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 over 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 (see the blog "Passive Battery Cell Balancing"). 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 below 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 lifetime faster. 30% represents fully discharged to prevent deep discharge of the cells.
Over time, some cells will become weaker than others, resulting in a discharge profile represented by the figure below.
It can be seen that even though there may be quite a bit of capacity left in several batteries, the weak batteries limit the runtime of the system. A battery mismatch of 5% results in 5% of the capacity 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 since it results in an increase in the number of battery charge and discharge cycles, which reduces the lifetime of the battery, leading 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.
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 our system can hold. The diagram below 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 important considerations.
Active Cell Balancers
Analog Devices Inc. has a family of active cell balancers, with each device targeting different system requirements. The LT8584 is a 2.5A discharge current, monolithic flyback converter used in conjunction with the LTC680x family of multichemistry battery cell monitors; charge can be redistributed from one cell to the top of the battery stack or 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 that provides up to 10A of balancing current; since it is bidirectional, charge from any selected cell can be transferred at high efficiency to or from 12 or more adjacent cells. A single LTC3300 can balance up to six cells.
The LTC3305 is a standalone lead acid battery balancer for up to four cells; it uses a fifth reservoir battery cell (AUX) and continuously places it in parallel with each of the other batteries (one at a time) to balance all battery cells (lead acid batteries are rugged and can handle this).
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 the charging efficiency. Active balancing requires a more complex, larger footprint solution; passive balancing is more cost effective. Whichever method works best in your application, Analog Devices Inc. offers solutions for both, integrated into our battery management ICs (such as the LTC6803 and LTC6804) and complementary devices that work in conjunction with these ICs to provide a precise, robust battery management system.