What is Battery Management System (BMS)? – Building Blocks, Working & Functions

Do you know, “What is a Battery Management System?” and
“How does it work?”

Maybe you are looking to design a custom BMS system for
a battery all by yourself.

In this article, we can help you with these questions
and problems!

With the evolving technology, the need for the batteries
is also growing at an astronomical pace. They are found everywhere, nearly in
all devices from small earbuds to high-speed electric cars.

Researches are going on to make batteries as compact as
possible without decreasing the efficiency.

Benefits of using BMS

  • Ensure
    reliable battery operations
  • Continuous
    battery health monitoring to avoid explosion
  • Increases
    the life span of the battery
  • Indicates
    battery level

The Battery Management System (BMS) Technology is so
useful. Unfortunately, we have experienced that there is very less information
available on the internet, so we have decided to round-up an article on BMS in
details.

So stay tuned and read till the end.

What is Battery Management System?

What is Battery Management System - Robu.inWhat is Battery Management System - Robu.in

A Battery Management System AKA BMS monitors and regulates internal operational parameters, i.e. temperature, voltage and current during charging and discharging of the battery.

In technical terms, the BMS estimates the SoC (State of
Charge) and SoH (State of Health) of the battery to improve safety and
performance.

It avoids over-charging and over-discharging of the
battery pack. This way, it maintains charge level within maximum and minimum
allowed capacity to prevent sudden accidents [explosion].

Hence a BMS is a highly crucial device to ensure the
safety of the battery and user.

There are many other benefits of using BMS; we will
discuss them later in this article.

A BMS is an electronic board consisting of a variety of
components and circuitry. After detecting a problem in operational parameters
(voltage, temperature etc.) BMS triggers input to the alarm system followed by
disconnection the battery pack from the load or charger.

Building Blocks of Battery Management System

The design of the BMS is board is a bit complicated. To
keep this article short and informative, we have briefly defined building
blocks of the BMS.

If you want in-depth coverage of BMS board construction,
refer to this article BMS Tutorial by
Renesas.

As you are still here, let us discuss the BMS Building Blocks briefly,

Building Blocks of Battery Management System - Robu.inBuilding Blocks of Battery Management System - Robu.in

There are four main functional blocks,

  • Cut-off
    FETs
  • Fuel
    Gauge Monitor
  • Cell
    voltage monitor
  • Temperature
    Monitor

Along with this, there are few other blocks we will
describe them as well.

So without any further ado, let’s get started.

1. Cut-off FETs

A FET-driver acts as isolation between the battery and
the charger. It is used to connecting the high-se and low-side of the battery
pack.

  • High-side
    – Activates NMOSFET using the charge pump driver
  • Low-side
    – Activates NMOSFET without charge pump driver

This integrated Cut-off FETs reduce the overall cost of
the BMS. It is also eliminating the use of high voltage devices that could
consume a large die area.

2. Fuel Gauge Monitor

This helps in keeping track of the charge entering and
exiting the battery pack. The charge flowing is calculated by multiplying
current and time.

Although several methods are used to measure the current
flow, the most efficient and cost-effective solution is to measure the voltage
of sense resistor using a 16-bit ADC with low offset and high common-mode
rating.

Higher ADC is beneficial to obtain an extensive dynamic
range at more speed.

3. Cell Voltage Sensors

Cell voltage monitoring is the most standard function of
a Battery Management System. It is useful in determining the health of the
battery.

All cells in a battery should operate at standard
voltage levels during charging and discharging for safety and improving the
lifecycle.

To know how battery cells are connected in series and
parallel to make a battery pack check out this blog Series Parallel Configuration of Lithium Batteries.

4. Temperature Monitoring

As technology is evolving, batteries are made to supply
high currents, thereby keeping the voltage constant.

Since a high current flow through batteries can cause
sudden temperature rise and force them to blast accidentally. It needs to be
avoided. For this reason, the BMS continuously monitor the temperature of the
battery and regulate to the rated value.

This feature is handy because if the temperature rises
above the rated value, it will inform you to start/stop charging or
discharging.

5. Other Building Blocks

Few more of the available blocks are,

  • Battery Authentication –
    prevents the connection of BMS electronics to the third-party battery pack.
  • Real-time Clock (RTC) –
    used in black-box application
  • Memory –
    used in black-box application
  • Daisy Chain –
    simplifies the connection between stacked devices

Working of Battery Management System

The working of a battery management system depends upon
the complexity of the components available on board.

Let we discuss them briefly,

The microcontroller of the BMS measures the cell voltage and current in real-time and based on that it switches the Mosfets. In the BMS only one bus is used for charging and discharging

Initially, both charging and discharging FEts are off so there is no flow of current.

The microcontroller of the BMS senses the voltage at the input and it turns on the charging Mosfet which starts the battery charging.

If the voltage at input pin is not present then BMS determines the load is connected and it turns on the discharging FET.

Normally there are two types of cell balancing are used in BMS.

  • Passive cell balancing
  • Active cell balancing.

In Passive cell balancing, the bypass resistors are used to discharge the excess voltage and equalize with other cells.

In the active cell balancing the excess charge of one cell transferred to another cell which has a low charge to equalize them. It uses charge storing capacitors and inductors.

To give you the general idea of the working of the BMS, let’s consider a simple circuit as shown below,

Working of Battery Management System - Robu.inWorking of Battery Management System - Robu.in

The image shows that a 3S BMS is connected with a 3-cell
battery pack. Cells are connected in series.

  • Storage
    Mode Voltage – 3.7V
  • Full
    charge voltage – 4.2V

So the charging voltage for the three-cell battery pack
is 12.6V.

The heavy gauge wires are connected to the power supply.
The small wires are used as the cell balancing wires and carry less current.

A BMS has rated capacity. It measures the electric
charge available and relates to the capacity.

As the capacity decreases the count of the electric
charge drops. This lowered count of the charge

Functions of BMS

1. Safety

Due to the higher density in lithium-ion battery packs,
there is an increased risk of catching fire. So as mentioned earlier, it is
essential to operate batteries at rated value.

A BMS does this work for you. It avoids over-charging
and over-discharging of the battery pack to extend the battery life.

It also offers short-circuit protection, charging and
discharging over current protection, anti-reverse charging protection etc.

Modern BMS are equipped with Bluetooth and UART
communications.

2. Battery Performance Optimization

For a battery to perform at its best, it must operate
somewhere between the maximum and minimum rated values, i.e. current, voltage,
temperature etc.

As we learned earlier, a BMS helps batteries to operate
within these critical rated values.

In case of the battery packs, it helps in maintaining
equal charging and discharging of the cells. This massively improves the
performance of the battery pack.

Battery Management System is useful in improving
performance as well as the life of the battery.

3. Health Monitoring and Diagnostics

The charge level of a battery determines the charging
and discharging time. A BMS is capable of calculating and indicating the charge
available in battery.

A BMS checks for the oddity in the battery parameter by
comparing them with rated values. Also, it is capable of taking corrective
actions to increase the health of the battery.

Most Popular BMS Boards

There are a variety of BMS boards available in the
market.

Also, with the right knowledge of this technology, you
can design your own BMS. Yes, of course! It will take lots of time and efforts.
But it is for designing BMS for big batteries.

If you have a small lithium-ion battery pack, then we
can suggest few BMS that are popular in 2021.

#1 – TP4056 Li-ion BMS

TP4056 Li-ion BMS - Popular Battery Management System Board - Robu.inTP4056 Li-ion BMS - Popular Battery Management System Board - Robu.in

This BMS is perfect for charging a single Li-ion cell.
The charge current is 1A and supplied by the TP4056 charger IC and DW01 battery
protection IC.

It automatically disconnects the battery from the power
supply when finished.

#2 – 1S 18650 Li-ion BMS

1s 18650 Li-Ion BMS - Popular Battery Management System Board - Robu.in1s 18650 Li-Ion BMS - Popular Battery Management System Board - Robu.in

This is one more popular BMS for single-cell lithium-ion
batteries. It comes with short circuit protection, over-current protection,
over-charge and over-discharge protection.

#3 – 3S 10A 18650 Li-Ion BMS

3S 10A 18650 Li-Ion BMS - Popular Battery Management System Board - Robu.in3S 10A 18650 Li-Ion BMS - Popular Battery Management System Board - Robu.in

In contrast to the above BMS boards, this can be used
for 3-cell li-ion battery. You can connect this BMS with a battery of 11.1V
(storage mode voltage) or 12.6 (full-charge voltage).

It offers over-charge and over-current protection.

Conclusion

We hope that you could understand the Battery Management
System completely. In case of any doubts, don’t forget to comment below.

With the use of lithium-ion batteries in automobiles,
the popularity and the design of a BMS is continuously improving. It is
becoming increasingly important to know about this technology.

There are various types of BMS available which are
categorized into 1S, 2S…. to 20S BMS. Here the word “S” stands for the number
of cells. So it means that you can use BMS with the battery packs that have 20
cells.

At Robu, we have series of BMS available that you can
use. Don’t forget to check them by clicking here!