Reviving Old Phones: How to Power a Smartphone Without Its Battery for DIY Projects

In an era of planned obsolescence, tech enthusiasts are finding innovative ways to repurpose outdated devices. One developer successfully modified an old Samsung Galaxy S5 to run without its original battery, transforming it into a dedicated controller for a 3D printer using a clever "fake battery" circuit.

The Challenge: Beyond the Built-in Battery

The journey began with a common problem for device tinkerers: an old Samsung Galaxy S5 with a worn-out battery that couldn't last through extended use. The author had a specific goal in mind – using the phone to run OctoPrint, a popular open-source software for controlling 3D printers.

As the author explains, "Using an old smartphone for OctoPrint is a perfect fit - it has USB OTG support to connect to the printer, WiFi to access the controls and upload models, and a camera to monitor the print progress." However, the Galaxy S5's kernel limitation prevented charging while connected to a USB device, making the original battery impractical for multi-hour printing sessions.

Understanding the Battery: More Than Just Positive and Negative

The first step in the project was to understand the battery's terminal functions. Unlike simple two-terminal batteries, modern smartphone packs often include additional connections for safety and features.

The author identified four terminals on the Samsung battery:
1. Positive (+): The main power output
2. Negative (-): The ground connection
3. Thermistor (T): A temperature sensor for safety monitoring
4. NFC Terminal: For near-field communication functionality

Through careful measurement, the author determined that the thermistor terminal has a resistance of approximately 2350 ohms at room temperature. This safety feature prevents the phone from starting if the thermistor connection is missing – a critical detail for building a compatible replacement.

Designing the "Fake Battery" Circuit

With the battery's functions understood, the next challenge was to create a circuit that could replicate its behavior. The requirements were clear:

  1. Provide a variable voltage between 3.4V and 4.4V across the positive and negative terminals
  2. Create a resistance of approximately 2350 ohms between the thermistor and negative terminals

The solution was elegantly simple, leveraging the phone's existing USB power connection:

  • A silicon diode was added between the 5V USB power supply and the positive terminal to drop the voltage by approximately 0.7V, bringing it within the acceptable range
  • A series combination of 2200Ω and 150Ω resistors created the required 2350Ω resistance for the thermistor connection

The schematic shows this elegant solution:


(+5V)
  |
  |          silicon diode
  +---------------|>|------(+)
  |
  |        +---[2350ohm]---(T)
  | 1000uF |
  +---||---+---------------(-)
           |
        (ground)

Power Management Challenges and Solutions

Initial testing revealed that the phone demanded significant power – more than typical USB chargers could provide. "Basic phone charger <1A USB power supply was not enough to even finish booting, but a ~2A was enough to boot and launch octo4a," the author noted.

To address these power fluctuations, several optimizations were implemented:

  1. Capacitance Boost: A 1000μF capacitor was added to smooth out power spikes during high-load operations
  2. Software Optimization: Battery Saver mode was activated with automatic triggering below 75% battery level to reduce processing demands
  3. Camera Management: The camera was either disabled or used at minimal settings (320x240 resolution at 5 FPS) to reduce system load

Physical Implementation and Results

The final step was creating a physical housing for the circuit. The author crafted a custom base from flooring material to fit precisely in the battery compartment. The circuit was built on a through-hole prototype board and secured with double-sided tape. The battery terminals were connected via bent pins that made contact with the compartment's springy terminals.

The result was a fully functional, battery-free smartphone controller for a 3D printer, housed in a 3D-printed holder. "This setup has been working fairly well for me for a few weeks now," the author reported.

Technical Implications and Broader Applications

This project demonstrates several important technical principles:

  1. Device Repurposing: Old smartphones can serve as dedicated IoT devices with minimal modification
  2. Power Management: Understanding voltage requirements and current draw is crucial for embedded projects
  3. Safety Circuit Emulation: Replicating safety features like thermistor monitoring is essential for device compatibility

Beyond 3D printing, this approach could be applied to other projects requiring a compact, touchscreen-enabled controller with WiFi connectivity. From home automation dashboards to retro gaming consoles, the possibilities are limited only by the maker's imagination.

Conclusion: Extending Device Lifespans Through Innovation

As electronic waste continues to be a global concern, projects like this offer a compelling alternative to disposal. By understanding the underlying hardware and creatively solving technical challenges, developers can breathe new life into devices that might otherwise be discarded.

The project's success hinges on a deep understanding of both the hardware requirements and software limitations – a testament to the value of technical curiosity and problem-solving in the maker community.

This article is based on a project originally published by yaky.dev.