In a BESS system, thousands of battery cells are connected together in long strings to produce the required voltage and power. In theory, all these battery cells are identical. But in reality, due to small differences in manufacturing and operating conditions, each battery cell will have a slightly different capacity and internal resistance.
This small difference, if left unchecked, will grow larger with each charge/discharge cycle and create a serious problem: imbalance. This is where one of the smartest and most important functions of a BMS comes into play: Cell Balancing.
1. Why is Imbalance Dangerous?
Imagine a battery chain as a chain, and each battery cell as a link. The strength of the entire chain is determined by its weakest link.
- When charging: In an unbalanced chain, the battery cell with the lowest capacity (weak link) will be fully charged first. To protect this cell from overcharging, the BMS will have to stop the entire charging process, even though the other battery cells are not yet fully charged.
- When discharging: Conversely, this weakest battery cell will also run out of energy first. To protect it from being over-discharged, the BMS will have to disconnect the entire battery string, even though the other cells still have energy.
Consequence:
- Reduce available capacity: You can never use the full capacity of the entire system.
- Reduced lifespan: A weak battery cell will be constantly “stressed” at the upper and lower thresholds, causing it to degrade faster and dragging down the life of the entire system.
2. Cell Balancing: The Solution to Restore Balance
Cell balancing is a process by which the BMS actively adjusts to ensure all battery cells in a string have the same State of Charge (SoC). There are two main methods to do this:
a. Passive Balancing
- Principle: This is the simpler and more common method. The BMS will determine which battery cell has the highest voltage (fullest). It will then connect a resistor in parallel with that cell to “burn” off a small amount of the excess energy as heat, allowing the remaining cells to continue charging until they all reach the same voltage level.
- Advantage: Low cost, simple design.
- Disadvantages: Wastes energy (albeit very small) and can only work while charging.
b. Active Balancing
- Principle: This is a more advanced and efficient method. Instead of burning off excess energy, the active balancing system takes energy from fuller cells and “transfers” it to the empty ones. It’s like spreading water between tanks.
- Advantage: No wasted energy, higher efficiency and can work during charging, discharging and even when the battery is at rest.
- Disadvantages: Higher cost, more complex electronic circuit design.
3. The Importance of Cell Balancing to Your Investment
The battery cell balancing function may seem like a small technical detail, but it has a direct and huge impact on the economic efficiency of the BESS project:
- Maximize Available Capacity: By ensuring all battery cells are fully charged and discharged evenly, cell balancing helps you get the most out of your system's storage capacity.
- Extend System Life: It prevents individual battery cells from being “overused”, helping all cells “age” more evenly, thereby significantly extending the overall life of the entire battery pack.
- Enhanced Safety: Keeping all battery cells operating within a uniform voltage range reduces the risk of overcharge/discharge problems.
Conclusion: Cell balancing is a fundamental function that represents the sophistication of a modern BMS. It is not just a “nice to have” feature, but a mandatory requirement to maximize performance, extend the life and protect the value of your investment in a BESS. A BESS equipped with a BMS with an effective balancing algorithm will deliver superior value in the long run.
