| Battery mismatch issue, fatal if large capacity battery
| Frequent battery charging and discharging due to mismatch reduces battery life
| Active balancing, less charging time and heat generation than passive

As applications using batteries increase in number, from smartphones to electric vehicles, engineers are now starting to think about how to use batteries efficiently and reduce replacement costs.

Monitoring each cell in the battery stack using passive and active cell balancing maintains a healthy battery state of charge (SoC). This not only extends battery cycle life, but also provides an additional level of protection by preventing battery cell damage due to over-discharge.

Passive balancing dissipates excess charge through external resistors to ensure that all battery cells have similar SoCs, but does not increase system runtime.

Active balancing is a more complex balancing technique. It increases the total charge available in the battery stack by redistributing charge between battery cells during the charge and discharge cycles, thereby increasing the system runtime. It reduces charge time and reduces heat generated during balancing compared to using passive balancing.


Active cell balancing during discharge
The diagram in Figure 1 shows a typical battery stack where all cells start at full capacity. In this figure, full capacity is shown as 90% charge. This is because if a battery is kept at close to 100% capacity for a long period of time, its lifespan will decrease rapidly. The maximum discharge is 30%, which is to prevent over-discharge.

Over time, some cells become weaker than others, resulting in a discharge profile similar to that in Figure 2.

As you can see in the picture, there is considerable capacity left in many cells, but weak cells are the limiting factor, limiting the system usage time.

A 5% battery mismatch means 5% of the capacity is unused. Therefore, for large capacity batteries, the amount of unused energy can be significant.

Unused energy results in increased battery charging and discharging, which in turn reduces battery life and increases costs by requiring more frequent battery replacement.

By using active balancing, charge can be redistributed from strong cells to weak cells. This allows achieving a fully discharged battery stack profile.


Active cell balancing during charging
When charging a battery stack without balancing, the weaker cells will reach full capacity before the stronger cells. In this case, the weaker cells will also limit the total charge that can be stored in the system. Figure 4 shows this limitation during charging.

Active balancing allows the stack to be fully charged by redistributing charge during charging. This article does not address the time allocated to balancing or the effect of balancing current on balancing time, but these issues are also important to consider.


ADI Active Cell Balancer Products
ADI offers a variety of active cell balancer products to meet a variety of system requirements. The LT8584 is a 2.5A discharge current monolithic flyback converter ideal for use with the LTC680x multi-material battery cell monitors.

Charge can be redistributed from one cell to the top cell of the battery stack, to another battery cell or combination of cells within the stack. One LT8584 is used per battery cell.

The LTC3300 is a standalone bidirectional flyback controller for lithium and LiFePO4 batteries, providing up to 10A of balancing current.

This product supports bidirectional, allowing charge from one cell to be transferred to 12 or more adjacent cells with high efficiency. A single LTC3300 can balance up to six cells.

The LTC3305 is a standalone lead acid battery balancer that can balance up to four cells. It uses a fifth storage battery cell (Aux) and places it in parallel with each of the other batteries, balancing all the battery cells one at a time. This is possible because lead acid batteries are robust.


Depending on the application, you need to choose the appropriate method.
Active and passive balancing are effective ways to improve system health by monitoring and matching the SoC of each cell.

Active cell balancing redistributes charge during charging and discharging, whereas passive cell balancing dissipates charge during the charge cycle. Therefore, active cell balancing increases the system usage time and improves the charging efficiency. However, active balancing requires a more complex and larger footprint solution. Passive balancing is more economical. Which technique is more suitable will depend on the application.

ADI offers solutions that can be applied to both techniques, either integrated into battery management ICs (LTC6803 and LTC6804) or as devices that can be paired with these ICs to achieve a precise and robust battery management system.


This article is adapted from an article titled “Active Battery Cell Balancing” by Kevin Scott, Director of Product Marketing for Analog Devices’ Power Products Group, and Sam Nork, Director of Analog Devices’ Boston Design Center.