Lithium Ion Battery Management and Protection Module (BMS ) Teardown - Schematics, Parts List and Working

Published  May 9, 2022   11
4S 40A BMS Module

In this article we will be learning about the features and working of a 4s 40A Battery Management System (BMS), we will look at all the components and the circuitry of the module. I have done complete reverse engineering of this module to find out how it works so that I can show how the BMS works. We also have another article and video where we have tested the safety parameters of this BMS. The image below shows the battery pack which also has a voltmeter, load (bulb), and a female DC jack for the charger, you can read more about it here.

Battery Pack using BMS Module

This BMS comes in 3 variants, the standard version, the enhanced version, and the balanced version.

Different Versions of BMS Module

We will be looking at the Balanced version. The balanced version has 4 resistors that are capable of load balancing, this feature is not available in the other versions. The standard version and enhanced version are almost similar with just a difference of 1 passive component, these variants are not capable of actively balancing the cells, whereas the balanced version has circuitry for balancing the cells.

Protection Features of 4S 40A BMS Circuit Diagram

A BMS is essential for extending the service life of a battery and also for keeping the battery pack safe from any potential hazard. The protection features available in the 4s 40A Battery Management System are:

  • Cell Balancing
  • Overvoltage protection
  • Short circuit protection
  • Undervoltage protection

Circuit Diagram of BMS

The schematic of this BMS is designed using KiCAD. The complete explanation of the schematic is done later in the article.

4S 40Amp BMS Circuit Diagram

BMS Connection with the Battery Pack

The BMS module has a neat layout with markings for connecting the BMS with different points in the battery pack. The image below shows how we need to connect the cell with BMS.

Battery Pack

Markings on the BMS

Connection with the BMS


Negative Terminal Connection for the battery pack for charging and connecting the load.


Positive Terminal Connection for the battery pack  for charging and connecting the load


Negative terminal of the 1st  cell


Positive terminal of the 1st cell


Positive terminal of the 2nd cell


Positive terminal of the 3rd cell


Positive terminal of the 4th cell

The BMS acts like 4 separate modules for 4 separate cells and then these 4 modules are very smartly integrated together with transistors and passive components to make a complete BMS that is able to deliver current up to 40A and protect individual cell’s parameters.

Digging deeper into the BMS

The BMS has 2 ICs, DW01, and BB3A; some variants of this BMS may have the same ICs or similar ICs from different manufacturers. But all the ICs will have the same pinouts and functioning. I will be discussing the 2 ICs later. The figure below shows the parts of BMS responsible for different operations.

BMS Parts responsible for Different Operations

From the above image, it is clear that one IC is responsible for overvoltage, overcurrent, and short circuit protection and that IC is DW01-A, whereas another IC BB3A is responsible for the cell balancing.

DW01-A: Battery Protection IC 

DW01-A is a 1 cell Li-ion/ Polymer battery protection IC. It is responsible for all the protection features of the BMS. Each individual cell has 1 DW01-A connected which monitors the health of the particular cell. It comes in a 6 pins sot-23-6 package. You can refer to the IC’s datasheet to see the functional diagram and other data. It has an internal voltage divider circuit that is responsible for measuring the undervoltage and overvoltage of the cell. The short circuit and overcurrent are detected by the comparators which compare the voltage between CS pin input and VSS.

Electrical Characteristics of DW01-A

The working of any Integrated circuit depends on how it has been designed, which is given by the manufacturer, the electrical characteristics of DW01 is given in the table below:

DW01 IC Protection Parameters

Protection Circuit 

The protection circuit of this battery pack is shown here. Here, the Batt+ and S3 denote the positive and negative terminals of the cell respectively. The IC measures the voltage of the cell using an internal voltage divider circuit between VCC and ground pin and based on the electrical characteristics table shown above control the Over-discharge (OD) and Overcharge (OC) pin thus controlling the transistors Q2 and Q3 in the figure below.
The DW01-A constantly monitors the overcurrent or short-circuit by measuring the voltage at the current sense pin. In the case of a short circuit the voltage exceeds VSIP, and the fault, i.e. the short circuit is inhibited by turning off the discharge control MOSFET. An explanation of overcurrent is given later in this article.   

Protection Circuit of Battery Pack

In the above figure, you can observe that the VSS pin is connected to the positive terminal of the cell with a resistor R24 and VSS and VDD have a capacitor C1 parallel to them. The capacitor and resistor are essential for suppressing the ripples and disturbance from the charger.

HY2212 BB3A: Cell Balancing IC 

Coming to the cell balancer circuit, the heart of this circuit is HY2212 BB3A, 1 cell Li-ion/polymer battery charger balance IC. This IC is capable of active balancing of a cell by electrical level monitoring and it comprises a very high-accuracy voltage detection circuit and delay circuit.

The series of HY2212 is created for a single-cell lithium-ion or can also be used for multi-cell battery packs with individual cells. It features charge balance control, electrical level monitoring ICs and it also comprises a high-accuracy voltage detection circuit and delay circuit The functional block diagram of the IC is given below, as you can see, the IC has a voltage divider circuit connected to the input VSS and VDD which is being fed to the overcharge detection comparator, which is used for controlling the enhancement MOSFET. You can refer to the datasheet of the IC to see the internal block diagram of this IC. It has a very simple circuit that just measures the voltage using a voltage detection comparator and gives an output. The output is used to control the gate of a MOSFET. Either P-type or N-type MOSFET can be used and the effective operation of both MOSFET are given in the table below.  

MOSFET Out Effective Operation

In this BMS an N-channel BMS is being used which is then connected to a Resistor of 480 ohm, the circuit used in the BMS is shown in the image below:

N Channel BMS Circuit

In the above circuit, the MOSFET used with this IC is A2SHB which is an N-channel Enhancement MOSFET. When the out pin from pin 6 of BB3A gives a high signal to the gate of this enhancement type MOSFET, the MOSFET connects a low resistance path through this 480-ohm resistance which acts as a load resistor and starts depleting the battery.

The rate of discharge can easily be found by Ohm’s law. V=IR

rate of discharge

 So, the battery can be discharged at a rate of 91 mill-Amp per hour. We can change the discharge rate by changing the value of the resistor.

Full 4S 40A BMS Circuit Diagram

4S 40A BMS Circuit Diagram

The above image shows the complete circuit diagram of the BMS circuit, as discussed above the circuit can be divided into smaller modules for balancing and monitoring every single cell.
As shown in the image below, we can see that the Balancer IC is connected in parallel with the cell. Similarly, the Battery charging IC, DW01 is also connected in parallel to the cell.

Balancer IC and DW01 IC Connected in Parallel with Cell

As explained above the VSS and VDD of DW01 are connected with the negative and positive of the cell respectively, and pin 2 which is the current sense pin is connected to the negative rail. According to the received input from the current sense pin, the Overcharge and Over-discharge transistors are controlled.

How does the 4s 40A BMS Circuit work?

The 10 MOSFET AOD472 are actually connected as 2 sets of 5 MOSFETs each.  The first set is for overcurrent protection and the other set is responsible for over-discharge protection. All cells in the circuits can trigger the overcurrent or over-discharge protection, this is required as the cell health degrades at different rates for different cells. The Gate of all the parallel MOSFETs are connected together and so is the source pins in order to trigger them together. All the 10 MOSFETs have their drain pins connected together, which means that the circuit will only work when all the MOSFETs are in the on state, else no current will flow and the Battery pack will neither power the output nor charge at that time.

Why multiple MOSFETs are connected? 

Since the BMS is designed for drills motor operations, the starting current is generally higher than the rated current. Starting current of motors can be as high as 4-8 times its rated current. The current reduces and comes back to its rated speed as the motor accelerates and reaches its synchronous or base speed. So, even though it is rated for 40A if a 500Watt motor that consumes around 40A is connected the surge current can be higher than 240 Amps for a very small time, hence multiple MOSFETs are connected in parallel.

Note:  When placing the MOSFETs in parallel, make sure that all the MOSFETs have very close actual values of V​GS(TS) since you want all the parallel-connected MOSFETs to turn on at the same time so as to avoid damages to the MOSFET.

BMS Module Circuit

The above image shows the flow of current when all the MOSFETs are in the On State. The current from the battery flows through the battery pack and from the series-parallel connection of MOSFET AOD 472s.

Controlling the MOSFETs 

The MOSFETs are controlled by controlling the overcharge and overdischarge pins of the DW01 IC. The source on the left side MOSFETs are connected to the ground, the current sense pin of DW01 is connected with the source, hence when there is a short circuit or an overcurrent is detected by the DW01 IC it turns on Q9 which turns on the transistor pair giving a signal to the gate terminal thus turning the MOSFETs off.

The Gate of the right pair of MOSFETs which are responsible for protecting the battery pack from overcharging is connected to the positive terminal of the battery pack. When the battery is overcharged, the DW01 IC will sense the overcharge condition using the internal potential divider circuit and will turn on the OD transistor.

Internal Potential Divider Circuit

Taking the IC 1 in this condition, it will turn on transistor Q2, the flow of the current will turn on Q21 connecting the gate of the combination of parallel MOSFETs responsible for overcharge protection with the ground thus turning it off and hence disconnecting the whole circuit. The below graph shows us the working of the DW01 IC during the charge condition.

Components used in the 4S 40A BMS Module

Coming to the components of the BMS, the BMS has 2 ICs, DW01-A which is a battery protection IC and a BB3A which is a cell balancing IC. Apart from the 2 ICs, we have this component with text G1 which is MMBT5551 a High voltage NPN transistor, 2L which is a high voltage PNP transistor, PMST5401 apart from this we have a Schottky rectifier and here at the bottom we have 10 N-channel enhancement MOSFET D472 with 2 parallel sets of 5 MOSFET connected in series which enables the high transfer of current and is a very crucial component for the overcurrent protection and overcharge protection. 

Components used in 4S 40A BMS Module

All the components used in the BMS are given in the table below:


Battery protection IC


1 Cell Li-ion/Polymer Battery Charge Balance IC

G1 (MMBT5551)

SMD High Voltage Transistor (NPN)

2L (PMST5401)

PNP high-voltage transistor


N-Channel Enhancement mode Field Effect Transistor)


Schottky rectifier

D472 (10 different types)

N-Channel Enhancement Mode Field Effect Transistor

Protection  Features of 4S 40A BMS Module

As discussed above, the BMS module has all the necessary features to protect the battery pack, it provides overcharge protection, overdischarge protection, short circuit protection along cell balancing. More details about the protection features are given below.

Overcharge Condition

When the cell is charged beyond a safe charging voltage, the cell’s health is affected and the lifecycle of the cell is reduced. To protect the cell from overcharging, this BMS employs the overcharge protection mechanism which disconnects the battery pack from the charger. The working of the overcharge protection is shown in the graph below

Overcharge Condition Graph

From the above graph, you can see that when the charger is connected, the battery voltage continues to increase, and as soon as it goes over  VOCP(Overcharge protection voltage), it waits for the TOC (overcharge delay time) and opens the overcharge protection transistor thus switching off the overcharge protection MOSFETs. The IC won’t turn off the OC pin unless the cell’s voltage falls below the VOCR(Battery Overcharge release Voltage).

Over Discharge Condition

When the cell voltage goes below a safe operating voltage, the cell’s health is affected and the lifecycle of the cell is reduced. To protect the cell from over discharge, this BMS employs over discharge protection. The working of the over discharge protection is shown in the graph below-

 Over Discharge Condition Graph

From the above graph, you can see that when the load is connected, the battery voltage continues to decrease and as soon as it goes under VODP(Over-discharge protection voltage) it waits for the TOD(over dis-charge delay time) and open the over discharge protection transistor thus switching off the over-discharge protection MOSFETs. Hence no current flows through the BMS. And till the time the battery is not recharged and the voltage of the cell does not cross beyond the VODR(Over-discharge release voltage), the BMS doesn’t allow the usage of the battery pack, thus increasing the life of our battery pack.

4S 40A BMS Circuit

The above image shows the flow of current during overdischarge conditions. As you can see the transistors are connected in parallel because when the transistors are connected in parallel, they act as a single big transistor that can handle a higher amount of current. 

Transistors in parallel

Over Current Protection 

Over current protection in a BMS is necessary to safeguard the battery systems from overcurrent or short circuit when a short circuit fault occurs or there’s a surge in current from the load which is higher than the battery packs’ specification. This condition can affect the cell’s health or even cause damage to the cell leading to fires. To protect the cell from over current, this BMS employs overcurrent protection. The working of the overcurrent protection is shown in the graph below

Over Current Protection Graph

In normal conditions, the CS pin monitors the discharge current by constantly monitoring the voltage of CS pin. When there is a surge in current demand from the cell and the voltage in CS pin exceeds the VOIP(Over-current protection voltage) for longer than TOI1(Over-current delay time) the overcurrent protection circuit operates and turns off the OC MOSFET thus disconnecting the circuit. The overcurrent discharged protection is released only when the load is released or the impedance of the battery between the positive and negative terminal becomes greater than 500k ohm.


The 4s 40A BMS is an economical as well as very effective module to protect the Li-ion cells from getting damaged. The design can be modified to be used for a higher or lower number of cells making it a very versatile design. The components used in the BMS are readily available and a lot of replacement parts are available in the market making it a really good BMS to try for your next project.

I hope that you enjoyed reading this article and learned something new, if you have any other doubts, you can post it in the comment section below.


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are both mosfet on when when charging or only the charge mosfet and the body diode is passing current around off discharge mosfet?


like wise when discharging are both mosfets on or only discharge mosfet on and body diode of charge mosfet passing current around off charging mosfet?

Sould the 5x parrallel transistors be flipped?

By that I mean the right MOSFETs (on schematic) are controlled by the discharge control pin of the DW01 and the left by the charge control pin. According to the datasheet, this sould be reversed? or am I missing something. 

As I understand it, if the overcharge protection kicks in and the Left pair is turned OFF, then the body diode will still conduct the current. Like wise, if during discharging the undervoltage protection kicks in then the right pair will turn off but the body diode will still allow current. That is why I think there is an error in the schematic.

8S BMS module.

Hello,this is a 8s bms circuit. I have a few questions.Is this shematic correct? How do I provide current control in this circuit? So how do I determine the current going to each cell (battery)?  Thank you in advance if you reply. <3 <3 <3