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3S 4A Lithium-Ion Battery BMS – 18650 Charger Protection Circuit Board with Balance Function

SKU: FA2193-07-0
Compatible Battery

3S 11.1V lithium-ion (18650 cells)
Full Charge Voltage

12.6V
Continuous Discharge Current

4A
Overcharge Protection Voltage

~4.25V – 4.3V per cell
Over-discharge Protection Voltage

~2.5V – 3.0V per cell
Balance Function

Yes
Connection Type

Solder pads (spot welding supported)
Module Dimensions

Compact PCB for 18650 packs

The 3S 4A Lithium-Ion Battery BMS is a compact, essential safety module designed to protect 3-cell (3S) lithium-ion battery packs — specifically those using 18650 cells — from overcharge, over-discharge, and overcurrent conditions. It combines protection circuitry with a balance function and 4A current handling, making it an indispensable component for DIY battery packs, power banks, portable electronics, and energy storage systems.

At the heart of the board is a BMS (Battery Management System) protection circuit that continuously monitors the voltage of each cell and the total pack current, disconnecting the load or charger if any parameter exceeds safe limits. This protects both the battery pack and your connected devices from damage, extending battery lifespan and ensuring safe operation.

The board provides three critical protection functions:

  • Overcharge protection — Disconnects the charger when any cell voltage reaches approximately 4.25V to 4.3V, preventing damage from excessive charging.

  • Over-discharge protection — Disconnects the load when any cell voltage drops to approximately 2.5V to 3.0V, preventing deep discharge that can permanently damage the battery.

  • Overcurrent protection — Disconnects the output when current exceeds 4A, protecting against short circuits and excessive loads.

The balance function is a key feature of this BMS. During charging, the board monitors the voltage of each cell and bleeds off excess charge from cells that reach full voltage before others. This ensures all cells in the pack reach the same voltage, maximizing pack capacity and lifespan. Without proper balancing, one cell can become overcharged or over-discharged, leading to premature failure or safety hazards.

The board features a solder pad design that supports both direct soldering and spot welding. The exposed pads allow you to solder wires directly to the board, while also being compatible with spot-welded nickel strips for professional battery pack assembly. The board includes B+, B-, BM1, BM2 (mid-points), P+, and P- pads for battery and output connections.

The 18650 compatibility makes this BMS ideal for building custom battery packs using the most popular lithium-ion cell format. 18650 cells are widely available, reliable, and offer excellent energy density, making them the preferred choice for DIY power banks, portable speakers, RC vehicles, and e-bikes.

The 3S 4A BMS is widely used in DIY power banks, 18650 battery packs, portable speakers, RC vehicles, solar garden lights, energy storage systems, and any application where 3S lithium battery safety and balancing are critical.

Important Note: This BMS is designed for 3S (3-cell) 11.1V/12.6V lithium battery packs using 18650 cells, with a maximum continuous current of 4A. For higher-current applications, a BMS with a higher current rating should be used. Always verify the voltage and current ratings match your application before installation.

Whether you are building a custom 3S 18650 battery pack, assembling a portable power bank, or adding safety protection to an energy storage system, the 3S 4A Lithium-Ion Battery BMS provides a reliable, cost-effective solution. It is suitable for individual makers, students, and businesses purchasing in bulk for product development or educational programs.

Features

  • 3S triple-cell protection — Manages three 18650 cells in series

  • 4A current handling — Suitable for moderate-power applications

  • Balance function — Ensures even charge across all three cells

  • Comprehensive protection — Overcharge, over-discharge, and overcurrent protection

  • 18650 compatibility — Designed for the most popular lithium-ion cell format

  • Solder pad design — Supports both direct soldering and spot welding

  • Compact PCB design — Sized for space-constrained battery packs

  • Wide application range — Power banks, RC vehicles, portable speakers, energy storage

Technical Specifications 

# Parameter Specification
1 Compatible Battery 3S 11.1V lithium-ion (18650 cells)
2 Full Charge Voltage 12.6V
3 Continuous Discharge Current 4A
4 Overcharge Protection Voltage ~4.25V – 4.3V per cell
5 Over-discharge Protection Voltage ~2.5V – 3.0V per cell
6 Balance Function Yes
7 Connection Type Solder pads (spot welding supported)
8 Module Dimensions Compact PCB for 18650 packs

Usage

Getting Started

  1. Identify the battery pack terminals — Locate the positive (+), negative (-), and mid-point (BM1, BM2) terminals of your 3S 18650 battery pack.

  2. Connect the battery — Solder or spot weld the B+ pad to the pack positive terminal, B- pad to the pack negative terminal, BM1 to the first mid-point, and BM2 to the second mid-point.

  3. Connect the output — Solder your load or output wires to the P+ and P- pads on the protection board.

  4. Test the protection — Verify that the output voltage is present when the pack is within safe limits, and that the output disconnects when any cell voltage drops too low or rises too high.

  5. Charge the pack — Use an appropriate 3S lithium battery charger. The BMS balance function will ensure all cells reach full voltage evenly.

Protection Thresholds (Typical)

Parameter Threshold Action
Overcharge ~4.25V – 4.3V per cell Disconnect charger
Over-discharge ~2.5V – 3.0V per cell Disconnect load
Overcurrent 4A Disconnect output

Connection Points

Pad Function
B+ Battery pack positive terminal
B- Battery pack negative terminal
BM1 First mid-point between cells
BM2 Second mid-point between cells
P+ Protected output positive
P- Protected output negative

Typical Applications

  • DIY power banks — Protect 3S 18650 packs in custom portable chargers

  • RC vehicles — Protect 3S batteries in remote-controlled models

  • Portable speakers — Protect batteries in audio devices

  • Solar garden lights — Protect 3S batteries in outdoor lighting

  • Energy storage systems — Protect 3S battery packs in solar storage

  • Portable electronics — Add safety protection to 3S battery devices

  • Educational projects — Teaching battery safety and protection principles

Notes for Business Buyers

  • 18650 optimized — Designed specifically for the most widely used lithium cell format.

  • Balance function included — Ensures even charge and discharge for longer pack life.

  • Essential safety component — Protects 3S battery packs and devices from damage.

  • Suitable for bulk purchasing — Ideal for battery pack assemblers, RC hobby shops, and educational institutions.

Q: What is the 3S 4A BMS used for?

It is a safety module that protects 3-cell 18650 lithium battery packs from overcharge, over-discharge, and overcurrent conditions, with a balance function for even charging.

Q: What batteries are compatible with this BMS?

It is designed for 3S (3-cell) 11.1V lithium-ion battery packs using 18650 cells, with a full charge voltage of 12.6V.

Q: What are the protection thresholds?

Overcharge protection activates at approximately 4.25V–4.3V per cell. Over-discharge protection activates at approximately 2.5V–3.0V per cell. Overcurrent protection activates at 4A.

Q: What is the balance function?

The balance function monitors each cell during charging and bleeds off excess charge from cells that reach full voltage before others, ensuring all cells reach the same voltage.

Q: What is the difference between B+/-, BM1/BM2, and P+/-?

B+ and B- connect to the pack positive and negative terminals. BM1 and BM2 connect to the mid-points between the three cells. P+ and P- provide the protected output to your load or charger.

Q: Can I spot weld this BMS to the battery?

Yes. The board supports both direct soldering and spot welding. The exposed pads are compatible with spot-welded nickel strips.

Q: What is the maximum continuous discharge current?

The BMS is rated for 4A continuous discharge. For higher-current applications, a BMS with a higher current rating should be used.

Q: Can I use this BMS with a 2S or 4S battery pack?

No. This BMS is specifically designed for 3S (3-cell) 11.1V battery packs. For other configurations, a different protection board or BMS is required.

Q: Does the BMS protect against short circuits?

Yes. The BMS includes overcurrent protection that disconnects the output if a short circuit or excessive current is detected.

Q: Can I purchase in bulk for a product development project?

Yes. This BMS is suitable for both individual users and business purchasing, including battery pack assemblers, RC hobby shops, and educational institutions.

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