How to design a 12C Battery Management System using Quickboards’ reference designs

A complete 12-cell lithium-ion Battery Management System (BMS) can be designed using modular Quickboards schematic blocks. This guide outlines how to architect and assemble each part of the system using proven reference designs for voltage monitoring, current and temperature sensing, relay control, power conversion and distribution.

Overview of the Battery Management System Architecture

Above is a hierarchical diagram of the Battery Management System (BMS), illustrating the key circuit building blocks. At the core is the Processor Block, which interfaces with several critical modules:

• A Power Management Block (020052024) for cell voltage measurement and balancing
• A Power Management Block (020012024) containing several buck converters for powering the different circuits of the BMS
• An ADC Block (070012024) for monitoring temperature and current sensors
• A Sensor Block (040222024) for temperature measurement
• A Sensor Block (040082024) for Current measurement
• A Power Management Block (020222025) for driving MOSFETs to control mechanical relays

Processor Block

Microcontroller Selection — PIC24FJ128GA006

Start your design by adding the Processor Block reference design, built around the PIC24FJ128GA006 microcontroller. This 16-bit MCU is ideal for battery management systems due to its combination of processing power, I/O flexibility, and integrated peripherals.

The PIC24 operates at 3.3V and features:

• A 16 MIPS CPU core
• 128KB of program Flash memory and 8KB of RAM
• 53 general-purpose I/O pins, enabling multiple interface and control options
• A 10-bit ADC, multiple timers, and SPI/I²C/UART peripherals for system integration

Included Peripherals in the Processor Block

The Processor Block schematic includes the following essential components and interfaces:

• ICSP Port (In-Circuit Serial Programming):
  Used to program the microcontroller using a debugger such as the PICkit or MPLAB ICD.
• UART Interface:
  Provides a serial communication link to external devices such as a PC, Bluetooth module, or data logger. Ideal for diagnostics or telemetry output.
• External 20 MHz Oscillator:
  Provides a stable clock source for precise timing and protocol synchronization.
• Heartbeat LED:
  Connected to a GPIO pin, this LED is toggled by firmware to indicate that the processor is active and running (a simple watchdog/debug feature).
• Decoupling Capacitors:
  Placed close to power supply pins to suppress voltage transients and noise, improving stability and reducing EMC issues.

Power Management and Cell Monitoring

LTC6811 for Cell Voltage Measurement and Balancing

Next, integrate the Power Management Block featuring the LTC6811, a high-performance battery stack monitor IC from Analog Devices. This block is responsible for measuring the voltages of up to 12 series-connected lithium-ion cells and performing passive cell balancing to extend the battery pack’s capacity and lifespan.

The LTC6811 includes:

• 12 independent voltage measurement channels with high precision (±1.2 mV max error)
• Built-in passive balancing through onboard balancing switches and external resistors
• Configurable watchdog timer and fault detection logic for enhanced safety
• Built-in redundancy and self-test features, making it suitable for safety-critical applications like EVs, drones, and medical equipment

By balancing the cells regularly, the system ensures that all cells remain within a safe voltage window, preventing overcharging, undercharging, and capacity imbalance between cells — key factors in battery degradation.

Isolated Communication via LTC6820 and isoSPI

To enable safe, high-speed communication between the LTC6811 and the main processor, this block also incorporates the LTC6820 isoSPI transceiver. isoSPI (isolated SPI) provides:

• Robust galvanic isolation between the processor and high-voltage battery domain
• Improved noise immunity in electrically noisy environments
• Long communication length capability with twisted pair (up to 100 meters)
• Simplified daisy-chaining of multiple LTC6811 ICs in large battery packs

The LTC6820 acts as a bridge between the standard SPI port on the PIC24 and the isoSPI interface on the LTC6811, preserving signal integrity and system safety.

The LTC6811 peripherals include the voltage measurement RC filter and the PMOS discharge circuit.

Current and Temperature Sensing

Current Sensing Block

Accurate current measurement is essential for reliable state-of-charge (SoC) estimation and overcurrent protection in lithium-ion battery systems. The Current Sensing Block uses the ACS781KLRTR-150U-T, a compact, bidirectional Hall-effect current sensor capable of measuring currents up to ±150A.

This sensor provides a linear analog voltage output, centered around 1.65V for a 3.3V supply, which varies proportionally with current in either direction. Its galvanic isolation ensures safe operation, while the low-resistance internal path (100 µΩ) minimizes power loss.

Temperature Sensing Block

To ensure safe operation and prevent overheating or thermal runaway, the Battery Management System includes a Temperature Sensing Block built with eight analog temperature sensors.

In this design, MCP9700 SMT temperature sensors are used. These are compact, surface-mount analog sensors ideal for directly mounting onto the PCB alongside 18650 lithium-ion cells. Their linear voltage output simplifies integration with ADC channels, making them well-suited for close-proximity thermal monitoring.

Alternative Approach — NTC Thermistors for Remote Packs

In most conventional battery pack designs, the BMS circuit is physically separated from the battery cells. In such cases, NTC thermistors are preferred due to their low cost and high sensitivity. These thermistors are typically Mounted directly on the battery cell surface using adhesive or thermal tape, Wired back to the BMS PCB using long leads, and configured in a resistive voltage divider, with the output voltage fed into the ADC for measurement.

This configuration enables accurate monitoring of each cell’s temperature, triggering safety mechanisms in response to abnormal thermal conditions.

Power Control with MOSFETs

Controlling Battery and Charge Relays

Incorporate a Power Control Block with eight N-Channel MOSFETs, where two MOSFETs are used to energize two mechanical relays, one for connecting the battery pack to the primary bus, and a second relay for connecting the charger to the bus. The remaining six MOSFETs can support protection, precharge, or additional control functions.

ADC Integration for Sensor Data

10-Bit ADC Block

To read analog voltages from current and temperature sensors, use a 10-bit ADC Block based on the ADC108S022CIMTX, an 8-channel SPI-compatible ADC. Assign two channels to monitor charge and discharge currents, and the remaining six to analog temperature sensors.

Connect the ADC’s CS, DIN, DOUT, and SCLK pins to the SPI2 peripheral on the PIC24 microcontroller. This setup enables the Processor Block to efficiently poll all sensor channels, ensuring accurate, real-time monitoring for current regulation, thermal management, and fault detection.

Power Regulation and Supply

Buck Converters for Subsystem Power

Finally, integrate a Power Supply Block using the AOZ1280, a high-efficiency synchronous buck converter, to provide stable, regulated voltages for all BMS subcircuits. This ensures reliable operation across digital logic, analog sensors, and communication interfaces.

In this application, the system requires only two output voltage rails. The 5V rail supplies components such as the LTC6811, while the 3.3V rail powers the PIC24 microcontroller, ADC, and temperature sensors. The AOZ1280 supports high switching frequencies and uses compact external components, making it well-suited for space-constrained PCB designs.

Although the reference design includes multiple buck converter outputs, only the 5V and 3.3V rails are necessary for this implementation. The unused converters can be removed from the schematic. This block delivers clean, isolated power essential for robust system performance throughout the BMS.

Conclusion

Using Quickboards modular design blocks simplifies the development of a complex BMS. By combining standard components like the LTC6811 and PIC24 microcontroller with reusable power and sensing modules, you can rapidly prototype and scale reliable lithium-ion battery systems.

Interested in the schematics? Request them below.