Power Bank Build: Custom Capacity 18650 Power Pack

Why buy a power bank when you can build one with exactly the capacity you need? Using 18650 cells, a boost converter, and a charging module, you can create a custom power bank for a fraction of the retail price. This guide covers the complete build process, from cell selection to final assembly.

Designing Your Power Bank

First, decide on your specifications:

• Capacity: Each 18650 cell gives about 2000-3000mAh at 3.7V. For a 10,000mAh power bank, you need 3-5 cells in parallel (1S3P to 1S5P).
• Output: Standard 5V USB at 1A-2A for phones and tablets.
• Input: 5V USB for charging (Micro USB or Type-C).
• Size: Each 18650 is 18x65mm. Plan your enclosure accordingly.

Components List

• 3-4x 18650 cells (capacity matched within 5%)
• TP4056 charging module with protection (one per cell, or use a parallel config with one module)
• MT3608 or similar boost converter (3.7V to 5V, 2A capable)
• USB-A female connector for output
• On/Off switch
• Battery holders or nickel strip for connections
• LED charge indicator (optional)

Circuit Assembly

For a simple 1S3P (3 cells in parallel) power bank:

1. Connect three 18650 cells in parallel (all positives together, all negatives together). Use capacity-matched cells.
2. Connect the parallel bank to the TP4056 BAT+/BAT- pads.
3. Connect the TP4056 OUT+ to the boost converter input+.
4. Connect boost converter output to USB-A connector.
5. Add a switch between the boost converter and USB output.

Important: For parallel cells, you can use a single TP4056, but charging at 1A for a 6600-9000mAh bank is very slow. Consider using the dedicated power bank module instead.

Charging Circuit with TP4056

The TP4056 handles safe lithium-ion charging with automatic termination. For faster charging of larger parallel banks, consider using a 2A or 3A charger module instead of the standard 1A TP4056. The charging time for a 3-cell parallel bank at 1A is approximately 7-10 hours. At 2A, this reduces to 4-5 hours.

Boost Converter for 5V Output

The MT3608 boost converter steps up the 3.0V-4.2V battery voltage to a stable 5.1V for USB output. Adjust the potentiometer on the MT3608 to set exactly 5.1V (slightly above 5V to account for cable drop). For 2A output capability, consider the larger SX1308 or IP5306 dedicated power bank IC.

Enclosure and Safety

• Use an enclosure that protects cells from physical damage.
• Include ventilation holes for heat dissipation.
• Ensure no metal parts can short-circuit the cells.
• Use hot glue or Kapton tape to insulate connections.
• Add fuse wire (2A-3A) in the positive line as final protection.

Testing and Usage

Test your power bank by charging a phone and measuring the actual delivered capacity. A well-built 3-cell (1S3P, ~7500mAh) power bank should charge a 3000mAh phone about 1.5 times. Energy loss in the boost converter (typically 10-15%) and cable resistance account for the difference.