Why Build Your Own Power Bank?

Commercial power banks are convenient, but building your own teaches you hands-on lessons about battery management, boost conversion, and USB power delivery. You also get full control over capacity, cell quality, and form factor. This project is suitable for anyone comfortable with basic soldering and electrical safety.

What You'll Need

  • 18650 Li-Ion cells (2 cells for ~5,000–7,000 mAh depending on cell rating)
  • 18650 battery holder (2-cell, in parallel configuration)
  • TP4056 charging module with built-in over-discharge protection
  • MT3608 or similar boost converter module (steps up 3.7 V to 5 V USB output)
  • USB-A female connector (or a pre-wired USB breakout board)
  • Micro-USB or USB-C female connector for charging input
  • Toggle switch (optional, for on/off control)
  • LED indicator + 100Ω resistor (optional charge indicator)
  • Soldering iron, solder, wire, heat shrink tubing
  • Multimeter for testing

Step 1: Understand the Circuit Flow

The power path works like this:

  1. Charging input (USB) → TP4056 module → charges the 18650 cells
  2. Battery output → Boost converter → 5V USB output

The TP4056 with protection handles safe charging and prevents the cells from going below ~2.5 V. The boost converter raises the cell voltage (3.0–4.2 V) up to a steady 5 V for USB devices.

Step 2: Wire the Battery Cells in Parallel

Place both 18650 cells in the holder. In a parallel configuration, positive to positive and negative to negative. This keeps the voltage at 3.7 V nominal but doubles the capacity. Label your wires and double-check polarity with a multimeter before proceeding — reversed polarity can damage your modules instantly.

Step 3: Connect the TP4056 Module

The TP4056 module has clearly labeled pads:

  • IN+ / IN−: Connect to your charging USB input
  • BAT+ / BAT−: Connect to the battery pack
  • OUT+ / OUT−: This is the protected battery output — connect this to your boost converter input

Use the OUT terminals (not BAT) for downstream circuits. The OUT terminals are protected against over-discharge and short circuits.

Step 4: Set Up the Boost Converter

Connect the boost converter's input to the TP4056's OUT terminals. Before connecting anything else, power the converter and use a multimeter to adjust the output trimmer potentiometer until you read exactly 5.0–5.1 V on the output. This is critical — too high a voltage can damage connected devices.

Step 5: Wire the USB Output

Connect the boost converter's output to your USB-A socket:

  • VBUS (pin 1) → Boost converter OUT+
  • GND (pin 4) → Boost converter OUT−
  • For basic charging compatibility, short D+ and D− together or use a dedicated charging data resistor network

Step 6: Test Before Enclosing

Before putting everything in a case, test the full system:

  1. Plug in a charging cable — the TP4056 LED should indicate charging (usually red).
  2. Connect a phone or USB device — it should begin charging.
  3. Measure output voltage under load — it should stay close to 5 V.

Safety Reminders

  • Only use cells from reputable suppliers — avoid no-brand cells with inflated capacity claims.
  • Never short-circuit lithium cells, even momentarily.
  • Store partially charged (around 50%) if not using for an extended period.
  • Always use heat shrink on exposed solder joints inside the enclosure.

This project is an excellent weekend build that gives you real insight into how commercial power banks work — and the satisfaction of charging your phone from something you built yourself.