3S 12V Li-ion 18650 BMS Battery Protection Board – 25A

The 3S Li-ion Battery Management System (BMS) Protection Board is designed to protect a 3-cell series lithium-ion battery pack against overcharging, over-discharging, excessive current, and short circuits.

It is suitable for building rechargeable battery packs using 3.7V Li-ion cells such as 18650 batteries, producing a nominal pack voltage of approximately 11.1V and a fully charged voltage of 12.6V.

The compact protection board continuously monitors the battery pack and disconnects the load or charger when potentially unsafe electrical conditions are detected.

Technical Specifications

• Product Type: Li-ion Battery Protection Board / BMS
• Battery Configuration: 3S
• Compatible Cell Type: 3.7V Li-ion / 18650
• Number of Series Cells: 3
• Nominal Pack Voltage: 11.1V
• Fully Charged Pack Voltage: 12.6V
• Maximum Operating Current: 0–25A as Specified for This Board
• Maximum Transient Current: Approx. 34–40A
• Overcharge Detection Voltage: 4.25–4.35V ±0.05V per Cell
• Over-Discharge Detection Voltage: 2.3–3.0V ±0.05V per Cell
• Quiescent Current: <30µA
• Internal Resistance: <100mΩ
• Short-Circuit Protection: Yes
• Short-Circuit Recovery: Delayed Self-Recovery
• Operating Temperature: Approx. -40°C to +50°C
• Storage Temperature: Approx. -40°C to +80°C
• PCB Dimensions: Approx. 56 × 45 × 1.2mm
• Protection Functions: Overcharge, Over-Discharge, Overcurrent & Short Circuit

3S Lithium-Ion Battery Configuration

This BMS is designed for three 3.7V lithium-ion cells or cell groups connected in series.

The resulting battery pack has:

Nominal Cell Voltage: 3.7V

Nominal Pack Voltage: 3 × 3.7V = 11.1V

Maximum Cell Charge Voltage: 4.2V

Maximum Pack Charge Voltage: 3 × 4.2V = 12.6V

A suitable 12.6V lithium-ion charger designed for a 3S pack should be used.

Battery Protection Functions

The board provides several important protection functions for a 3S lithium-ion battery pack.

Overcharge Protection

If the voltage of an individual cell rises above the protection threshold during charging, the BMS disconnects the charging path to help protect the battery.

Over-Discharge Protection

If a cell voltage falls below the permitted level, the board disconnects the load to help prevent excessive battery discharge.

Overcurrent Protection

The BMS monitors load current and can disconnect the output when excessive current is detected.

Short-Circuit Protection

If the output is accidentally short-circuited, the protection circuit can disconnect the battery pack from the load.

Typical Battery Connections

The board uses connections for the individual voltage points of the three-cell battery pack:

• B- → Battery Pack Negative / 0V
• B1 → First Cell Junction
• B2 → Second Cell Junction
• B+ → Battery Pack Positive

For three nominal 3.7V cells connected in series, these points correspond approximately to:

• B- → 0V
• B1 → 3.7V
• B2 → 7.4V
• B+ → 11.1V Nominal / 12.6V Fully Charged

The exact pad arrangement should always be confirmed from the markings on the supplied PCB before soldering.

Cell Matching

For best performance and safer pack operation, the cells used to build the battery pack should be closely matched.

Use cells with similar:

• Chemistry
• Capacity
• Voltage
• Age
• Internal Resistance
• Charge / Discharge Characteristics

Avoid mixing new and heavily used cells or cells with significantly different capacities.

Before assembling the pack, the three cell groups should be brought to similar voltage levels.

Applications

Suitable for DIY 3S lithium-ion battery packs, 18650 battery assemblies, rechargeable electronics, portable equipment, backup power projects, LED systems, robotics electronics, embedded projects, battery-powered controllers, and custom power systems where the load characteristics are compatible with the board.

Important Note

Despite this board commonly being marketed as a “3S 30A BMS,” the specific Robot Pi Shop version lists a maximum operating current of 0–25A and a transient current of approximately 34–40A. Do not treat 30A as a guaranteed continuous operating current.

This board is designed for standard 3.7V Li-ion cells and should not be used with LiFePO₄ cells, which have different charge and protection voltage requirements.

The current Robot Pi Shop listing also specifically warns against using this version for electric drills, electric bicycles, children's ride-on vehicles, high-current motors, and similar high-surge loads.

Battery-pack construction can involve high short-circuit currents and fire risk. Verify cell polarity and every B-, B1, B2, and B+ connection before connecting the board. Do not intentionally short-circuit the output for testing.

A BMS provides battery protection but does not replace a proper 12.6V CC/CV lithium-ion charger.

Package Includes

1 × 3S Li-ion 18650 Battery Protection / BMS Board