Product Description

3S 40A Balanced BMS Lithium Battery Protection Board

= High-Performance Battery Management System with Cell Balancing =

The 3S 40A Balanced BMS is a highperformance battery management system designed to protect and balance 3series lithiumion battery packs. Featuring 40A continuous discharge current, 100mA balancing current, and auto recovery protection, this board provides comprehensive protection against overcharge, overdischarge, overcurrent, and short circuits. Perfect for power tools, electric drills, e-bikes, and DIY battery projects.

Designed for 3.6V/3.7V Liion and LiPo battery cells (18650, 26650, polymer)

Supports 3S configuration with 12.6V full charge voltage

Not suitable for LiFePO4 batteries Liion/Li-Po only

= Technical Specifications =

Parameter	Value
Battery Configuration	3S (3 cells in series)
Supported Battery Types	Liion / LiPo (3.6-3.7V cells)
Full Charge Voltage	12.6V - 13.6V
Continuous Discharge Current	40A
Continuous Charge Current	Up to 20A
Overcurrent Protection	80A (starting current)
Balancing Current	~100mA (passive balancing)
Overcharge Detection	4.25V ± 0.025V per cell
Over-discharge Detection	2.5V ± 0.08V per cell
Short Circuit Protection	Yes (auto recovery)
Static Current	< 24uA
Board Dimensions	~41 x 55 x 3.4mm
Mounting	Through-hole pads

= Key Features =

High Discharge Capacity - 40A continuous discharge with 80A overcurrent protection for demanding applications

Cell Balancing - 100mA passive balancing ensures cells remain at equal voltage for extended battery life and performance

Comprehensive Protection Overcharge, overdischarge, overcurrent, and short circuit protection with auto recovery

Auto Recovery - After short circuit or overcurrent protection, automatically recovers when load is removed

Low Standby Current - Only 24uA static current minimizes battery drain during idle periods

Wide Application Range Supports power tools up to 170W, ebikes, LED lights, and low-power inverters

== Applications ==

Power tools (electric drills, screwdrivers, sprayers)
E-bikes and electric scooters
Portable power supplies and generators
LED lighting systems
Low-power inverters and solar systems
DIY battery pack builds
Power banks and backup systems
Industrial and automation equipment

= Wiring Guide =

Connection	Description
B+ (Battery Positive)	Positive terminal of cell 3
B1	Positive terminal of cell 1 (balance wire)
B2	Positive terminal of cell 2 (balance wire)
B3	Positive terminal of cell 3 (balance wire - also B+)
B- (Battery Negative)	Negative terminal of cell 1
P+ (Load/Charger Positive)	Positive output/charge input
P- (Load/Charger Negative)	Negative output/charge input

Connect in series: Cell 1: B- to B1, Cell 2: B1 to B2, Cell 3: B2 to B3

Use appropriate gauge wire for 40A current (minimum 3mm² copper wire)

Connect balance wires in strict order: B0 (B-) then B1 then B2 then B3 (B+)

= Wiring Sequence =

Mark black cable as B0 (B-), red cables sequentially as B1, B2, B3 (B+)
Solder B0 to battery pack total negative
Solder B1 to connection between cell 1 positive and cell 2 negative
Solder B2 to connection between cell 2 positive and cell 3 negative
Solder B3 to battery pack total positive (cell 3 positive)

After wiring, measure voltage between adjacent cables with a multimeter to confirm correct voltage collection

Voltage difference of each string should not exceed 1V before connecting BMS

Always ensure correct voltage is detected before plugging in the BMS

= How It Works =

The 3S 40A Balanced BMS protects your battery pack through multiple mechanisms:

1. **Overcharge Protection**

Detects when any cell exceeds 4.25-4.30V and disconnects charging

2. **Over-discharge Protection**

Disconnects the load when any cell drops below 2.5V to prevent cell damage

3. **Overcurrent Protection**

Monitors current flow and disconnects during short circuits or excessive loads

4. **Cell Balancing**

Passive balancing equalizes cell voltages during charging to maintain pack health

5. **Auto Recovery**

After a fault condition is resolved, the BMS automatically reconnects the circuit

The BMS uses passive balancing discharges highervoltage cells through "430" resistors (heating up to about 1.29W) to match lower-voltage cells

For optimal performance, use matched cells with similar capacity and internal resistance - "2 good battery + 1 poor battery = 3 poor batteries"

The board will not function if balance wires are connected incorrectly - follow wiring order carefully

= Installation Tips =

1. Cell Matching	Use cells from the same batch with similar voltage before assembly
2. Balance First	Connect all balance wires in correct order before connecting main leads
3. Awaken BMS	After wiring, connect a charger to activate the BMS before first use
4. Heat Management	For continuous high-current operation, ensure adequate cooling and airflow
5. Wire Gauge	Use minimum 3mm² copper wire for main power connections (do not use nickel strips)

Before assembly, ensure all cells have similar voltage (within 0.1V) for optimal balancing

Mount the BMS on a flat, non-conductive surface away from heat sources

For brushed motor loads, connect a nonpolar capacitor (25V+, 10100uF) to prevent reverse spikes from damaging the MOS tube

= Why Choose 3S 40A Balanced BMS? =

High 40A discharge capacity for demanding power tool applications

100mA passive balancing extends battery pack lifespan

Auto recovery after fault conditions reduces maintenance

Comprehensive protection ensures safe operation

Compact 41x55mm design fits standard battery pack configurations

Suitable for drills with starting current up to 80A and power below 170W

√ Compatible with all 3.6V/3.7V Liion and LiPo cells

√ Supports 18650, 26650, and polymer battery formats

√ Direct replacement for standard battery management systems

√ Easy installation with clear wiring labels

√ Proven reliability for DIY and professional use

= Safety Precautions =

Do not use with LiFePO4 batteries only Liion/LiPo (3.63.7V per cell)

Do not exceed 40A continuous discharge current

Ensure correct polarity and wiring order 0V, 4.2V, 8.4V, 12.6V

Keep away from moisture and flammable materials

Use only with appropriate charger (12.6-13.6V for 3S)

Do not short circuit output terminals

Ensure adequate ventilation for heat dissipation

The BMS should be mounted in a protected area away from physical damage

For applications exceeding 40A continuous, consider a higher-current BMS model

Using a brushed motor? Connect a nonpolar capacitor (25V+, 10100uF) across motor terminals to prevent reverse spikes from damaging the MOS tube

The 3S 40A Balanced BMS provides reliable battery protection and management with integrated cell balancing for your 3series Liion battery packs. With comprehensive protection features, passive balancing, and auto recovery, it is the ideal solution for power tools, e-bikes, portable devices, and DIY battery projects.