1. Product Overview

This 4S 20A BMS is a battery-management and protection board designed for four LiFePO₄ cells or parallel cell groups connected in series.

A completed 4S LiFePO₄ battery pack has a nominal voltage of approximately 12.8V and reaches 14.6V when fully charged.

The BMS monitors all four series-connected cell groups and helps protect the battery against unsafe charging, excessive discharge, overcurrent, short-circuit and abnormal-temperature conditions.

The board includes cell-balancing circuitry to help control voltage differences between the four series groups. Eight CTD3080-marked power MOSFETs are used for charge and discharge switching.

2. Technical Specifications

The CTD3080 marking identifies the power MOSFETs fitted to the board. It is not the BMS model number or current rating.

Protection thresholds, balancing current, delay times and recovery behaviour should be confirmed from the supplier’s specification for the exact board.

3. Visible Connection Points

Confirm the exact load and charger connections using the supplier’s wiring diagram. Do not identify terminals only by their physical position.

4. Protection Functions

The BMS is intended to provide essential protection for compatible 4S LiFePO₄ battery packs, including:

• Monitoring of four series-connected cell groups
• Cell-overcharge protection
• Cell-over-discharge protection
• Discharge-overcurrent protection
• Short-circuit protection
• Charge and discharge switching through onboard MOSFETs
• Cell-voltage balancing
• Battery-temperature monitoring
• Recovery after supported protection events

A BMS protects a correctly designed battery pack but cannot repair damaged, mismatched, weak or deeply discharged cells.

5. Cell-Balancing Function

The onboard balancing circuitry helps reduce voltage differences between the four series-connected cell groups during charging.

Cell balancing may improve:

• Cell-group voltage consistency
• Battery utilisation
• Charging stability
• Long-term pack performance
• Protection against one group reaching the upper-voltage limit too early

Balancing current is normally much lower than the main charging current. All four cell groups should therefore be closely matched before assembling the battery.

6. Suitable Applications

This protection board may be suitable for compatible 4S LiFePO₄ battery packs used in:

• Solar-lighting systems
• Small inverter and UPS batteries
• Emergency backup systems
• Rechargeable LED lights
• Robotics and automation projects
• Portable electronic equipment
• Camping and outdoor power systems
• Security and monitoring equipment
• DIY 12.8V LiFePO₄ batteries
• Battery repair and replacement projects

The continuous operating current and starting current of the connected equipment must remain within the verified limits of the BMS and battery cells.

7. Battery and Charger Compatibility

Use this board only when:

• The battery contains exactly four series-connected LiFePO₄ cell groups.
• Each cell group has a nominal voltage of approximately 3.2V.
• The completed pack has a nominal voltage of approximately 12.8V.
• The charger is designed specifically for a 4S LiFePO₄ battery.
• The charger’s maximum output voltage does not exceed 14.6V.
• All four cell groups are at closely matched voltages before installation.
• The battery cells can safely supply the required current.
• The equipment’s continuous and peak current remain within the verified system limits.

Do not use this BMS with 4S lithium-ion or lithium-polymer cells rated at 3.6V or 3.7V nominal. Those chemistries require different protection thresholds and normally charge up to 16.8V.

8. Installation Guidance

• Measure all four cell-group voltages before installation.
• Confirm the correct connection sequence using the supplier’s wiring diagram.
• Connect B− to the battery-pack negative terminal.
• Connect B1 to the junction between cell groups 1 and 2.
• Connect B2 to the junction between cell groups 2 and 3.
• Connect B3 to the junction between cell groups 3 and 4.
• Connect B+ to the battery-pack positive terminal.
• Connect the charger and load through the specified external terminals.
• Verify the voltage between every consecutive battery connection before applying power.
• Secure the temperature sensor against the battery cells using electrically insulating tape.
• Use cables and connectors suitable for the required current.
• Install an appropriately rated fuse close to the battery’s positive terminal.
• Keep soldering time short to avoid overheating the PCB or battery cells.
• Insulate every exposed terminal and conductive connection.
• Mount the board where it cannot contact nickel strips, busbars or a metal enclosure.
• Provide adequate ventilation and heat dissipation.

Incorrect B1, B2 or B3 connections can permanently damage the BMS.

9. Important Current-Rating Note

The 20A BMS rating does not increase the current capability of the battery cells.

The safe current of the complete battery system is determined by the lowest-rated component, including:

• Battery cells
• BMS
• Nickel strips or busbars
• Power cables
• Connectors
• Fuse
• Charger
• Connected load

Loads such as motors and inverters may draw starting currents substantially higher than their normal operating current. Confirm both continuous and peak current requirements before selecting this BMS.

Adequate heat dissipation is necessary when operating near the BMS current limit.

10. Safety Information

• Use only with a compatible 4S LiFePO₄ battery pack.
• Never reverse battery polarity.
• Never short the battery or BMS terminals.
• Do not connect the battery sensing points in the wrong sequence.
• Verify every intermediate voltage before connecting the board.
• Use only a compatible 14.6V LiFePO₄ charger.
• Do not exceed the verified charge or discharge-current rating.
• Use appropriately rated wires, connectors and a suitable fuse.
• Insulate the PCB from the battery cells and conductive surfaces.
• Keep the board away from water, conductive dust and loose metal objects.
• Do not use a burnt, cracked, corroded or unusually hot board.
• Stop using the battery if any cell becomes swollen, overheated or abnormally unbalanced.
• Installation should be performed by someone familiar with lithium-battery wiring and safety.