Active and Passive Battery Pack Balancing Methods

When a battery pack is designed using multiple cells in series, it is essential to design the system such that the cell voltages are balanced to optimize performance and life cycles. Typically, cell balancing is accomplished by means of by-passing some of the cells during the charge or discharge cycles. Adopting precise cell balancing achieves a larger capacity for the intended application because the state of charge (SoC) that can be accomplished is higher as explained in Understanding the Role of Cell Balancing in Battery Packs. 

Figure 1. Using multiple battery cells in series requires a design where the cell voltages are balanced, optimizing performance and life cycles.

Several techniques can be employed to achieve appropriate cell balancing based on the specifications and application in hand. Read on to learn more about the different techniques used for balancing interconnected cells.

Cell Balancing Techniques

The basic ways of balancing the cells can be classified as active cell balancing and passive cell balancing [1]. These are means for equalizing the voltage and state of charge when the cells are completely charged. 

Active Cell Balancing

The active cell balancing technique uses inductive charge shuttling or capacitive charge shuttling to transfer the charge between the cells. This technique is proven to be an efficient approach as it transfers energy to where the energy is needed instead of wasting it. However, this demands additional components to be added to the system which in turn translates to increased cost.

Passive Cell Balancing

The passive cell balancing technique uses the idea of discharging the cells through a bypass route that is mostly dissipative in nature. It is simple and easier to implement than active balancing techniques as the bypass can either be external or be integrated — keeping the system more cost-effective either way. However, since all the excess energy is dissipated as heat, battery run time is adversely impacted and is less likely to be used during discharge.

Hardware Implementations of Cell Balancing

One of the simple techniques to balance the cells would be by means of a current bypass [2]. The bypass transistors are placed in parallel with the cells and are turned-on when voltage difference is detected using a comparator. Voltage-based control algorithms are used for the detection of voltage differences. However, the energy of the bypassed charge is wasted in the process. 

The other approach is based on charge redistribution. This overcomes the disadvantage of charge wastage in the current bypass technique by offering an efficient cell-balancing approach that allows the high cells to drain to the bottom. The best way to accomplish this is by not having any cells connected in series in the battery pack. This enables the step-up converter to ensure that sufficient voltage is attained by the device. However, the drawback of this technique is increased design complexity, size, and poor efficiency of the power supply.

There are other solutions that enable energy transfer from high cells to low cells using specific circuits rather than using the bypass resistor for the same. One such approach is to redistribute the energy between the cells by connecting the capacitor to a high cell and a low cell and is typically referred to as the charge shuttles method. This method also allows faster equilibration in cells placed far apart in the pack by providing the capability of remote cell connection as shown in Figure 2 [3]. The drawback of this technique is high losses incurred during the charging phase of the capacitors. The known efficiency of this process is only about 50% and efficiency is higher only during the end of discharge as the transfer rate is proportional to voltage differences.

Figure 2. Shuttle circuit with remote cells connection capability

A cell-balancing method called inductive converters overcomes the disadvantage of small voltage differences between cells. In this method, the battery pack energy is transferred to a single cell by channeling the battery pack current through a transformer as shown in Figure 3 [4]. The transformer is connected to the cell that requires an additional charge. The downside of this approach is the use of an additional transformer which leads to an increase in cost and size along with reduced overall efficiency. 

Figure 3. Inductive converter cell balancing circuit

A Look at the Balancing Algorithm

Apart from the previously discussed details regarding the basic cell balancing methods and hardware implementation solutions, it is necessary to understand the balancing algorithm — specifically the decision on when to turn on the bypass switch or when to enable energy exchange for a particular cell based on the need. This is based on the voltage difference between the cells as discussed before and is applied based on a threshold value set. If the voltage difference exceeds the threshold value set, a bypass or energy transfer is initiated. 

The implementation of the cell-voltage-difference-based algorithms can be accomplished by balancing during the charging cycle only in portable applications in order to save the energy gained. It can also be achieved by balancing at high states of charge only in order to avoid the adverse effects of unbalance in impedance. Simultaneous cell balancing can also be accomplished for multiple cells at once by means of comparator-based circuit solutions which facilitate the decision of bypass or energy transfer considering the entire battery pack.

Key References

• Anton Beck, “Why proper cell balancing is necessary in battery packs”, Battery Power.
• Yevgen Barsukov, “Battery cell balancing: What to balance and how”, Texas Instruments.
• S. W. Moore and P. J. Schneider, Delfi application note 2001-01-0959.