ESP8266

Part 1 – System Architecture and Firmware State Machine Introduction 18650 lithium ion cells are widely used in portable electronics, IoT devices, and energy storage systems. However, determining the actual capacity and health of a cell is not trivial. Many cells available on the market—especially reclaimed or recycled ones—have significantly degraded performance. To address this challenge, I designed a scalable 18650 battery capacity tester built around the ESP32 running MicroPython. The goal of the project was to build a reliable, cost effective, and extensible testing platform that can evaluate battery capacity and health while remaining flexible for future improvements. The system was designed with extensibility in mind. Future upgrades may include: Temperature based current control Fan speed regulation based on thermal conditions LCD interface for real time monitoring and logging Power path control to allow the device to operate from a battery during idle phases and low power consumption states (the discharge phase is lengthy but has low power consumption) Battery temperature monitoring Sending all recorded logs from ESP32 local storage to a web server via Wi-Fi (Wi-Fi is disabled during testing because it affects ADC, DAC, and GPIO functionality) A key design principle of this project was predictable and safe operation. For this reason, the firmware was implemented as a state machine, where each stage of the testing process is clearly defined. Firmware Architecture Battery testing involves multiple stages: validation, charging, resting, discharging, and measurement. Implementing this workflow with a state machine architecture makes the firmware easier to maintain, debug, and extend. Each state controls: Which hardware blocks are active Which measurements are taken Which conditions trigger a transition to the next state Title in Other Languages German: Wie man einen ESP32 18650 Batterie-Tester baut: Architektur & Logik French: Comment construire un testeur de batterie 18650 ESP32 : architecture et logique Spanish: Cómo construir un probador de baterías 18650 con ESP32: arquitectura y lógica Italian: Come costruire un tester per batterie 18650 con ESP32: architettura e logica Portuguese (Brazil): Como construir um testador de bateria 18650 com ESP32: arquitetura e lógica Tip: Right-click anywhere on this page and choose “Translate to your language”. Firmware Architecture: State Machine Design Below, I have added three different state machine diagrams to make the concept easier to understand. Colored Diagram Simple Diagram Advanced Digram A reliable battery 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 minutesNext 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 minutesNext state: MEASURE_IR 9. MEASURE_IR (Internal Resistance Measurement) Battery internal resistance provides valuable information