Basic knowledge of battery management system
Today, lithium-ion batteries dominate, with an energy density of up to 265 Wh/kg. However, they do enjoy a notorious reputation. If they are subjected to excessive stress, they will sometimes burst out and burn all their energy. This is why they often need a battery management system (BMS) to control them.
In this article, we will discuss the basics of BMS concepts and introduce some basic parts that make up a typical BMS.
Basic BMS configuration
In the picture below, we see the basic box of the appearance of the BMS, which also has the function of preventing serious battery failure.
The picture above is a typical BMS block diagram
This example BMS can handle four lithium-ion batteries in series. The battery monitor reads all battery voltages and equalizes the voltages among them: this function is called the equalization function. This is controlled by an MCU that processes telemetry data as well as switch manipulation and balancing strategies.
In fact, the market offers different solutions for simpler designs, including a single battery without a balance or MCU, as shown in the figure below.
The picture above is a simple battery manager.
The disadvantage of these simpler systems is that the designer must be tied to the functionality provided by a given component (for example, high-end or low-end switches) without customization.
When more batteries are used, a balancing system is required. There are simple solutions that can still run without an MCU, as shown in the figure below.
The picture above is a battery balancer independent of the MCU.
When using a larger battery pack or any product that needs to connect batteries in series or perform fuel gauge calculations, an MCU is required. The picture below is the most integrated (and therefore the lowest cost) solution.
The picture above is a commercial BMS.
This is a BMS that uses an MCU with proprietary firmware to run all battery-related functions.
Components: battery management system components
Look back at the first picture to understand the basic parts that are essential to BMS. Now, let us browse the main parts of the above figure in more detail to understand the various elements involved in the BMS block diagram.
When a severe short circuit occurs, the battery cells need to be protected quickly. In the picture below, you can see the so-called SCP fuse. When the fuse is over-voltage, the fuse will be blown by the over-voltage control IC, thereby grounding pin 2.
The above figure is the control of SCP fuse and commercial BMS
The MCU can communicate the blown fuse, which is why the MCU power supply must be before the fuse.
Current sensing/Coulomb counting
The low-side current measurement is implemented here, allowing direct connection to the MCU.
The picture above shows the typical low current inductance of a commercial BMS
By maintaining a time reference and integrating the current over time, we obtain the total energy entering or leaving the battery and implement a coulomb counter. In other words, we can use the following formula to estimate the state of charge (SOC, not to be confused with the on-chip system):
Temperature sensors (usually thermistors) are used both for temperature monitoring and safety interventions.
In the picture below, you can see a thermistor used to control the input of the overvoltage control IC. This is an artificially blown SCP (the fuse shown in Figure 5) without the intervention of the MCU.
Pictured above. In the event of a serious thermal problem, the thermistor can control the SCP
The figure above shows two other thermistors used for telemetry.
The thermistor used by the firmware in the picture above
To be used as a switch, the MOSFET needs its drain-source voltage to be Vds≤Vgs−Vth. The current in the linear region is Id=k⋅(Vgs−Vth)⋅Vds, so that the resistance of the switch RMOS=1/[k⋅(Vgs−Vth)].
It is important that VGsVGs ensure low resistance and thus reduce losses.
In the figure above, the main switch of the battery pack (NMOS, high-end)
The NMOS type is also used for high-end switches by charge pumps because they usually have lower RMOS.
The capacity and impedance of the battery cell have a given tolerance. Therefore, during the cycle, the difference in charge will accumulate between the batteries connected in series.
If a group of weaker batteries has a smaller capacity, the charging speed will be faster than other batteries connected in series. Therefore, the BMS must stop the charging of other batteries, otherwise the weaker battery will be overcharged, as shown in Figure 10.
In the picture above, a low-capacity battery prevents the battery pack from being fully charged.
Conversely, the battery can discharge faster, putting the battery at risk of falling below its minimum voltage. In this case, the BMS without a balancer must stop the power supply in advance, as shown in the figure below.
In the picture above, a battery with a lower capacity hinders the use of energy in the battery pack.
Like the circuit in the figure below, as shown in Figure 10, this circuit will discharge other batteries in series with a higher SOC (state of charge). This is achieved by using a passive balancing method called charge shunting.
Above: An example of a passive balancing strategy
Since the current flows through the transistor in the on state and is dissipated through R, and since the reference voltage is CELL1 (negative), only such a unit can release excess energy.
This article aims to introduce the basic concepts of a battery management system and introduce the basic components used in its design. Hope you now have a better understanding of the work to be done by the battery management system and how to use it in power supply design.