Resistors, capacitors, inductors, and dc/dc converters can all be used in various topologies to provide cell balancing for battery packs. Cell balancing is needed to obtain the maximum performance since performance is limited by the weakest cell in the pack. Once the weakest cell is depleted, the pack stops delivering energy. The various cell balancing circuits are designed to maintain equal voltages for each individual cell forming a battery pack, ensuring maximum efficiency of the pack.

An important parameter used to measure and control cell balancing is state of charge (SoC), which quantifies the amount of charge in a battery relative to its capacity. The goal of cell balancing is to have the same SoC for every cell at a given time.

In the figure below, the battery pack (a) would behave like a pack with a nominal voltage of 3.7V, pack (b) would behave like a pack with a nominal voltage of 3.1V, while pack (c) would behave like a pack with a nominal voltage of 3.4V. Pack (a) would provide significantly more energy than either pack (b) or pack (c). Cell balancing would result in all three packs achieving 3.7V for all four cells, delivering improved performance for packs (b) and (c).

The need for cell balancing arises from several sources. Even for cells that are well-matched when initially assembled into a battery pack, several forms of degradation occur at different rates for the various cells. For example, the actual lithium content can vary slightly due to manufacturing tolerances. And when in the field, operating temperature, the uniformity of the temperature distribution, vibration, and the uniformity of vibration distribution between the various cells in a battery pack can result in varying rates of cell degradation. Temperature is often the most important factor, so thermal management is a key consideration in maximizing cell lifetimes. But no matter how well temperature, vibration, and other factors are managed, cell balancing is key to maximizing battery pack performance and lifetimes.

Regardless of the cell balancing approach used, precision battery management system (BMS) ICs are available, which combine battery monitoring with cell balancing to improve overall pack performance. Performance considerations for BMS ICs include accuracy of SoC measurements and the ability to measure the overall state of health, balancing speed, efficiency, cost, and solution size. The goal of all BMS systems and cell balancing schemes is to minimized cell-to-cell SoC mismatches to improve pack performance and minimize the impact of cell aging, which can result in lost capacity.

Various active and passive techniques are used to achieve a balanced SoC for packs of battery cells. Passive cell balancing can be lower in cost, but it is quite inefficient since it involves bleeding off “excess” charge across a resistor from the cells with higher SoC. The shunt resistor can be constantly connected across the cells, or it can be switched in and out of the circuit, which is more efficient but more complex. While either the fixed or switched resistor methods may be okay for some low-cost systems, they cannot be used with Li-based batteries since they bring the risk of internal cell damage resulting in fires. Various active cell balancing techniques are often employed to improve efficiency, increase cell lifetimes and promote safety.

Active cell balancing techniques can use capacitors, inductors, or dc/dc converters to efficiently transfer charge from high SoC cells to low SoC cells as needed. Active cell balancing control topologies can be subdivided into several subcategories, including cell bypass, cell-to-cell, cell-to-pack, and pack-to-cell.

The cell bypass method can be split into three approaches: complete shunting, shunt resistors, and shunt transistors. As the name implies, in cell bypass equalization, current bypasses the cells that have reached their maximum SoC to the remaining cells until all cells are at maximum SoC. Cell bypass techniques tend to be easy to implement and relatively low in cost. However, they can only be implemented toward the end of the charging process when one or more cells have reached maximum SoC and the overall efficiency is good. Cell-to-cell methods pass the extra energy stored in a cell to adjacent ones if they have lower stored energy. While this may be more efficient than cell bypass, it is still complex to implement and slow.

In cell-to-pack equalization, energy is drawn from the most-charged cell in the pack and equally spread between the remaining cells. Pack-to-cell implementation is the mirror image of cell-to-pack. In pack-to-cell, energy is transferred from the entire pack to the least charged cell. Both cell-to-pack and pack-to-cell are lower in efficiency than cell bypass and cell-to-cell, and the complexity is relatively high. There are multiple ways to implement each of these cell balancing methods. The following are a few examples.

So-called flying capacitors can be used for cell-to-cell balancing. The capacitor is initially connected to the higher voltage cell to charge, then switched to the lower voltage cell to discharge. Since the charge is being shuttled between this method is sometimes referred to as charge shuttling. There are several disadvantages to using charge shuttling; charge can only be transferred between adjacent cells, it is less efficient since it involved energy loss during charging and discharging of the capacitor and any switching losses in the switch, and the charging, switching, discharging cycle takes more time compared with alternative methods.

Alternatively, inductors can be used in place of the capacitors to move the charge between adjacent cells. While this method is generally faster and more efficient than capacitor shuttling, it still suffers from several disadvantages; charge can be transferred only from higher cells to lower cells. There are still switching losses and a diode voltage drop to be considered.

The figure below supports cell-to-pack balancing, as well as other balancing topologies. The LT8584 monolithic flyback converter is rated 2.5A. In this case, it is used in the LTC680x family of multi-chemistry BMS IC. These devices support various cell balancing techniques; charge can be redistributed from one cell to the top of the battery pack (cell-to-pack) or to another battery cell or a combination of cells within the stack.

Summary

Cell balancing is needed to get maximum battery pack performance since performance is limited by the weakest cell in the pack. Cell balancing can be performed using passive or active techniques. Active techniques are required for lithium chemistries and are inherently more efficient than passive approaches. Active cell balancing control topologies can be subdivided into several subcategories, including cell bypass, cell-to-cell, cell-to-pack, and pack-to-cell, each offering tradeoffs in efficiency, speed, and other performance parameters.

References

Active Cell Balancing, Analog Devices
Cell Balancing for Maximum Battery Pack Performance, Ion Energy
Review of Battery Cell Balancing Methodologies for Optimizing Battery Pack Performance in Electric Vehicles, IEEE
Single Switched Capacitor Battery Balancing System Enhancements, Researchgate