BMS Theory | Cell Balancing

Date Published: February 20, 2024

Cell balancing plays a pivotal role in maintaining the health efficiency and safety of lithium batteries which is integral to Battery Management System (BMS) technology. When individual lithium cells, each with slight manufacturing differences and unique characteristics, are linked together in series to achieve the desired output voltage for a system, imbalances in charge levels can occur during the battery's charge and discharge cycles. These imbalances can lead to cell divergence, resulting in a decrease in the battery's overall usable capacity. Moreover, significant imbalances can create safety risks, including the potential for overheating or failure of the cells. Recognizing the critical nature of this issue, cell balancing emerges as a key component of quality BMS technology. It ensures that all cells within a battery maintain equal charge levels, thereby maximizing the battery's capacity, prolonging its lifespan, and safeguarding against hazardous situations. This discussion will delve into the significance of cell balancing, highlight its advantages, and outline several strategies manufacturers employ to achieve effective cell balancing.

Cell Assembly

To assemble a usable Lithium battery, individual cells are connected in series to increase the voltage. For example, a nominal LiFePO4 12V (12.8V) battery will have four cells in series, LiFePO4 24V (25.6V) will have eight series, and LiFePO4 48V (51.2V) will often have sixteen cells in series.

Note: To lower costs, some manufacturers try to pass off fifteen-cell batteries as 48V batteries, but these will often function less efficiently with 48V inverter systems than sixteen-cell batteries. We recommend using batteries with 16 cells, 51.2VDC, and when comparing battery, it’s especially important to compare kWh of storage rather than Amp-Hours.

Causes of Imbalance

The causes of imbalance among cells within a battery pack during charging and discharging cycling stems from a combination of factors. Manufacturing variations play a significant role, as no two cells are identical; slight differences in material composition, size, and assembly can affect each cell's performance and capacity. Additionally, the inherent characteristics of lithium cells, which include their chemical makeup and how they age over time, contribute to discrepancies in cell behavior. A critical factor exacerbating imbalance is the variation in internal resistance within each cell, and the resistance between cells, known as busing resistance. These resistances affect how easily electrons flow through the cells and the connecting material, leading to uneven charge and discharge rates among the cells. As some cells may charge or discharge faster than others, this discrepancy can result in a state of imbalance across the battery pack.

The Benefits of Balanced Cells

Lithium-ion cells are sensitive to extreme conditions, especially high voltage situations. Without balancing, some cells can become overcharged or discharged more than others. This imbalance can reduce the overall capacity of the battery since the battery management system (BMS) will stop charging if any cell reaches a critical maximum voltage, and stop discharging if any cell reaches critical depleted voltage.

Balancing attempts to ensure that all cells reach their full capacity simultaneously, maximizing the usable capacity of the battery. Overcharging or deep discharging even a single cell can significantly harm the cell. Overcharging can cause extreme temperature rise, posing a major risk of thermal runaway – a condition where one cell's failure rapidly cascades to adjacent cells, potentially leading to a fire or explosion. Cell balancing helps to avoid these extremes by ensuring that all cells stay within a safe operating range.

By keeping all cells in balance, the risk of such catastrophic failures is significantly reduced. Consistently operating cells within their optimal voltage range through balancing extends their lifespan. It prevents scenarios where weaker cells degrade faster than stronger ones, which would otherwise lead to premature battery pack failure. There are functionally two ways our industry achieves effective balancing of cells: active and passive.

Active Cell Balancing

Active balancing is by far the most advanced, most accurate, and fastest balancing principle; it redistributes charge among the cells in a battery pack to ensure that the cells all have the same state of charge throughout the charging process.

An active balancing BMS monitors the voltage of each cell and adjusts the charging and discharging current on each cell accordingly, using inductive or capacitive charge shuttling to transfer the charge between cells. This is a very efficient and effective approach as it transfers energy to where it is needed instead of wasting it through resistors. However, this method requires sophisticated components, which can substantially increase the cost of the BMS components. A good quality BMS with active balancing, capable of high current power distribution, is not a cheap component, and these are most often found in very large ESS or advanced high-voltage automotive battery applications where batteries may not be fully charging often or ever, and/or where quick, high current cycling is required. It is highly unlikely that one will find this advanced functionality executed in a quality manner within low-cost ESS battery solutions. With residential ESS systems (especially with Lithium Iron Phosphate batteries), it’s often unnecessary to have active balancing; passive balancing is most often used.

Passive Cell Balancing

Passive balancing, or top balancing, essentially uses the principle of discharging the cells through a bypass route as each cell reaches a defined top voltage. This principle relies on Ohm’s Law and balances resistor characteristics to bring cells all up to the same state of charge. During the charging process, cells will start to diverge at the top end of charge; as they diverge, the BMS will apply resistance (a load) to individual cells, diverting the current from these higher cells, allowing the lower cells to continue charging. We call this the balancing state, and it occurs during what would normally be the absorption (Constant Voltage) stage of lead-acid battery charging. Passive balancing is generally a slower process than active balancing and may take longer to achieve completely balanced cells. However, this is rarely a concern with renewable energy applications because, in most cases, the batteries are held at a full charge for significant periods (giving the cells plenty of time to balance) before they are discharged, and they are charged at a relatively slow rate compared to other industries like automotive supercharging. Top balancing circuits are simpler and easier to implement than active balancing techniques, keeping the system more cost-effective.

Cell Balancing Conclusion

Cell balancing is a crucial aspect of lithium battery technology that ensures the efficiency, health, and safety of the battery. Imbalanced cells can reduce the overall capacity of the battery and pose a safety risk. Balancing ensures that all cells reach their full capacity simultaneously, maximizing the usable capacity of the battery and extending the lifespan of the cells. There are two ways to achieve cell balancing: active and passive balancing, with active balancing being the most advanced and accurate but also the most expensive. Passive balancing, on the other hand, is slower but a more cost-effective method that is suitable for most applications, including residential ESS applications.

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Posted in Batteries & Charging