewing these conditions first can help distinguish between a real battery performance issue and a misleading test result.

Dr. Emily Li

Principal Scientist

Graduated from Peking University,Dr. Emily Li has 10 years of experience in lithium battery material research and over 10 years of background in new materials application. She is experienced in the lithium battery materials specific application and performance.