Designing a Li-ion Battery Management System (BMS)
Last updated: 5/13/19
In the process of designing a photovoltaic battery charger, I realized that I needed a BMS to safely operate and monitor the 18650 Li-ion cells that would be storing the solar energy. So, I decided to design my own.
The Battery Management System
I decided to prioritize the protection components of the circuit (like overcharge and discharge protection) before building the performance components (like the SoC estimation and cell balancing). I have not made an overall circuit schematic for the Li-ion v2 charger yet, but each individual circuit is discussed in detail with a current progress update.
Battery Management System Status:
✅ State of Charge (SoC) Estimation
🕐 Cell Balancing Mechanism
🕐 Thermal Control
🕐 State of Health (SoH) Estimation
What is a Battery Management System (BMS)?
The primary function of a BMS is to ensure that cells are operated within safe limits to prevent cell damage, premature degradation & capacity fade, and potentially dangerous fires or explosions. Additionally, the BMS serves as the interface between the user and the state of the cells, communicating the remaining charge of the cell (state of charge, SoC) and its health (state of health, SoH).
The BMS only has access to the battery voltage, current in/out, and temperature of the cells. It accomplishes the above functions with 6 key components:
Overcharge protection: ensures that each Li-ion cell is not charged past 4.2V. This prevents a cell from potentially catching on fire or exploding.
Overdischarge protection: ensures that each Li-ion cell is not discharged past 2.5V. This extends the operational lifetime of a cell and preserves capacity.
State of charge (SoC) estimation: estimates the percent of charge remaining in the cell.
State of health (SoH) estimation: estimates the capacity loss of the cell over time.
Cell balancing: prevents cells of different voltages from charging each other instead of delivering power to load.
Thermal management: monitors the temperature of a cell to ensure safe operation.
The photovoltaic battery charger the BMS is being designed for
In this system, a 12V solar panel will charge 2 18650 Li-ion cells, which can then be used to charge an iPhone with sunlight.