A Guide to Designing A BMS Circuit Diagram for Li-ion Batteries

Table of Contents
An illustration guide showing a BMS (Battery Management System) circuit diagram for lithium-ion batteries with a series connection of multiple battery cells and a BMS module connected to monitor and balance the cells.

Lithium-ion batteries have become extremely popular due to their wide application in portable electronics. However, unlike lead-acid or nickel batteries, lithium-ion batteries require precise control of the charging and discharging process. Improper charging can cause lithium-ion batteries to swell or even explode. Deep discharge can also lead to battery failure. An ideal lithium-ion battery charger should have voltage and current stabilization as well as a balancing system for battery banks. The voltage of a fully charged lithium-ion cell is 4.2 Volts. Once the bank reaches this voltage, charging should stop. In this article, we will examine a circuit that allows charging Li-ion cells connected in series while also balancing them during the charging process. This BMS circuit diagram is not only simple but also highly effective.

Knowing the Components of BMS Circuit First

A. Battery Management Unit (BMU)

A Battery Management Unit (BMU) is a critical component of a BMS circuit responsible for monitoring and managing individual cell voltages and states of charge within a Li-ion battery pack. The BMU collects real-time data on each cell’s voltage and state of charge, providing essential information for overall battery health and performance. It constantly monitors and assesses the voltage levels of each cell to ensure uniform charging and discharging, preventing imbalances that could impact battery life. Accurate data from the BMU is crucial for making informed decisions regarding charging, discharging, and overall battery management.

B. Voltage Balancing Circuit

The Voltage Balancing Circuit is a key element in Li-ion battery management, addressing the need to balance individual cell voltages to enhance overall battery pack performance. Its primary goal is to equalize the voltage across all cells, preventing overcharging or over-discharging of specific cells that could lead to premature battery failure. Different strategies, such as passive and active balancing, are employed by voltage balancing circuits to ensure that cells maintain similar voltage levels during charging and discharging.

C. Temperature Monitoring

Temperature Monitoring is a critical aspect of BMS design, ensuring that the Li-ion battery operates within optimal temperature ranges for safety and performance. Extreme temperatures can affect battery performance, lifespan, and safety. Temperature sensors are employed to monitor and control the battery’s thermal conditions. Various sensors and probes are utilized to measure temperature at different points within the battery pack, providing comprehensive data for effective thermal management. It is important to maintain the right temperature range to prevent issues such as thermal runaway, ensuring the battery operates efficiently and safely.

D. Current Sensing and Control

Current Sensing and Control mechanisms play a vital role in BMS circuits, monitoring and regulating charge and discharge currents for optimal battery usage. Adding current sensors can measure the flow of electric charge, providing essential data for managing the charging and discharging processes. Accurate measurement of currents helps in preventing overcharging and over-discharging, contributing to prolonged battery life. So BMS circuits implement control mechanisms to regulate currents, optimizing the overall efficiency and safety of Li-ion batteries.

E. Protection Circuits

Protection Circuits are crucial components in a BMS, safeguarding Li-ion batteries from potential risks such as overcharge, over-discharge, and short circuits. These protection circuits monitor and prevent overcharging, a condition that can lead to thermal runaway and damage. They may include voltage limiters and disconnect switches. Also, over-discharging can damage cells and reduce battery lifespan. Protection circuits implement mechanisms to disconnect the load when the voltage drops to a critical level. Circuits are also designed to detect and mitigate the risks of short circuits, preventing potentially hazardous situations and maintaining the integrity of the battery pack.

Overview of BMS Circuit Diagram Symbols and Notations

BMS circuit diagrams use standardized symbols and notations to represent various components, ensuring clear communication and understanding.

-Common Symbols: Symbols such as resistors, capacitors, and specific icons for BMU, voltage balancing, temperature sensors, and other components are universally recognized in BMS circuit diagrams.

-Standardized Representation: Following a standardized representation helps engineers and technicians accurately interpret and implement the circuit design.

Circuit diagram 1

Design A BMS Circuit Diagram with Adjustable Voltage

Circuit diagram 2

This is a Zener diode circuit that opens when a certain voltage threshold is reached in the battery, turning off any unnecessary components. The circuit uses a Zener diode regulator based around a TL431 chip. When the threshold voltage is reached, a power transistor opens up. Together with the diodes in the collector circuit, this forms the equivalent of a dummy load. In other words, any excess power will be dissipated as heat through these elements, so a heat sink may be needed for the transistor.

In my design, I’m using a BD140 transistor, though the choice isn’t too critical – any PNP transistor with a collector current rating above 1 Amp would work. I have 3 of these circuit units collected on a single board, which lets me charge 3 lithium-ion battery banks simultaneously.

In theory, you could have any number of these circuit units. The board has a trimmer potentiometer to adjust each unit for the desired cutoff voltage. An LED indicator on the transistor’s collector lights up when the transistor opens, signaling that charging is complete. I’m using 5mm LEDs; you can pick any color, but it’s best to use the same color for all units.

The circuit adjustment process is simple. First, set a power supply to around 4.2V output. Connect the board and slowly turn the trimmer resistor until the LED lights up. Adjust all units this way until the current draw is balanced across units. In my case, it’s 160-180 milliamps per unit. For best accuracy, I recommend multi-turn trimmer resistors. Of course, there are calculators available for the TL431 chip, but resistor tolerances mean each unit needs individual tweaking anyway. By the way, the TL431 chip can be salvaged from old PC power supply boards.

The power supply voltage itself is regulated to a stable level by an LM317 linear regulator. A second chip provides current stabilization, feeding into the balancer circuit board. As mentioned earlier, the charge current is also stabilized, with the value set by resistor R18.

The proper resistor values can be calculated using LM317 calculator tools available online. With quality components, this charging system can match commercial lithium-ion chargers, though it will produce more heat.


The experiments demonstrated that the balancing circuit functions optimally. The charging process reaches completion upon attaining the designated voltage of 4.2 Volts.

Overall, I would recommend utilizing this circuit. Additionally, the circuit can also balance batteries independently of the charging unit.


Hope you will like this guide for designing the BMS circuit diagram for Li-ion battery charging. MOKOEnergy has several highlights in its BMS circuit diagram design. As electric vehicles and renewable energy systems become more popular, the importance of BMS will continue to be emphasized, and MOKOEnergy continues to innovate and improve its BMS board manufacturing technology, so please feel free to contact us with any questions you may have.


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