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Extending hybrid/EV battery packs' lifetime (Part 2)

Posted: 04 Jan 2012     Print Version  Bookmark and Share

Keywords:passive balancing  battery pack  charge/discharge cycles 

Adjusting the state of charge (SOC) of all cells in the pack is the goal of passive balancing, such that the maximum amount of energy can be safely extracted from the battery pack. Passive balancers do not create or contribute charge to the pack, which means that the cells with the lowest capacity in the battery pack will determine the pack's useful capacity. To maximise the pack capacity the balancer needs to ensure that batteries with lower capacity and SOC are allowed to fully charge and discharge.

A battery's total stored energy will only be used if it is allowed to be fully charged and fully discharged, meaning that the weakest cell should be the cell that finishes charging and discharging first. The main concern of a passive balancing scheme is to be able to identify the cells with higher capacity.

The SOC of a battery is reflected in the open circuit voltage of the battery and is an indicator of the percentage of the remaining energy. Two cells having the same SOC does not mean the cells are storing the same amount of energy; a higher capacity cell will always have more stored energy at a given SOC than a lower capacity cell.

The balancing software control algorithm is designed to coordinate balancing with the charger, and is enabled at the beginning of the charge cycle. Because passive balancing can only remove energy from the battery pack, it makes no sense to balance while the pack is discharging. This also eliminates the possibility that a cell with lower capacity will be equalised to the SOC of a higher capacity cell, which would decrease available capacity during discharge.

Once the charge cycle has started, the cell voltages are stored before the charger is connected; the balancer should determine which cell had the lowest cell voltage at the beginning of the charge cycle, this cell will be referred to as Clow. When the charge cycle is complete, indicated by one cell reaching the predetermined maximum voltage limit, the cell voltages are again stored. In both cases the cell voltages are measured with no load current and after a short period of settling.

Balancing is required if the Clow's measured voltage isn't the highest voltage after the charge cycle is completed. Clow's voltage after the charge cycle is set to Vbalance. The bleed resistors should be activated for cells in the pack that have a measured voltage that is higher than Vbalance. Balancing switches should remain on until all individual cell voltages match the Vbalance voltage. After balancing has occurred, the batteries resume charging to completely charge the cells. To see the impact of passive balancing, two tests were conducted and the results follow.

Figure 1: Pack 1 cell voltages after charge cycle.

Test results: Battery pack 1
Battery pack 1 was cycled through 100 charge/discharge cycles. Figure 1 shows the six cell voltages recorded after a number of cycles.

The figure shows the measured cell voltages at the end of a complete charge cycle after a short period of relaxation. The imbalance between cell voltages after charge is related to small variations in capacity and internal resistance. On the first complete cycle the battery pack's capacity was measured to be 2.072 AHrs, after 100 cycles the capacity measured 2.043 AHrs, a small decrease in capacity as cycle count increased.


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