SPIKE Battery Charger

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

Parameter Value Input Voltage 5V DC Input Current 2A (recommended) Charging Current 1.2A Compatible Battery SPIKE™ Prime 7.4V Li-ion (45610) PCB Layers 2-layer Charging Efficiency 94%

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

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

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

3. FM4256 is more cost-effective and easier to source

Research findings reveal the 3-pin configuration:

1. Pin 1 (Left)   —— BAT+ (Battery Positive, 7.3-8.4V)

2. Pin 2 (Middle) —— NTC (Temperature Sensor, 10kΩ @ 25°C)(Seems)

3. 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.

Main Components

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

2. 3-Pin Connector: Connects to SPIKE battery

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

4. SPIKE Prime Hub Battery (45610)

5. Type: 2S Lithium-ion battery

6. Capacity: 2100mAh

7. Nominal Voltage: 7.3-7.4V

8. Charging Voltage: 8.3-8.4V

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

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

Blue LED blinking, red LED on → Standby mode

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

Charging complete → Blue LED on, red LED off

Remove battery

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

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

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

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

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

Please understand Li-ion battery charging safety