LEGO® SPIKE™ Battery Charger (FM4256)

Free Your Hub - Charge Batteries Independently

A DIY USB charger designed for LEGO® SPIKE™ Prime batteries, allowing you to charge robot batteries independently without tying up your SPIKE hub.

⚡ Features

• Direct USB Charging: Charge using any 5V/2A USB adapter

• Independent Operation: No need to connect to SPIKE hub

• Smart LED Indicators: Visual charging status display

• Safety Protection: Overcharge, overcurrent, short-circuit, and temperature protection

• DIY Friendly: Simple 2-layer PCB design, easy to manufacture

• Easy to Use: Uses FM4256 charging module with 3D-printed enclosure

📊 Specifications

🔬 Technical Background: Why FM4256?

The SPIKE Prime Hub internally uses the MPS MP2639A charging IC. After reverse engineering research, we selected FM4256 for this standalone charger because it has nearly identical electrical specifications:

Key Differences

• MP2639A has bidirectional operation (charge/discharge) → Not needed for standalone charger

• MP2639A has MID pin for cell balancing → SPIKE battery seems not use this feature

• FM4256 is more cost-effective and easier to source

About SPIKE Battery's 3 Pins

Research findings reveal the 3-pin configuration:

Pin 1 (Left)   —— BAT+ (Battery Positive, 7.3-8.4V)
Pin 2 (Middle) —— NTC (Temperature Sensor, 10kΩ @ 25°C)(Seems)
Pin 3 (Right)  —— BAT- (Battery Negative, GND)

• Pin 2 is an NTC thermistor, used for temperature protection during charging (0°C ~ 50°C)

• Not a cell balancing pin (MID), SPIKE battery doesn't expose cell midpoint

• FM4256 module may not have NTC input, but this doesn't affect basic charging functionality

• Battery has internal protection board, safe for normal indoor use

Conclusion

FM4256 provides all necessary charging functions and is ideal for this DIY project.

🛠️ Hardware Design

Main Components

• FM4256 Charging Module: 2S Li-ion battery USB boost charging IC

• 3-Pin Connector: Connects to SPIKE battery

• 3D-Printed Enclosure: Uses M3 screws for case

SPIKE Prime Hub Battery (45610)

• Type: 2S Lithium-ion battery

• Capacity: 2100mAh

• Nominal Voltage: 7.3-7.4V

• Charging Voltage: 8.3-8.4V

🚨 LED Status Indicators (FM4256 S22)

Note

• Some modules only show Red LED on first power-up

• LED behavior normalizes after inserting and removing battery once

• IC models S21 and S22 have different LED patterns; most are S22

📖 Usage Instructions

Preparation

1. Connect 5V/2A USB power adapter

2. Insert SPIKE Prime battery into connector

3. Ensure correct battery orientation (guided by 3D frame to prevent reverse connection)

Charging Process

1. Blue LED blinking, red LED on → Standby mode

2. After connecting battery → Blue LED off, red LED on → Charging

3. Charging complete → Blue LED on, red LED off

4. Remove battery

Important Notes

• Charging time: ~0.5-1.5 hours (depending on remaining battery capacity)

• Monitor during charging, do not leave unattended

• Remove battery when fully charged, do not leave on charger for extended periods

• Do not use continuously for more than 24 hours

⚠️ Safety Warnings

• For LEGO® SPIKE™ Prime / RI battery only

• Use 5V / 2A USB charger

• Do not reverse polarity or short terminals

• Charge under adult supervision

• Keep ventilated – do not cover

• Stop use if heat, smoke, or odor occurs

• Avoid charging in extreme temperatures (<5°C or >40°C)

• Safe for normal indoor use

⚠️ Disclaimer

• DIY / prototype – no testing or certification

• No BMS or balancing included

• Misuse may cause fire, explosion, or injury

• For educational/technical use under adult guidance

• User assumes all risks and responsibility for use

• Provided "AS IS" without warranty or liability

• Manufacturer not liable for any loss, damage or injury

📚 Resources & References

• FM4256 Datasheet

• SPIKE Prime Battery Specifications

• SPIKE Prime Hub Reverse Engineering

• Li-ion Battery Charging Best Practices

📝 Legal Notice

LEGO® is a trademark of the LEGO Group which does not sponsor, authorize or endorse this project.

📄 License | 授權協議

This project is open source and available under the BSD-3-Clause license.

🤝 Contributing | 貢獻

Contributions, issues, and feature requests are welcome!

⚠️ Use at your own risk - Please understand Li-ion battery charging safety