Product Description

4S 40A Balance BMS Lithium Battery Protection Board

= Intelligent Battery Management System for 4Cell Liion Packs =

The 4S 40A Balance BMS is a highperformance battery management system designed to protect and balance 4series 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 portable power supplies .

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

Supports 4S configuration with 16.8V full charge voltage

Not suitable for LiFePO4 batteries Liion/Li-Po only

= Technical Specifications =

Parameter	Value
Battery Configuration	4S (4 cells in series)
Supported Battery Types	Liion / LiPo (3.6-3.7V cells)
Full Charge Voltage	16.8V - 18.1V
Continuous Discharge Current	40A
Continuous Charge Current	20A
Balancing Current	100mA
Overcharge Detection	4.28V ± 0.05V
Over-discharge Detection	2.5V ± 0.08V
Short Circuit Protection	Yes (auto recovery)
Board Dimensions	~60 x 45mm
Mounting	Through-hole pads

= Key Features =

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

Passive Cell Balancing - 100mA balance current 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 protection activates, disconnecting the load allows automatic restoration of output

Enhanced Design Features 10 lowresistance MOS tubes for improved current handling and reliability

Wide Application Range Supports electric drills, ebikes, power tools, 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 4
B1	Positive terminal of cell 1 (balance wire)
B2	Positive terminal of cell 2 (balance wire)
B3	Positive terminal of cell 3 (balance wire)
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, Cell 4: B3 to B+

Use appropriate gauge wire for 40A current (minimum 8-10 AWG)

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

= Wiring Sequence =

Mark black cable as B0 (B-), red cables sequentially as B1, B2, B3, B4 (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 connection between cell 3 positive and cell 4 negative
Solder B4 to battery pack total positive (cell 4 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 4S 40A Balance BMS protects your battery pack through multiple mechanisms:

1. **Overcharge Protection**

Detects when any cell exceeds 4.28V ± 0.05V and disconnects charging

2. **Over-discharge Protection**

Disconnects the load when any cell drops below 2.5V ± 0.08V to prevent cell damage

3. **Overcurrent Protection**

Monitors current flow and disconnects during short circuits or excessive loads

4. **Cell Balancing**

100mA balance current 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 resistors to match lower-voltage cells

For optimal performance, use matched cells with similar internal resistance and capacity

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 thick wires for main power connections to handle 40A current

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

Improper wiring can damage the BMS and connected batteries

= Why Choose 4S 40A Balance BMS? =

High 40A discharge capacity for demanding power tool applications

100mA passive balancing extends battery pack lifespan

Auto recovery after fault conditions reduces maintenance

Enhanced design with 10 MOS transistors for improved reliability

Comprehensive protection ensures safe operation

Compact 60x45mm design fits standard battery pack configurations

√ 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

√ Overcharge, over-discharge, overcurrent, and short circuit protection

√ 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

Keep away from moisture and flammable materials

Use only with appropriate charger (16.8-18.1V for 4S)

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

The 4S 40A Balance BMS provides reliable battery protection and management for your 4series 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.