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USB-C Fast-Charging Power Bank with Replaceable 18650 Cells

Prototype of a USB-C power bank using replaceable 18650 cells with fast-charge/USB Power Delivery support.

Advanced Project — This is advanced because a real USB-C fast-charge power bank combines battery charging, power-path management, high-current boost conversion, and USB-C/PD negotiation, all of which require careful power and thermal design.
Assumptions:
  • Single-cell 18650 pack architecture (1S1P) rather than multi-cell series pack.
  • User wants both charging the battery from USB-C and fast-charge output to external devices over USB-C.
  • Prototype can use a dev board or controller module rather than a fully custom power-bank ASIC design.
  • Battery protection, cell holder, and enclosure are not included in the database parts and will need to be sourced separately.

Bill of Materials

Microcontroller
Top Pick ESP32-S3-DevKitC-1 Espressif Systems From our database
ESP32-S3-DevKitC-1 is the best overall prototype MCU here because it has native USB support, plenty of GPIO, and enough flexibility to supervise USB-C/PD control, battery monitoring, and user interface features without making the design complicated. Note: this MCU includes built-in WiFi and/or Bluetooth — no separate connectivity module needed.
Digikey $15.00 (483 in stock) Mouser $13.30 (1,570 in stock)
ESP32-DevKitC-32U Espressif From our database
Good prototype controller with built-in USB programming, plenty of GPIO for status LEDs, buttons, and I2C/SPI peripherals, and easy firmware development. WiFi/BLE are available if you later want battery telemetry or app connectivity, though they are not required for the power-bank core.
ESP32-C6-DEVKITC-1-N8 Espressif Systems From our database
Low-cost dev board with modern connectivity and enough processing headroom for a power-bank controller prototype. Good if you want a compact board and may later add wireless telemetry, but it is less directly convenient than the S3 for USB-centric work.
USB Type-C / Power Delivery Controller
Top Pick STUSB4500QTR STMicroelectronics From our database
STUSB4500QTR is the best fit for a prototype because it is simpler to integrate, supports standalone USB-C PD sink negotiation, and is easier to use than the more complex an alternative part family when you mainly need fast-charge input over USB-C.
Digikey $2.02 (10,709 in stock) Mouser $1.96 (9,098 in stock)
Battery Charger / Power-Path Management
Top Pick MCP73871-2CCI/ML Microchip Technology From our database
MCP73871-2CCI/ML is the best overall choice because it directly addresses the core power-bank requirement: charging a single 18650 cell while simultaneously powering the system load through integrated power-path management.
Digikey $2.41 (2,174 in stock) Mouser $2.41 (4,375 in stock)
MCP73871T-2CAI/ML Microchip From our database
Same MCP73871 family with autonomous power-path management, making it suitable for load-sharing between the battery and the system rail. Good choice if you want a proven single-cell charger/controller for a compact prototype.
Battery Fuel Gauge
Top Pick MAX17043G+T Analog Devices From our database
Top pick: MAX17043G+T (Analog Devices). Very compact fuel-gauge option for a single-cell pack, with simple serial interface integration. Good for a prototype where you want a straightforward state-of-charge estimate with minimal firmware overhead.
Boost Converter / 5V Output Stage
Top Pick TPS61040DBVR Texas Instruments From our database
Top pick: TPS61040DBVR (Texas Instruments). Simple boost converter that can generate higher voltage from a 1-cell battery, useful for a basic 5V rail. Less ideal than a buck-boost for a power bank because it cannot regulate as gracefully when battery voltage is near or above the target rail.
TPS61088 Texas Instruments
High-current boost converter commonly used in portable power designs when you need stronger 5V output capability from a single-cell battery. Good if your target is a more capable USB output stage and you are willing to design the surrounding power stage carefully.
Digikey $2.74 (7,718 in stock) Mouser $5.37 (670 in stock)

Compatibility Notes

  • The core design is a 1S 18650 system, so charger, fuel gauge, and boost stage must all support single-cell Li-Ion operation.
  • MCP73871-2CCI/ML and ESP32-DevKitC-32U are both ESP32-DevKitC-32U system parts, but the charger itself is not a USB-PD controller; STUSB4500QTR handles USB-C PD negotiation on the input side.
  • ESP32-DevKitC-32U is appropriate for a 1-cell battery to 5V rail, but you still need to verify peak output current against the USB fast-charge profile you want to advertise.
  • ESP32-S3-DevKitC-1 runs at 3.3V logic, so any external power-path or PD control signals must be checked for voltage compatibility and open-drain requirements.
  • If you want true USB-C fast-charge output as a source, the design becomes more complex than a simple 5V boost bank and may require additional source-side Type-C/PD circuitry beyond the parts listed here.

You'll Also Need

  • 18650 cell holder or spring contacts
  • Protected 18650 cells or a separate battery protection/BMS circuit
  • USB-C receptacle, cable, and ESD protection parts
  • Inductors, sense resistors, capacitors, and compensation components for the charger and boost converter
  • Status LEDs, pushbutton, and any enclosure/mechanical hardware
  • Thermal design and PCB layout for high-current paths
  • If you want USB-C PD output as a source, additional source-side PD controller circuitry may be needed beyond the listed parts
Estimated BOM Cost: $35-40 (based on live distributor pricing)
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Design Considerations

Power Path Architecture
A power bank should not simply charge the cell and separately boost the output without load sharing. The MCP73871-2CCI/ML is attractive because it can power the system while charging, which avoids brownouts when a load is plugged in during charging. For a prototype, keep the system rail and USB output rail clearly separated in the schematic and verify startup behavior with a nearly empty battery.
USB-C Fast-Charge Reality
USB-C fast charge can mean either higher-current 5V output or actual USB Power Delivery negotiation. If you only need 5V at higher current, the design is much simpler; if you want 9V/12V source profiles, you need source-side PD control and careful role management. The STUSB4500QTR is a good sink controller, but it does not by itself make the bank a full-featured PD source.
Battery Safety
Replaceable 18650 cells require a protection strategy, especially in a prototype that may see unknown cells from different vendors. Use protected cells or add a proper protection/BMS stage, and include reverse-polarity protection if the holder allows user replacement. Validate cutoff behavior at low voltage, overcurrent, and short-circuit conditions before connecting real phones or tablets.
Output Current and Thermal Limits
A single 18650 cell can deliver useful power, but sustained 5V high-current output will heat the boost converter, inductor, and battery. Expect efficiency losses and plan for thermal rise at 2A to 3A output; a compact enclosure can trap heat quickly. Measure inductor and switch temperatures at full load and derate the advertised output if needed.
State-of-Charge Accuracy
Voltage-only battery indication is poor for Li-Ion because the discharge curve is flat over much of the capacity. The ESP32-DevKitC-32U gives much better user feedback, but it still needs correct battery characterization and a stable sense path. Calibrate the gauge with the actual cell type you plan to use, and test across temperature and load conditions.
Prototype Validation
Test the design in stages: first charger-only, then boost-only, then combined power-path operation, and finally USB-C negotiation. Use electronic load tests to verify startup, transient response, and cutoff behavior. A lot of power-bank failures show up only when a phone repeatedly connects and disconnects, so include hot-plug and short-circuit tests early.

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