Testing lithium battery capacity is important to determine the actual Ah (Ampere-hour) and Wh (Watt-hour) capacity of a battery. It can help identify weak cells, battery aging, reduced backup time and problems in a battery pack.

The most reliable practical method is a controlled discharge test.

Basic Process

Fully Charge → Connect Capacity Tester → Controlled Discharge → Measure Ah/Wh → Compare With Rated Capacity

What Is Lithium Battery Capacity?

Battery capacity is generally expressed in Ah or mAh.

For example:

• 3.7V 5000mAh cell = 5Ah
• 12.8V 20Ah LiFePO4 battery = 20Ah
• 48V 50Ah lithium battery = 50Ah

Battery energy can be estimated using:

Energy (Wh) = Voltage (V) × Capacity (Ah)

For example:

48V × 50Ah = 2400Wh

So a 48V 50Ah battery has approximately 2.4kWh nominal energy.

Equipment Required

For a proper capacity test, you generally need:

• Lithium battery charger
• Battery capacity tester
• Electronic load or programmable discharge tester
• Digital multimeter
• Suitable wires and connectors
• Temperature monitoring
• Safe testing area

A dedicated battery capacity tester is preferred because it can automatically record voltage, current, time, Ah and Wh.

Step 1: Check the Battery

Before starting the test, inspect the battery.

Check:

• Battery voltage
• Physical condition
• Connectors
• Wires
• BMS
• Cell-group voltages, where accessible
• Temperature
• Signs of swelling or damage

Do not capacity-test a swollen, leaking, physically damaged or abnormally hot lithium battery.

Step 2: Fully Charge the Battery

Use the correct charger according to the battery chemistry and series configuration.

For example:

These are typical values; always follow the cell/battery manufacturer’s specifications.

Step 3: Select the Discharge Current

The discharge current can be selected according to the required C-rate.

Formula

Discharge Current = Battery Capacity × C-rate

For a 20Ah battery:

For accurate capacity comparison, use the discharge current specified by the battery or cell manufacturer.

Step 4: Set the Cut-Off Voltage

Set the capacity tester to the correct end-of-discharge voltage.

The correct voltage depends on:

• Battery chemistry
• Cell specification
• Number of cells in series
• BMS settings
• Manufacturer’s test conditions

Do not use a generic cut-off voltage without checking the battery specification.

Step 5: Start the Discharge Test

Connect:

Lithium Battery → Capacity Tester / Electronic Load

Start the controlled discharge.

The tester records:

• Starting voltage
• Discharge current
• Battery voltage
• Discharge time
• Capacity in Ah
• Energy in Wh
• Temperature, if supported

The test stops when the programmed cut-off condition is reached or the battery’s BMS disconnects the load.

Step 6: Calculate Actual Capacity

The basic formula is:

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

Example

Suppose:

Discharge current = 10A

Discharge time = 1.9 hours

Then:

10 × 1.9 = 19Ah

The battery delivered approximately:

19Ah

Step 7: Calculate Capacity Percentage

To compare actual capacity with the rated capacity:

Capacity % = Measured Capacity ÷ Rated Capacity × 100

Example:

Rated capacity = 20Ah

Measured capacity = 19Ah

19 ÷ 20 × 100 = 95%

Therefore, the battery delivered approximately 95% of its rated capacity under the selected test conditions.

How to Calculate Battery Energy

Battery energy is expressed in Wh.

Formula

Energy (Wh) = Voltage × Capacity (Ah)

For example:

48V × 20Ah = 960Wh

However, because battery voltage changes during discharge, the actual Wh recorded by the capacity tester is preferable to simply multiplying nominal voltage by Ah.

Example: Testing a 48V 20Ah Battery

Suppose you have a:

48V 20Ah Li-ion battery

You perform a controlled discharge at 10A.

The tester records:

• Discharge current: 10A
• Test time: 1.9 hours
• Measured capacity: 19Ah
• Rated capacity: 20Ah

Capacity percentage:

19 ÷ 20 × 100 = 95%

Result:

Measured capacity = 19Ah

Capacity percentage = 95%

How to Test 18650 and 21700 Cells

Individual 18650 and 21700 cells can be tested using a suitable single-cell capacity tester.

Procedure:

Charge → Rest → Discharge → Measure mAh → Compare With Rated Capacity

For example:

Rated cell:

5000mAh

Measured:

4700mAh

Capacity percentage:

4700 ÷ 5000 × 100 = 94%

The cell delivered approximately 94% of its rated capacity under the test conditions.

How to Test a Complete Lithium Battery Pack

For a complete battery pack, follow this sequence:

1. Fully charge the battery

↓

2. Check pack voltage

↓

3. Check cell-group voltages if accessible

↓

4. Connect the capacity tester

↓

5. Set the correct discharge current

↓

6. Set the correct cut-off voltage

↓

7. Start the discharge

↓

8. Record Ah, Wh and test time

↓

9. Monitor temperature

↓

10. Recharge the battery

This provides a useful record of the battery’s actual performance.

Voltage Test vs Capacity Test

Checking battery voltage alone does not tell you the actual battery capacity.

A battery can show a normal voltage but still have significantly reduced capacity.

Why Does a Lithium Battery Have Low Capacity?

A battery may deliver less capacity than its rating because of:

• Aging cells
• Weak cells
• High internal resistance
• Cell imbalance
• BMS cut-off
• Incorrect charging
• Excessive discharge current
• High or low temperature
• Poor connections
• Incorrect capacity-test settings

For battery repair, capacity testing should ideally be combined with cell voltage, internal resistance and BMS testing.

Capacity Testing for Battery Repair

For a customer battery with reduced backup, a technician can use:

Visual Inspection → Voltage Test → BMS Check → Cell Testing → Internal Resistance Test → Capacity Test

This can help determine whether the problem is related to the cells, BMS, wiring, charging system or battery capacity.

Important Safety Precautions

When testing lithium batteries:

• Use a correctly rated tester.
• Use suitable cables and connectors.
• Never short-circuit the battery.
• Do not exceed the manufacturer’s discharge current.
• Use the correct cut-off voltage.
• Monitor battery temperature.
• Do not test swollen or damaged batteries.
• Stop the test if the battery becomes abnormally hot.
• Keep the testing area suitable for lithium battery testing.

Quick Capacity Test Formula

Capacity

Ah = Current (A) × Time (Hours)

Capacity Percentage

Capacity % = Measured Ah ÷ Rated Ah × 100

Nominal Energy

Wh = Voltage × Ah

Example:

Battery: 48V 20Ah
Measured: 19Ah
Capacity: 95%
Nominal energy: 48 × 20 = 960Wh

Contact Lion Battery

For lithium battery capacity testing, battery repair, custom battery packs, lithium battery manufacturing and OEM/ODM requirements, contact Lion Battery.

Phone / WhatsApp: +91-9724991737

Email: info@lionbattery.in

Location: Umargam, Valsad, Gujarat

Contact Lion Battery