2S 20A Battery Management Systems (BMS)

BMS For Two Battery Strings And 20A 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.

When connecting batteries to your BMS, make sure to use wires with sufficient diameter for the anticipated high currents.

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 20A, you need a bigger board with more MosFETs that share the load.


It is recommended to use a balanced board when you plan to charge your batteries through the BMS. Balanced boards are slightly larger and cost very little more. During charging, they make sure each battery is treated individually, so when one battery is fully charged while another one still lags behind, it can balance the charging and distribute charging power individually to individual cells.


Unbalanced boards are physically smaller and fine when you need a BMS for discharging only.

The unbalanced version of the particular board presented here goes by the name HW392 or X403 whereas the balanced version is often labelled HW391. These labels may or may not be printed on your boards.


All terminals are accessible both from top and bottom.

Wiring is similar in both versions. Here is the wiring for the balanced board:

This is what the backside of the unbalanced board looks like:


Connect the batteries like this:

  • First battery string to 0V (-) and 4.2V (+)
  • Second battery string to 4.2V (-) and 8.4V (+)

The output voltage is available at + and - located in the inside of the board.


Protection Threshold
Over-Charge >4.3V
Over-Discharge <2.3-2.5V
Over-Current 25A
Short Circuit yes,resettable, 200mS delay
Continuous Current 20A (may need heat sink)
Size 48x20x3.4mm


When charging batteries through this BMS, the charging voltage needs to be 8.4-9.0V. The BMS limits the charging current to 10A.


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(content created Apr 02, 2024)