ESP32 0-24V Voltage Monitor

An advanced, industrial-grade voltage monitoring system designed for the ESP32 platform. This project samples analog voltage (0-24V range) at a high frequency on a dedicated CPU core, averages the data, and securely uploads the data to ThingSpeak and/or ThingsBoard.

Featuring a dual-core FreeRTOS architecture, NTP real-world time synchronization, and an offline LittleFS flash storage buffer, this system guarantees zero data loss and uninterrupted sampling even during total network or power outages.

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📌 Example Use Cases

This monitor is especially useful for vehicles that sit for long periods and are more likely to develop battery issues over time:

• Watching any low-use vehicle that may slowly discharge while parked.
• Tracking the battery health of an old or vintage car where the battery is often the weak point.
• Monitoring other 6V-12V-24V battery-powered equipment such as motorcycles, scooters, boats, RVs, tractors, generators, and utility vehicles.
• Monitoring a battery while it is connected to a battery charger.

ThingSpeak can also be used to trigger an alert when battery voltage drops to a level where it is time to recharge, while ThingsBoard can be used for dashboards, rules, and telemetry replay.

Example live data from my spare car: ThingSpeak Channel 3405145

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🚀 Key Features

• Dual-Core Isolation (FreeRTOS):
  • Core 1 (TaskMeasure): Dedicated entirely to strict, time-critical 10ms hardware ADC sampling.
  • Core 0 (TaskSend): Handles heavy, blocking network tasks (WiFi reconnection, NTP sync, HTTP POST payloads).
• Zero-Loss Persistency (LittleFS): If WiFi is down, data points are immediately appended as raw binary structures to the ESP32's onboard Flash memory.
• Power-Loss Proof Time Tracking (NTP): Uses Network Time Protocol to log data with absolute Unix Epoch timestamps. If booted offline, it maintains a precise relative timeline and auto-corrects/backdates points before cloud upload once the network recovers.
• Timezone-Aware Local Logs: Serial output uses a real timezone rule with automatic daylight saving time adjustment, while ThingSpeak uploads remain in UTC and ThingsBoard telemetry uses client-side timestamps in milliseconds.
• Smart Bulk Uploading: Uses ThingSpeak's Bulk Update JSON API and ThingsBoard's batched telemetry format to push up to 30 accumulated points at once, bypassing free-tier rate limits and avoiding RAM exhaustion.

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🔌 Hardware Configuration

Voltage Divider Circuit

Since the ESP32 ADC safe input limit is around 3.1V (when configured with maximum 11dB attenuation), a voltage divider is required to safely scale down the battery input.

The divider used in this project is well suited for monitoring a 12V battery and still leaves some headroom above the normal charging range.

• Resistor 1 ($R_1$): $330\text{ k}\Omega$
• Resistor 2 ($R_2$): $47\text{ k}\Omega$
• Analog Pin: GPIO 34 (ADC1)

$$\text{Attenuation Ratio} = \frac{R_1 + R_2}{R_2} = \frac{330\text{k} + 47\text{k}}{47\text{k}} \approx 8.0212$$

Maximum Measurable Voltage: $3.1\text{V} \times 8.0212 \approx 24.86\text{V}$.

                   GPIO 34
                      |
                      |
12V -----/\/\/\/------+-----/\/\/\/----- GND
          330k                47k

For a 24V battery system, use a higher-ratio divider so the ESP32 still has safe headroom when the battery is charging. A simple alternative is:

• Resistor 1 ($R_1$): $470\text{ k}\Omega$
• Resistor 2 ($R_2$): $47\text{ k}\Omega$

This gives an attenuation ratio of about $11.00$ and raises the measurable range to roughly $34.1\text{V}$, which is a better fit for a 24V battery setup.

                   GPIO 34
                      |
                      |
24V -----/\/\/\/------+-----/\/\/\/----- GND
          470k                47k

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💾 Installation & Setup

1. Prerequisites

Ensure you have the following libraries and configurations ready in your Arduino IDE / PlatformIO environment:

• Board Manager: ESP32 Arduino Core (v2.x or v3.x fully supported).
• Filesystem: LittleFS (built into the ESP32 core).

2. ThingSpeak / ThingsBoard Configuration

1. Log into your ThingSpeak Account.
2. Create a new Channel and enable Field 1.
3. Note your Channel ID and Write API Key.
4. If you want ThingsBoard reporting, create a device in your ThingsBoard tenant and copy the device access token.

3. Firmware Configuration

Open the source code file and update the configuration sections at the top:

// WiFi Credentials
const char* ssid     = "Your_WiFi_SSID";
const char* password = "Your_WiFi_Password";

// Cloud Reporting Parameters
const bool ENABLE_THINGSPEAK = true;
const bool ENABLE_THINGSBOARD = false;

const char* writeApiKey = "YOUR_THINGSPEAK_WRITE_KEY";
const char* channelID   = "YOUR_CHANNEL_ID_NUMBER";
const char* thingsBoardServer = "https://thingsboard.cloud";
const char* thingsBoardToken  = "YOUR_THINGSBOARD_DEVICE_ACCESS_TOKEN";

// NTP Parameters (Adjust for your timezone rule)
const char* timeZone = "CET-1CEST,M3.5.0/2,M10.5.0/3"; // Example: CET/CEST with automatic DST

// Example alternatives:
// const char* timeZone = "EST5EDT,M3.2.0/2,M11.1.0/2"; // US Eastern with automatic DST
// const char* timeZone = "UTC0"; // No daylight saving time

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🛠️ Software Architecture

Data travels safely across cores and into the cloud through the following pipeline:

[VOLTAGE INPUT]
│
▼  (Core 1 - Precision 10ms Timer Loop)
┌────────────────────────────────────────┐
│ TaskMeasure: Continuous ADC Sampling   │
└────────────────────────────────────────┘
│
▼  (Averages 1500 samples every 15 seconds)
┌────────────────────────────────────────┐
│ FreeRTOS Inter-Task Queue (Safe Pipe)  │
└────────────────────────────────────────┘
│
▼  (Core 0 - Asynchronous Network Core)
┌────────────────────────────────────────┐
│ TaskSend: Writes immediately to Flash  │
└────────────────────────────────────────┘
│
├──► [WiFi Offline] ──► Keep buffering in LittleFS. Retry WiFi silently.
│
└──► [WiFi Online]  ──► Sync NTP Clock ──► Cloud Uploads ──► [ThingSpeak and/or ThingsBoard]

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Fault Tolerance Behavior Matrix

This document outlines how the ESP32 Voltage Monitor responds to various network, power, and infrastructure anomalies while maintaining high-frequency data sampling integrity.

Scenario	System State	Hardware Sampling Core (Core 1)	Network & Storage Core (Core 0)	Data Preservation	Recovery Action
Normal Operation	Online	Samples ADC every 10ms. Queues 15-second average voltage.	Translates data to absolute timestamps and uploads to whichever cloud targets are enabled.	Real-time transmission. Local storage file remains empty.	N/A
WiFi Access Point Disconnected	Offline	Continues uninterrupted 10ms sampling and 15s averaging.	Appends raw data structure to /offline_data.bin in LittleFS Flash memory. Initiates a non-blocking WiFi.begin() loop.	100% Preserved locally on internal flash memory.	Periodically checks WiFi status without stalling the system.
ThingSpeak or ThingsBoard Cloud Down	Offline	Continues uninterrupted 10ms sampling and 15s averaging.	Detects failed HTTP response from the enabled cloud target(s). Leaves data intact within LittleFS.	100% Preserved locally on internal flash memory.	Retries transmission on the next 15-second loop iteration.
Power Loss during Offline Phase	Critical Failure	Hardware turns off.	Hardware turns off.	100% Preserved up to the last completed 15-second mark. LittleFS is non-volatile.	On reboot, reads current NTP time. If internet is unavailable, constructs a reliable relative timeline based on internal milestones.
WiFi Connection Re-established	Recovery	Continues uninterrupted 10ms sampling and 15s averaging.	Fetches real-world time via pool.ntp.org. Extracts up to 30 past data points from Flash, matches them to absolute time metrics, and sends the batch to each enabled cloud target.	100% Recovered. History is populated chronologically.	Deletes processed entries from the LittleFS binary file after all enabled uploads succeed.

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Core Isolation Visual Design

The absolute stability of this fault tolerance architecture relies on FreeRTOS task prioritization and core pinning:

• Core 1 (High Priority Task): Strictly isolated from network instabilities. It will never drop a sample because of a slow server response or poor WiFi signals.
• Core 0 (Low Priority Task): Designed to handle network bottlenecks, delays, or blocking socket requests without interfering with Core 1 operations.

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Operational Notes

• Time handling & DST: Configure the timeZone POSIX string in CarBatteryMonitor.ino to match your locale (examples are included in the source). The device calls configTzTime() and prints local timestamps on Serial with automatic daylight saving adjustments; ThingSpeak uploads are produced from UTC (gmtime()), while ThingsBoard telemetry uses Unix milliseconds in the ts field.

• Fallback timestamps & correction on sync: If the device boots without network/NTP, it stores measurements immediately to LittleFS using a sentinel fallback epoch (FALLBACK_EPOCH = 1) and keeps incrementing a local counter to preserve ordering. When NTP time becomes available, the firmware computes an offset and applies it only to placeholder-era records (those with timestamp < MIN_VALID_EPOCH, currently 1700000000) so already-valid timestamps are not rebased. Corrected points are then uploaded in chronological order.

• Upload gating until NTP locked: To avoid sending placeholder timestamps or creating duplicate/far-future timestamps, the firmware will buffer data locally and will not attempt ThingSpeak uploads until isTimeSynchronized() confirms a valid NTP clock.

• Boot-button flash delete: If you need to clear the offline buffer, hold the BOOT button (GPIO0) while powering on / resetting the device. On startup the firmware checks that pin and will delete /offline_data.bin from LittleFS when detected.

• LED indicator semantics: The onboard status LED indicates network/time state independently of the 15s sampling loop:

  • Slow blink (~500 ms on/off): WiFi is offline.
  • Fast blink (~250 ms on/off): WiFi is connected but NTP time sync is pending.
  • Steady off: Device fully online (WiFi + NTP) and operating normally. During HTTP POST the LED briefly pulses HIGH to signal an upload in progress.

• Offline storage file: The LittleFS binary buffer is stored at /offline_data.bin. Processed records are removed or truncated only after all enabled cloud uploads succeed.

• Where to change divider / ADC constants: The voltage divider and ADC scaling constants are defined in CarBatteryMonitor.ino (look for DIVIDER_RATIO and the resistor value comments). For 12V systems the shipped values are R1=330k, R2=47k; for 24V systems use the suggested R1=470k, R2=47k alternative.

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