• • • BATTERIES & CHARGERS • • •


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


Figure 2. Mismatched 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 per cent of charge because keeping a battery at or near its 100 per cent capacity point for long periods of time degrades its lifetime faster. The 30 per cent 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 by 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


Figure 3. Full depletion with active balancing


ACTIVE BATTERY CELL BALANCING W


By Kevin Scott and Sam Nork, Analog Devices


ith passive and active cell balancing, each cell in the battery stack is monitored to


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 1. Full capacity


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 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 important considerations.


ANALOG DEVICES ACTIVE CELL BALANCERS


Figure 4. Charging without balancing


Analog Devices has a family of active cell balancers, with each device targeting different system requirements. The LT8584 is a 2.5 A 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 10 A 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


26 ELECTRICAL ENGINEERING • JULY/AUGUST 2021 electricalengineeringmagazine.co.uk


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