Protected Dual-Output Motor Supply, a Raspberry Pi Pico project

This project builds a protected dual-output power supply for robotics and maker applications, delivering regulated power to both a Motor 2040 controller and a Raspberry Pi Pico 2 W. The system combines a matched pair of 18650 lithium-ion cells with a dedicated BMS protection board, USB-C balance charger, inline fusing, and a buck converter to safely step down voltage for the Pico while maintaining full power delivery to the motor controller.

The guide provides a complete wiring diagram, parts list, and step-by-step assembly instructions to safely integrate the battery pack, protection circuitry, and charging system. All power management happens in hardware, allowing the Pico to focus on motor control logic without managing power distribution itself. Testing procedures ensure safe operation before connecting any external boards or motors.

Wiring diagram

Gather all the parts

Qty	Component
1	2S 18650 battery holder with matched cells 2 × matched, unprotected 18650 Li-ion cells; 7.4 V nominal, 8.4 V full A holder that connects two matched rechargeable 18650 lithium-ion cells in series to make a 7.4 V nominal battery pack.
1	2S lithium-ion BMS protection board 2S, common-port, 10 A or greater continuous rating A 2-cell battery protection board that disconnects the pack during overcharge, over-discharge, short circuit, or excessive current.
1	USB-C 2S lithium-ion balance charger 2S / 8.4 V balance charger, USB-C input, 1–2 A charge rate A USB-C charger module that safely charges a two-cell lithium-ion pack to 8.4 V and balances the two cells.
1	Inline blade fuse holder 7.5 A automotive blade fuse A replaceable fuse holder that limits fault current to the motor-controller branch.
1	Inline blade fuse holder 2 A automotive blade fuse A replaceable fuse holder that limits fault current to the 5 V Pico supply branch.
1	24v Buck Converter Set output to 5.0 V; 3 A rated LM2596-based adjustable step-down buck converter module. Commonly used to regulate a higher battery rail, such as a 2S 18650 pack, down to 5V for Arduino logic. It is a regulator, not a charger or battery protection board.
1	Raspberry Pi Pico 2 W RP2350 wireless board A wireless Raspberry Pi Pico board that receives a regulated 5 V supply through its VSYS input.

Assemble it in 6 steps

1. Prepare the matched battery pair

Use two identical, unprotected 18650 lithium-ion cells of the same brand, capacity, and age. Charge them to the same voltage before putting them in the 2-cell holder. The holder creates one 7.4 V nominal battery pack: it reaches 8.4 V when full.

Label the two cells as a pair and keep them together for this project.
Do not mix old and new cells or different capacities; one cell can be overcharged or over-discharged even when the pack looks normal.
Never use damaged, swollen, or bare loose cells; a short circuit can make lithium-ion cells very hot or start a fire.

2. Wire the battery protection board

With both cells removed from the holder, connect the holder BAT- lead to B- on bms_2s, the middle tap marked MID to B1, and BAT+ to B+. These three wires must go to the matching labels in that exact order.

Use short, insulated wire sized for motor current, and check each label twice before soldering.
The BMS is the safety board that disconnects the pack if a cell is drained too far, overcharged, or shorted.
Do not reverse B+, B1, and B-; a wrong connection can permanently damage the protection board or battery cells.
Do not insert the cells until all three BMS battery wires have been checked with a meter.

3. Connect the USB-C charger

Connect usb_c_charger OUT+ to the BMS P+ terminal and charger OUT- to the BMS P- terminal. Plugging a USB-C power supply into the charger later charges the protected 2-cell pack through these two wires.

Use a USB-C wall adapter rated 5 V and at least 3 A.
The charger must specifically say it is for a 2S, 8.4 V lithium-ion pack and has cell balancing.
Do not use a common single-cell USB charger such as a TP4056; it cannot safely charge two cells connected in series.
Do not power the charger from a 9 V or 12 V adapter unless its printed input rating explicitly allows it; this design assumes normal 5 V USB-C input.

4. Make the Motor 2040 outlet

Connect BMS P+ to motor_fuse IN, then connect motor_fuse OUT to the Motor 2040 EXT PWR pad. Connect BMS P- directly to the Motor 2040 GND power-input pad. Fit a 7.5 A blade fuse in motor_fuse.

Keep these motor-power wires short and use heavier wire than the small signal wires.
The fuse protects the wiring if a motor lead is crushed or a motor stalls.
Do not connect this battery output to the Motor 2040 USB power input; use only EXT PWR and GND for the 2-cell battery.
Make sure the positive wire goes to EXT PWR and the negative wire goes to GND — swapped power can damage the Motor 2040.

5. Make the Pico 2 W outlet

Connect BMS P+ to pico_fuse IN. Connect pico_fuse OUT to pico_buck VIN+, and connect pico_buck VIN- to BMS P-. Before the Pico is attached, power the circuit and adjust pico_buck until a meter reads exactly 5.0 V between VOUT+ and VOUT-. Then connect VOUT+ to the Pico 2 W VSYS pin and VOUT- to a Pico 2 W GND pin.

Fit a 2 A blade fuse in pico_fuse.
The buck converter turns the changing 6.0–8.4 V battery voltage into the steady 5 V the Pico needs.
Do not connect the buck output to the Pico 3V3 pin; 5 V on that pin can damage the board.
Set and measure the converter output before connecting the Pico; an incorrectly adjusted converter can damage the Pico.

6. Test before inserting boards

Before attaching either board, insert the cells and use a meter. Measure roughly 6.0–8.4 V between the Motor 2040 outlet positive and ground, and exactly 5.0 V between the Pico outlet positive and ground. Then disconnect power, attach both boards, and reconnect power.

Test first with motors unplugged, then attach one motor at a time after the boards power normally.
If a fuse blows, a wire gets warm, or you smell hot plastic, disconnect the battery immediately and inspect for a short circuit.
Never leave lithium-ion charging unattended or charge the pack inside a sealed enclosure.

Review all connections

1. Connections between "battery_pack" and "Raspberry Pi Pico"

Function	battery_pack	Raspberry Pi Pico
`power`	BAT+ → 2S lithium-ion BMS protection board B+	EXT
`power`	MID → 2S lithium-ion BMS protection board B1	EXT
`ground`	BAT- → 2S lithium-ion BMS protection board B-	EXT

2. Connections between "bms_2s" and "Raspberry Pi Pico"

Function	bms_2s	Raspberry Pi Pico
`power`	P+ → Inline blade fuse holder IN	EXT
`ground`	P- → Pimoroni Motor 2040 GND power-input pad	EXT

3. Connections between "motor_fuse" and "Raspberry Pi Pico"

Function	motor_fuse	Raspberry Pi Pico
`power`	OUT → Pimoroni Motor 2040 EXT PWR pad	EXT

4. Connections between "pico_fuse" and "Raspberry Pi Pico"

Function	pico_fuse	Raspberry Pi Pico
`power`	OUT → 24v Buck Converter VIN+	EXT
`power`	IN → 2S lithium-ion BMS protection board P+	EXT

5. Connections between "pico_buck" and "Raspberry Pi Pico"

Function	pico_buck	Raspberry Pi Pico
`power`	VOUT+ → Raspberry Pi Pico 2 W VSYS	EXT
`ground`	VOUT- → Raspberry Pi Pico 2 W GND	EXT
`ground`	VIN- → 2S lithium-ion BMS protection board P-	EXT

6. Connections between "usb_c_charger" and "Raspberry Pi Pico"

Function	usb_c_charger	Raspberry Pi Pico
`power`	OUT+ → 2S lithium-ion BMS protection board P+	EXT
`ground`	OUT- → 2S lithium-ion BMS protection board P-	EXT
`power`	USB-C → USB-C wall adapter, 5 V / 3 A or greater	EXT

7. Connections between "pico2w_load" and "Raspberry Pi Pico"

Function	pico2w_load	Raspberry Pi Pico
`ground`	GND → 2S lithium-ion BMS protection board P-	EXT

Deploy the firmware

main.cppOpen in Schematik

#include <Arduino.h>
// Power-distribution project: the battery, BMS, charger, fuses, and buck converter
// operate in hardware. This sketch intentionally leaves all Motor 2040 outputs off.
void setup() {
}

void loop() {
  delay(1000);
}