3S 20A BMS 12.6V Lithium Battery Charger Protection Board

A 3S 20A BMS protection board for building 12V lithium-ion battery packs from three cells in series (3S). It protects each cell against overcharge and over-discharge, cuts the output on overcurrent or short circuit, and resets automatically once the fault is cleared. It suits 18650, 21700, 26650 and lithium polymer cells with a 3.6 to 3.7V nominal voltage (4.2V fully charged).

The board handles 10A continuously as standard, and up to 20A with good cooling. Overcurrent protection trips at 60A to 80A depending on the board version, which lets it start small DC motors and power tools without nuisance cutoffs.

Specifications

• Configuration: 3S (3 cells in series)
• Pack voltage: 11.1V nominal, 12.6V fully charged
• Cell chemistry: Li-ion and LiPo (3.6V/3.7V nominal). Not for LiFePO4
• Continuous discharge: 10A standard, 20A maximum with adequate heat dissipation
• Overcurrent protection: 60A (standard version) or 80A (enhanced version)
• Overcharge protection: 4.25V per cell (release 4.12V)
• Over-discharge protection: 2.5V per cell (release 3.0V)
• Short circuit protection: Yes, with auto recovery
• Internal resistance: 12mΩ typical
• Standby current: 18µA typical
• Operating temperature: -40°C to +85°C
• Dimensions: 59 x 20 x 3.4mm
• Weight: 4.3g

How to Wire It

The board connects to the pack at four points: 0V (pack negative), 4.2V (between cells 1 and 2), 8.4V (between cells 2 and 3) and 12.6V (pack positive). Your load and charger connect between pack positive and the board's output negative pad. Check the silkscreen on the board for the exact pad labels.

• Check that all three cells are at the same voltage (within about 0.05V) before assembly
• Connect the sense wires in order, starting from 0V and working up to 12.6V
• Use wire no smaller than 2.5mm² for the main negative and positive connections
• Charge with a 3S lithium-ion charger rated at 12.6V

Wiring the sense points out of order is the most common cause of a damaged board, so double check each connection before you connect the last cell.

Choosing Cells

The board can pass 20A, but your cells have to supply it too. For a pack running near 20A, use three high-drain cells rated at 10C to 20C, or six 5C to 10C cells arranged as 3S2P (two cells in parallel per group). Use matched cells of the same brand, capacity and age, and never mix used cells with new ones.

Troubleshooting

• No output after first assembly: connect the charger briefly to activate the board
• Won't charge: one cell has probably reached the 4.25V overcharge limit. Measure each cell; if one is much higher than the others, the cells are unmatched
• Won't discharge or cuts out: a cell has dropped below 2.5V, or the load's start current is above the board's overcurrent limit. Recharge, or use higher-drain cells or a larger BMS
• Gets hot under load: reduce the load current or improve airflow and cooling

Applications

• Custom 12V lithium-ion battery packs
• Cordless drills and small power tools (start current under 60A)
• LED lighting and backup power supplies
• Solar light and small energy storage packs
• Robotics and portable projects

Need a Different BMS?

• Higher current or motor loads: 3S 60A BMS Enhanced
• Cell balancing: 3S 60A BMS Balanced
• Smaller 2-cell packs: 2S 5A BMS
• Single cell projects: 1S 3A BMS or the TP4056 USB-C charger

What's Included

• 1x 3S 20A BMS protection board