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Supercapacitor Solar IoT for High-Power Actuation

Stores solar energy in supercaps to deliver 24V bursts—even after days without sunlight.

nelectraNelectra
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Most solar IoT systems can measure things. Very few can do things. This project shows a battery-free solar IoT system that runs for days without sunlight—and still delivers high-power bursts (9V–24V, up to 1.5A) to drive real-world actuators. By storing energy in supercapacitors instead of batteries, it enables maintenance-free operation while bridging the gap between low-power sensing and high-power control.

Overview

Heliotrax is a solar-powered, battery-less IoT platform designed for long-term outdoor operation. It enables reliable monitoring and control of sensors and actuators without the need for batteries or regular maintenance.

The system is built as a modular hardware platform for developers and system integrators, providing robust electronics and reference firmware while allowing full flexibility in software implementation.

The Problem

Many outdoor IoT systems rely on batteries, which introduce limitations in lifetime, reliability, and maintenance. In remote or hard-to-access locations, battery replacement becomes costly and impractical.

In addition, most solar-powered IoT nodes are designed only for low-power sensing. They can supply sensors requiring small voltages and minimal current, but are not capable of driving actuators.

There is currently a lack of compact systems that can:

  • harvest energy from small solar panels
  • store energy efficiently
  • deliver short, high-power bursts (e.g., up to 24V)

This limits the ability to control actuators such as latching solenoids, valves, or switching elements in autonomous outdoor system

The Solution

Heliotrax combines solar energy harvesting with efficient energy storage and controlled high-power delivery.

Energy is stored in two 22F supercapacitors connected in series, forming a storage stage up to 5V. The system accumulates energy over time and releases it as a controlled high-power burst when required.

This enables:

  • battery-less operation
  • long-term autonomous deployment
  • actuation of power-hungry devices

The platform bridges the gap between low-power energy harvesting and high-power actuation.

System Architecture

Transmitter Node (core hardware)

  • solar energy harvesting and storage
  • ultra-low quiescent current (~tens of µA)
  • sensor interfacing and data acquisition
  • controlled high-power output for actuators
  • wireless communication

Receiver Node

  • implemented on standard ESP32-C3 boards (e.g., XIAO, Dev Module)
  • receives data and triggers actions
  • requires only corresponding software

Performance & Technical Capabilities

  • Energy storage: 2× 22F supercapacitors (series, up to 5V)
  • Ultra-low quiescent current: a few 10 µA
  • No-light operation: up to ~5 days (foggy conditions, ultra-low-power mode)

High-power burst outputs:

  • 24V @ 500 mA
  • 12V @ 1 A
  • 9V @ 1.5 A

Expandability:

  • Additional supercapacitors or Lithium-Ion Capacitors via screw terminals
  • External solar panel or alternative energy source input

Hardware Platform Features

  • Modular hardware design
  • 7 Grove connectors for sensors and modules
  • Support for Grove ecosystem (e.g., LoRa modules)
  • Interface for SIM800L GSM/GPRS module
  • Designed for flexible integration and prototyping

Software Approach

Heliotrax follows a hardware-first approach.

The project provides:

  • reference firmware (Arduino-based PoC)
  • example implementations for transmitter and receiver

Users are expected to develop their own application software based on their requirements, ensuring full flexibility and independence from predefined ecosystems.

Why It Matters

Heliotrax extends the capabilities of autonomous IoT systems beyond sensing by enabling actuator control without batteries.

Key advantages:

  • maintenance-free deployments
  • operation in remote or hard-to-access locations
  • reduced environmental impact
  • ability to drive real-world devices (e.g., valves, relays, actuators)

This makes it suitable for applications such as:

  • water management systems
  • infrastructure control
  • distributed automation in outdoor environments

Documentation & Code

Reference firmware is available on GitHub:  https://github.com/Nelectra/Heliotrax-reference-firmware

The repository includes transmitter and receiver implementations for testing and development.

Project webpage: https://heliotrax.io/

Technical Specifications

PCB No.1 – Control and Communication

Energy storage:

  • Onboard 2x supercapacitors 22F/2.8V
  • Expandable with an additional pair...
Read more »

View all details

j4g_24V1.pdf

Schematics for PCB No. 2: Power & Actuation

Adobe Portable Document Format - 419.58 kB - 04/24/2026 at 19:50

Preview

j4g1.pdf

Schematics for PCB No. 1: Control & Communication

Adobe Portable Document Format - 414.02 kB - 04/24/2026 at 19:48

Preview

spacers.stl

3D-printable spacers for spacing PCBs and aligning the solar panel.

Standard Tesselated Geometry - 941.63 kB - 04/24/2026 at 19:43

Download

  • Unexpected Quiescent Current Increase Below 2.7 V

    Nelectra • 04/29/2026 at 12:42 • 0 comments

    During a multi-day “no sunlight” simulation, the test firmware was configured to:

    • communicate every 10 minutes
    • switch a 9 V DC latching Rain Bird solenoid valve every 30 minutes

    While analyzing the discharge curve, an unexpected behavior appeared once the supercapacitor voltage dropped below approximately 2.7 V. As shown in the chart, the voltage decay suddenly accelerated after crossing this threshold.

    After investigating the circuit, the issue was traced to the DRV8231A H-bridge IC. Following an output switching event at low supply voltage, the IC’s quiescent current increased from nearly 0 µA to more than 250 µA.

    The system uses a boost DC/DC converter for actuator drive, but below 2.7 V the converter can no longer regulate or boost the voltage. This places the DRV8231A outside its recommended operating range of 4.5 V to 33 V.

    Interestingly, the main issue is not failed switching itself. Instead, after being disabled at low voltage, the H-bridge occasionally remains stuck in a state where it continuously draws nearly 300 µA.

    The condition can be cleared by performing another switching event at a supply voltage above approximately 2.8 V, after which the quiescent current returns to nearly 0 µA.

    At the moment, this behavior appears to be an internal issue or undocumented edge case of the IC. I plan to contact Texas Instruments for further clarification.

    For this application, a simple software workaround is available: actuator switching will be disabled once the supercapacitor voltage drops below 2.9 V. This protection will be added in the next firmware revision.

  • Day-Night Cycles

    Nelectra • 04/24/2026 at 21:29 • 0 comments

    A long-term measurement shows the typical charge and discharge behavior following day–night cycles. Even under cloudy conditions, the supercapacitors are charged close to their maximum voltage levels.

    In this example, the ESP32-C3 operates in continuous deep sleep mode. Between February 19 and February 20, snowfall occurred in Bratislava, covering the solar panel. As a result, the supercapacitors could not be recharged for three days.

    Despite this, the voltage did not drop below 4.1 V, demonstrating the system’s robustness under unfavorable environmental conditions.

  • Maximum Runtime Without Sunlight

    Nelectra • 04/24/2026 at 21:20 • 0 comments

    In a continuous deep-sleep scenario, the system can operate for several days without sunlight.

    In this first experiment, a simplified firmware was used to keep the system in permanent deep sleep. The fully charged supercapacitors powered the ESP32-C3 through a DC/DC converter, operating down to an input voltage of approximately 1.5 V. Below this threshold, the converter disables output regulation and the ESP32-C3 can no longer maintain a stable supply.

    Under these conditions, total runtime reached nearly 6 days.

    This test demonstrates the feasibility of multi-day autonomous operation using only harvested solar energy stored in supercapacitors, without any battery buffer.

    For data logging, a custom-built hardware device was used to measure the supercapacitor voltage. The data samples were stored and visualized using the Home Assistant environment.

  • Real-World Test – First Prototype

    Nelectra • 04/24/2026 at 21:03 • 0 comments

    In the first real-world test, the system was connected to a 9 V DC latching solenoid valve (Rain Bird) and a DHT11 temperature and humidity sensor.

    The node communicated with a receiver (XIAO ESP32-C3) every 10 minutes. In addition, the solenoid was actuated in both directions every 30 minutes, with each switching event lasting approximately 1 second.

    This setup was used to evaluate the energy of real actuator operation under typical outdoor IoT conditions.

    The measurements show that each solenoid actuation caused the supercapacitor voltage to drop by approximately 11 mV, corresponding to an energy consumption of about 550 mJ at ~4.5 V.

    These results confirm that even high-power mechanical switching can be supported within a supercapacitor-based, battery-free energy budget when duty cycles are properly managed.

    This makes it suitable for applications such as remote irrigation systems where latching valves are used to minimize energy consumption.

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Discussions

neilh wrote 05/20/2026 at 17:55 • point

Wow great project - I've done some similar design

Thanks so much for sharing all the details of long term running and the real-world problems of low-voltage cutoff.

Having done something similar and also written software to manage the power - I would suggest that more detail be given on the energy stored in the S-CAP.  What is the power stored between max voltage defined by the Overvoltage circuit (with device tolerance % range) and the minimium voltage measurement - defined by component tolerance range. I used 0.1% R, though the C3 has a Vref tolerance of +/-9% uncalibrated. So a big difference between proof of concept and then can it be manufactured in qty and guaranteed to work with specified device ranges. 

  Are you sure? yes | no

Nelectra wrote 05/21/2026 at 12:31 • point

Thank you for your comments and for sharing your own experience.

You are absolutely right that there is a significant difference between a proof-of-concept and a design that can be manufactured in quantity with predictable tolerances.

The ADC1 input of the ESP32-C3 indeed has a Vref tolerance of about +/-9% when uncalibrated. Using the factory calibration data improves the typical accuracy significantly, usually into the +/-1% to +/-2% range. For a conservative production estimate over a larger quantity of devices, I would expect something around +/-3% to +/-4% total tolerance for nearly all units.

In my experience, however, the real-world variations in harvested solar energy (sun angle, clouds, fog, snow, dirt on the panel, seasonal changes, etc.) have a much larger impact on system reliability than the remaining ADC uncertainty.

From long-term measurements I learned that the energy storage should have a large safety margin anyway — typically around 200% to 300% of the “expected” energy requirement. Some applications tolerate temporary energy shortages quite well, while others are much more critical.

Because of that, there is limited practical value in measuring the supercapacitor voltage extremely precisely. Whether the node survives 25 hours or 30 hours without sunlight is usually less important than designing the system to survive perhaps 60 hours under poor real-world conditions.

Regarding the stored energy in the supercapacitors:

Assuming:

Supercapacitor bank: 11 F

Fully charged state: approximately 5.25 V

Minimum voltage for actuator operation: 2.9 V

the usable energy is:

E = 0.5 * C * (V_high² - V_low²)

So for actuator operation the usable stored energy is approximately 105 J.

For communication-only operation, the system can discharge further down to about 1.5 V, so the usable stored energy is approximately 139J. 

Depending on the operational mode, the usable energy is roughly between 105 J and 139 J.

This energy storage can also be extended by connecting additional supercapacitors to the screw terminals on the PCB.

  Are you sure? yes | no

neilh wrote 05/21/2026 at 14:10 • point

Yes you've got the engineering challenges - mapping  energy demand to energy available makes for some interesting algorithm's that can be implemented in software. Nice practical work on what the data sheets say about the voltage cutoff for specific devices.!  

For climate conditions, I think of  not getting any solar for 3 weeks when a storm system blows in during winter with low solar harvesting capability.!! I've looked at having a SCap as the short term - 12-36 hour energy available solar joules and then a Li battery as the longer term 3 weeks provider. The LiIon charger ICs are supporting that approach quite nicely. Hopefully the LiFePO4 chargers will have same functionality

Many thanks for sharing all your insights :)

  Are you sure? yes | no

Jesse Farrell wrote 04/25/2026 at 03:04 • point

Haha, we made the same project! What are the chances... and I even called mine niche.

  Are you sure? yes | no

Nelectra wrote 04/29/2026 at 14:26 • point

😄

I noticed you went with a more sophisticated harvesting approach using MPPT, which is really interesting. My main focus was a bit different: avoiding batteries completely and adding wireless communication/control while still being able to drive higher-power actuators from stored solar energy.

  Are you sure? yes | no

Jesse Farrell wrote 04/30/2026 at 15:26 • point

Totally fair. I actually looked into using supercaps instead of batteries (also LICs those things are cool). For my use case though I needed it to reliably last 2-3 months. The MPPT controller I used caused much chaos (startup issues), so I somewhat regret it... or atleast the TI one I selected.

  Are you sure? yes | no

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