4S 40A Battery Management Systems (BMS)

BMS For Four Battery Strings And 40A Max Current

BMS often ship in locked state: no output voltage is available at the output pins. Locked state is also entered whenever over-current protection was triggered. To unlock the BMS, connect it to a charger. If you did not add a dedicated charger board, apply the appropriate charging voltage to its output terminal.

This BMS is designed for high currents. Use copper 3mm2 copper wires to connect the batteries.

When designing battery packs, use batteries of same type and same state of charge only. It is recommended you fully charge all batteries before connecting. All batteries must have the same voltage (voltage difference less than 0.05V). Do not mix batteries from different vendors, types, capacity, or age.


LiIon LiPo

For currents up to 40A (i.e. for power tools like drills with a high initial current), here is a rugged and balanced BMS:

There are two very similar breakout board versions available. One shows the marking 4S 40A between the terminals 12.6V and 16.8V. The other one shows the marking on the left side between the connections 12.6V and 8.4V. They are called Enhance and Balance interchangeably, and it is not clear what the true differences are.


Connect the batteries like this:

  • First battery string to 0V (-) and 4.2V (+)
  • Second battery string to 4.2V (-) and 8.4V (+)
  • Third battery string to 8.4V (-) and 12.6V (+)
  • Forth battery string to 12.6V (+) and 16.8V (+)

The output voltage is available at the terminals marked + and - at the port labelled Discharge.

All terminals are also available at the bottom:


Protection Threshold
Continuous current 40A
Over-Charge >4.25V, recovery at <4.1V
Over-Discharge <2.5V (0.1C),<3V (@1C),<3.5V(@>2C)
Over-Current 80A (150ms delay)
Short Circuit yes,resettable
Continuous Current 20A
Size 45x55x3.4mm


Charging voltage is 16.8-18.1V. The BMS limits the charging current to 20A.


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(content created Mar 28, 2024)