How to Build an 18650 battery capacity tester: Architecture & Logic

A reliable 18650 battery capacity tester must operate through clearly defined stages. To achieve predictable behavior and simplify debugging, the firmware was implemented as a state machine. Each state represents a specific phase of the test process and defines which hardware components are enabled or disabled.

1. IDLE State

The IDLE state represents the safe standby mode of the device.

The system waits for:

• Battery insertion
• User button press

During this state:

• Disable all power paths
• Status LEDs remain off to indicate that the device is ready
• All outputs are disabled
• Battery, charge, and discharge circuits remain off

This ensures that no current flows unintentionally and the system remains safe while waiting for user interaction.

2. CHECK_BATTERY

Once a battery is detected, the system measures its voltage to determine whether the cell is safe to test.

The following conditions are checked:

• If $Vbat < 1 V$ → invalid battery or connection error
• If $1 V \le Vbat < 2.0 V$ → deep discharge condition (warning or rejection depending on policy)
• If $Vbat > 4.3 V$ → over‑voltage error

If the voltage is within a valid range, the system proceeds to the next stage.
Next state: DECIDE_CHARGE

3. DECIDE_CHARGE

At this stage, the system determines whether the battery must be charged before the discharge test begins.

Logic:

• If $Vbat < 4.10 V$ → the battery must be charged
• Otherwise → proceed directly to stabilization before discharge

Next state: CHARGE or REST_BEFORE_DISCHARGE

4. CHARGE

In this phase, the battery is charged using a TP4056 charger module.

Actions performed:

• Disable all power paths
• Enable the battery path
• Enable the charging path
• Periodically measure battery voltage
• Monitor the TP4056 status pins

Charging is considered complete when:

• The TP4056 indicates charge completion, or
• Battery voltage reaches approximately 4.18–4.20 V

The firmware also checks for abnormal conditions:

• Voltage not increasing over time
• Charge timeout
• Unexpected over‑voltage conditions

Next state: REST_BEFORE_DISCHARGE

5. REST_BEFORE_DISCHARGE

Before starting the capacity measurement, the battery is allowed to rest. This phase is important because immediately after charging, the battery voltage is temporarily elevated due to surface charge. A rest period allows the open‑circuit voltage to stabilize.

Actions:

• Disable all power paths
• Record voltage at the start and end of the rest period

Typical rest duration: 5 to 20 minutes
Next state: DISCHARGE

6. DISCHARGE (Capacity Measurement)

This stage performs the actual capacity measurement.

The system:

• Disables all power paths (resetting previous states)
• Enables the battery path
• Enables the discharge path
• Uses the ESP32 DAC output to control the discharge current
• Continuously measures battery voltage, shunt current, and elapsed time

The battery capacity is calculated by integrating current over time:

$$Capacity_{mAh} = \sum I(mA) \times \Delta t(hours)$$

The discharge process stops when:

• Battery voltage reaches 3.0 V
• The target current cannot be maintained
• A fault condition occurs

Next state: FINAL_RECHARGE

7. FINAL_RECHARGE

After the discharge test completes, the battery is recharged again. This step ensures the battery is returned to a usable state after testing.

Important clarification: This step is adjusted based on the battery’s intended use. For storage, the battery should be charged up to 3.9 V, but for immediate use, the battery is charged again to approximately 4.2 V. I chose the storage voltage for my design.

Actions:

• Disable all power paths
• Enable the battery path
• Enable the charging circuit
• Monitor voltage and TP4056 status

Completion occurs when:

• TP4056 indicates full charge, or
• Battery voltage reaches 3.9 V, checked by disabling the charge path every minute

Next state: REST_BEFORE_IR

8. REST_BEFORE_IR

After charging, another stabilization period is required before measuring internal resistance. This ensures that the voltage measurement reflects the true open‑circuit voltage of the battery.

Rest time: 5–20 minutes
Next state: MEASURE_IR

9. MEASURE_IR (Internal Resistance Measurement)

Battery internal resistance provides valuable information about cell health and aging.

The measurement procedure:

1. Measure the open‑circuit voltage ($V_{open}$)
2. Apply a short load pulse (approximately 0.5 A)
3. Wait about 100 ms
4. Measure loaded voltage ($V_{load}$) and discharge current ($I_{load}$)

Internal resistance is then calculated as:

$$R_{internal} = \frac{V_{open} – V_{load}}{I_{load}}$$

Optionally, a second longer pulse (e.g., 500 ms) can be used for additional diagnostics.
Next state: SHOW_RESULT

10. SHOW_RESULT

Once all measurements are completed, the device reports the results. Displayed or transmitted data may include:

• Measured capacity
• Total discharge time
• Internal resistance
• Capacity classification
• Internal resistance classification

Next state: IDLE

11. ERROR State

If any abnormal condition occurs during the process, the system enters the ERROR state.

Actions:

• Disable all power paths
• Activate buzzer (if available)
• Display error information

The system waits for a user reset. Afterward, the device returns to IDLE.