With passive and active cell balancing, each cell in the battery stack is monitored to maintain a healthy battery state of charge (SoC). This extends battery cycle life and provides an added layer of protection by preventing damage to a battery cell due to deep discharging because of overcharging. Passive balancing results in all battery cells having a similar SoC by simply dissipating excess charge in a bleed resistor; it does not, however, extend system run time. 1
Active cell balancing is a more complex balancing technique that redistributes charge between battery cells during the charge and discharge cycles, thereby increasing system run time by increasing the total useable charge in the battery stack, decreasing charge time compared with passive balancing, and decreasing heat generated while balancing.
Active Cell Balancing During Discharge
The diagram in Figure 1 represents a typical battery stack with all cells starting at full capacity. In this example, full capacity is shown as 90% of charge because keeping a battery at or near its 100% capacity point for long periods of time degrades its lifetime faster. The 30% discharge represents being fully discharged to prevent deep discharge of the cells.
Over time, some cells will become weaker than others, resulting in a discharge profile, as represented in Figure 2.
It can be seen that even though there may be quite a bit of capacity left in several batteries, the weak batteries limit the run time of the system. A battery mismatch of 5% results in 5% of the capacity being unused. With large batteries, this can be an excessive amount of energy left unused. This becomes critical in remote systems and systems that are difficult to access. As a result, there is a portion of energy that cannot be used, which results in an increase in the number of battery charge and discharge cycles. Furthermore, this unused energy reduces the lifetime of the battery and leads to higher costs associated with more frequent battery replacement.
With active balancing, charge is redistributed from the stronger cells to the weaker cells, resulting in a fully depleted battery stack profile (Figure 3).
Active Cell Balancing While Charging
When charging the battery stack without balancing, the weak cells reach full capacity prior to the stronger batteries. Again, it is the weak cells that are the limiting factor. In this case, they limit how much total charge the system can hold. The diagram in Figure 4 illustrates charging with this limitation.
With active balancing charge redistribution during the charging cycle, the stack can reach its full capacity. Note that factors such as the percentage of time allotted for balancing and the effect of the selected balancing current on the balancing time are not discussed here but are still important considerations.
Active Cell Balancers
Active cell balancing controllers perform battery management, with the ability to match different system requirements and battery chemistries. Balancing is achieved by redistributing charge from one cell to the top of the battery stack or to another battery cell or combination of cells within the stack (Figure 5). A 2.5-A discharge current monolithic flyback converter was used in conjunction with multi-chemistry battery cell monitors. One flyback converter is used per battery cell.
The use of a bidirectional controller allows charge from any selected cell to be transferred at high efficiency to or from 12 or more adjacent cells (Figure 6). This enables a single flyback controller to balance up to six cells.
For lead acid batteries, another reservoir battery cell (Aux) is used to balance cells (Figure 7). A balancer can balance up to four cells by continuously placing the Aux cell in parallel with each of the other batteries, one at a time. This is possible because lead acid batteries are rugged and can handle this method for balancing cells.
Both active and passive cell balancing are effective ways to improve system health by monitoring and matching the SoC of each cell. Active cell balancing redistributes charge during the charging and discharging cycle, unlike passive cell balancing, which simply dissipates charge during the charge cycle. Thus, active cell balancing increases system run time and can increase charging efficiency. Active balancing requires a more complex, larger footprint solution; passive balancing is more cost-effective and provides a precise, robust battery management system.
This article was written by Kevin Scott, product marketing manager for the Power Products Group, and Sam Nork, director, Boston Design Center, for Analog Devices (Norwood, MA). For more information, visit here .
- Kevin Scott and Sam Nork. “Passive Battery Cell Balancing.” Analog Devices, Inc.