4S 16.8V Lithium Battery Protection Board – BMS with Balance for 18650 Cells

The 4S-16.8V-BMS is a battery management system (BMS) protection board designed for 4-series lithium-ion battery packs, with a nominal voltage of 14.8V and a full-charge voltage of 16.8V. It is compatible with common 18650, 26650, and polymer lithium cells, and is widely used in power tools, portable devices, and custom battery packs where reliable cell protection is required. The board continuously monitors each cell and the pack as a whole, disconnecting the load or charger when any parameter exceeds safe limits.

The protection circuit provides overcharge protection, over-discharge protection, overcurrent protection, and short-circuit protection. The board also includes a cell balancing function (on balance-equipped versions) that equalizes the voltage across the four series cells during charging, helping to prevent a single cell from drifting out of balance and reducing long-term capacity loss. The balanced version typically activates at around 4.2V per cell and can handle a balancing current in the range of approximately 95–105mA. Overcharge detection is typically in the 4.25–4.35V range per cell, while over-discharge detection is typically in the 2.5–3.0V range per cell, depending on the specific model and manufacturer. Protection release (recovery) is automatic—the board restores normal operation once the fault condition is removed, without requiring a charger to reset it.

The 4S 16.8V lithium battery protection board is compact, typically measuring in the range of 45–65mm in length, 22–35mm in width, and 3–4mm in thickness, with a weight of roughly 4–10g depending on the version. Continuous discharge current varies by model, with common ratings of 5A, 8A, 10A, 20A, 30A, and 40A. The board uses soldering pads or nickel strip connection points for the battery terminals (B−, B1, B2, B3, B+, P−, P+), and it is a bare PCB module without a plastic enclosure. It is not a charger; it is a protection and balancing board that is used together with an external charger and the battery pack.