Description
The 4S 40A 18650 Li-ion Battery BMS Balance Protection Board is designed for 4-cell series (4S) lithium-ion and lithium-polymer battery packs. It provides battery protection, cell balancing, and current management for compatible high-drain battery applications.
A 4S Li-ion battery pack has a nominal voltage of approximately 14.4V–14.8V and reaches 16.8V when fully charged. This BMS is designed to help protect the battery pack during charging and discharging while keeping the four series-connected cell groups more evenly balanced.
With a rated continuous discharge current of up to 40A, this board can be used for suitable high-current applications such as cordless drills, power tools, sprayers, LED lighting, and DIY lithium battery packs. Proper cell selection and heat dissipation are essential when operating at high current.
Key Features
- Designed for 4S Li-ion / LiPo battery packs
- Suitable for compatible 18650 lithium-ion cells
- 14.4V–14.8V nominal pack voltage
- 16.8V maximum charging voltage
- Up to 40A continuous discharge current
- Up to 20A charging current
- Approximately 100mA balancing current
- Cell balancing function for the four series-connected cell groups
- Overcharge protection
- Over-discharge protection
- Over-current protection
- Short-circuit protection
- Automatic recovery from certain protection conditions
- Compact PCB design
- Suitable for high-drain battery applications
4S Battery Configuration
A 4S battery pack consists of four compatible Li-ion cells or cell groups connected in series.
Typical voltage values are:
- Per-cell nominal voltage: 3.6V–3.7V
- 4S nominal voltage: 14.4V–14.8V
- Fully charged voltage: 16.8V
The BMS monitors the individual series cell groups and provides protection and balancing according to the board’s design.
Balance Charging Function
This is the balanced version of the 4S BMS. Its balancing circuit is designed to help reduce voltage differences between the four series-connected battery groups during charging.
The listed balancing current is approximately 100mA. Cell balancing can help maintain more consistent cell-group voltages and support more reliable operation of a multi-cell battery pack.
Protection Functions
The BMS provides several protection functions for the connected battery pack:
Overcharge Protection
The listed overcharge detection voltage is approximately 4.25V ± 0.025V per cell.
Over-Discharge Protection
The listed over-discharge detection voltage is approximately 2.50V ± 0.08V per cell.
Over-Current Protection
The board is designed to provide protection against excessive current conditions.
Short-Circuit Protection
The BMS provides short-circuit protection and can automatically recover under suitable conditions after the fault is removed.
Current Capability
The board is listed with a 40A maximum continuous discharge current and 20A maximum charging current.
Actual current capability depends on several factors, including:
- Battery cell discharge rating
- Battery pack construction
- Wiring and connectors
- Load characteristics
- PCB temperature
- Heat dissipation
- Operating environment
For sustained high-current operation, adequate cooling or a suitable heatsink may be required.
Suitable Applications
This 4S 40A BMS can be used in:
- 4S 18650 Li-ion battery packs
- Cordless power tools
- Electric drills
- Battery-powered sprayers
- LED lighting systems
- DIY lithium battery packs
- Portable power projects
- High-drain electronic equipment
- Custom rechargeable battery systems
For power tools and other high-starting-current loads, use high-drain cells that can safely supply the required current.
Wiring Guide
Correct wiring is essential when installing a 4S BMS. Based on the listed terminal configuration, the connections are:
- B- / 0V: Connect to the negative terminal of the battery pack.
- B1: Connect to the positive terminal of the first cell/group.
- B2: Connect to the positive terminal of the second cell/group.
- B3: Connect to the positive terminal of the third cell/group.
- B+: Connect to the positive terminal of the fourth cell/group.
- P+ / P-: Connect to the compatible charger and load when using the common-port configuration shown for this board.
Important: Always follow the wiring diagram and terminal labels printed on the actual PCB. Incorrect BMS wiring can damage the board or battery cells.
Important Installation & Safety Notes
- Use four compatible Li-ion cells or cell groups with suitable specifications.
- Ideally use cells with similar capacity, age, condition, and voltage before assembling the pack.
- Do not mix unsuitable or damaged cells.
- Use a suitable 16.8V Li-ion charger for a 4S battery pack.
- Do not rely on the BMS as a substitute for the correct charger.
- For continuous high-current operation, provide adequate heat dissipation.
- Make sure the wiring and connectors are rated for the expected current.
- Verify every BMS connection before applying power.
Specifications
| Specification | Details |
|---|---|
| Product Type | 4S Li-ion BMS Balance Protection Board |
| Battery Chemistry | Li-ion / Lithium Polymer |
| Battery Configuration | 4S |
| Compatible Cells | 18650 and compatible Li-ion cells |
| Cell Nominal Voltage | 3.6V / 3.7V |
| Pack Nominal Voltage | 14.4V–14.8V |
| Maximum Charging Voltage | 16.8V |
| Continuous Discharge Current | Up to 40A |
| Continuous Charge Current | Up to 20A |
| Balance Current | Approx. 100mA |
| Overcharge Detection | 4.25V ± 0.025V per cell |
| Over-Discharge Detection | 2.50V ± 0.08V per cell |
| Protection | Overcharge, over-discharge, over-current and short circuit |
| Operating Temperature | -40°C to +85°C |
| Dimensions | Approx. 46 × 60 × 3.4mm |
| Application | High-drain Li-ion battery packs |
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