Technical Article

Introduction to Battery Management Systems

February 08, 2021 by Enrico Sanino

Learn the high-level basics of what role battery management systems (BMSs) play in power design and what components are necessary for their basic functions.

Nowadays, Li-ion batteries reign supreme, with energy densities up to 265 Wh/kg. They do, however, have a reputation of occasionally bursting and burning all that energy should they experience excessive stress. This is why they often require battery management systems (BMSs) to keep them under control.

In this article, we'll discuss the basics of the BMS concept and go over a few foundational parts that make up the typical BMS.

 

Basic BMS Configurations

In Figure 1, we see the basic blocks of how a BMS can look while serving the function of preventing major battery malfunctions. 


Figure 1. A typical BMS block diagram

 

This example BMS can handle four Li-ion cells in series. A cell monitor reads all the cell voltages and evens out the voltage among them: this function is called balancing (more on that later). This is controlled by an MCU that handles telemetry data, as well as switch manipulation and balancing strategy.

In practice, the market offers different solutions for simpler designs, including for single cells with no balancing or MCUs, as shown in Figure 2.

 

Figure 2. A simple battery manager. Image used courtesy of Texas Instruments

 

The downside of these simpler systems is that a designer is bound to what the given part offers (e.g., a high or low side switch) without customization.

When using more cells, a balancing system is needed. Simple schemes that still function without an MCU exists, as shown in Figure 3.

 

Figure 3. An MCU-independent cell balancer. Image used courtesy of Texas Instruments

 

When using bigger battery packs or anything which requires cells in series or a fuel gauge calculation, an MCU is needed. The most integrated (and therefore low cost) solution is the one in Figure 4.



Figure 4. A commercial BMS. Image used courtesy of Renesas

 

This is a BMS that uses an MCU with proprietary firmware running all of the associated battery-related functions.

 

The Building Blocks: Battery Management System Components

Look back at Figure 1 to get an overview of the fundamental parts crucial to a BMS. Now, let's go through the main parts of Figure 4 in a bit more detail to understand the various elements involved in a BMS block diagram.

 

Fuse

When a violent short circuit occurs, the battery cells need to be protected fast. In Figure 5, you can see what's known as a self control protector (SCP) fuse, which is mean to be blown by the overvoltage control IC in case of overvoltages, driving pin 2 to ground.

 

Figure 5. SCP fuse and control of a commercial BMS

 

The MCU can communicate the blown fuse's condition, which is why the MCU power supply has to be before the fuse. 

 

Current Sensing/Coulomb Counting

Here is implemented a low side current measurement, allowing direct connection to the MCU.

 

Figure 6. Typical low current sense of a commercial BMS

 

Keeping a time reference and integrating the current over time, we obtain the total energy entered or exited the battery, implementing a Coulomb counter. In other words, we can estimate the state of charge (SOC, not to be confused with a system-on-chip) by using the following formula:

 

 

where

  • $$SOC(t_0)$$ is the initial SOC (in Ah)
  • $$C_{rated}$$ is the rated capacity (in Ah)
  • $$I_b$$ is the battery current
  • $$I_{loss}$$ takes into account the cell reaction losses
  • τ is the averaging period of the electric current samples

 

Thermistors

Temperature sensors, usually thermistors, are used both for temperature monitor and for safety intervention.

In Figure 7, you can see a thermistor that controls an input of the overvoltage control IC. This artificially blows the SCP (the fuse shown in Figure 5) without MCU intervention.

 

Figure 7. A thermistor can control the SCP, in case of severe thermal problems

 

Figure 8 shows two additional thermistors for telemetry.


Figure 8. Thermistors used by the firmware

 

Main Switch

To act as switches, MOSFETs need their drain-source voltage to be $$V_{ds} \leq V_{gs} - V_{th}$$. The electric current in the linear region is $$I_d = k \cdot (V_{gs} - V_{th}) \cdot V_{ds}$$, making the resistance of the switch $$R_{MOS} = 1 / [k \cdot (V_{gs} - V_{th})]$$.

It's important to drive the $$V_{gs}$$ accordingly to ensure low resistance and hence low losses.

 

Figure 9. Battery pack main switch (NMOS, high-side)

 

NMOS types are used also on high side switches through a charge pump, since normally they have lower $$R_{MOS}$$.

 

Balancer

Battery cells have given tolerances in their capacity and impedance. So, over cycles, a charge difference can accumulate among cells in series.

If a weaker set of cells has less capacity, it will charge faster compared to others in series. The BMS has to therefore stop other cells from charging, or else the weaker cells will get overcharged, as seen in Figure 10.


Figure 10. Lower capacity cells impeding pack full charging. Image used courtesy of Analog Devices

 

Conversely, a cell can get discharged faster, risking that cells going under its minimum voltage. In this instance, a BMS without a balancer has to stop the power delivery earlier, as seen in Figure 11.


Figure 11. Lower capacity cells impeding usage of full pack energy. Image used courtesy of Analog Devices

 

A circuit like the one in Figure 12 will discharge the cell with higher SOC (state of charge) as shown in Figure 10 at the level of the other cells in series. This is accomplished by using a passive method of balancing called charge shunting.

 

Figure 12. Example of passive balancing strategy

 

Because current flows through the transistor in the ON state and dissipates through R, and because the voltage reference is CELL1 (a negative pole), only such a cell will discharge its energy excess.

 


 

This article has aimed to introduce the basic concept of a battery management system and introduce the basic components used in their design. Hopefully, you now have a better understanding of what a battery management system is meant to accomplish and how it can be used in a power design. 

If you have additional concepts you'd like to learn more about regarding BMS design, please leave a comment below.