Large stacks of series-connected battery cells are increasingly used to power electric vehicles or bank energy in wind and solar power systems. It is not uncommon to have 100 cells connected in series in an electric vehicle, and more than that in energy storage units for an alternative energy systems. Typically, the stack is treated by the charge-discharge system as a single battery—cells are charged and discharged as a series stack and the state of charge (SOC) of each cell depends on its ability to store and maintain charge. Treating the cell stack as a single battery composed of capacity-matched cells can work well in the short term, but becomes increasingly inefficient in the long run.
The LTC3300 balances the states of charge of individual cells in large battery stacks, increasing capacity, extending run time and prolonging lifetime of the stack
When a battery stack is first constructed, the capacities of its component cells can be well matched, but over time, individual cells lose capacity at different rates due to temperature variations and other factors. In a straightforward stack charge-discharge implementation, the cell with the least capacity—the weakest cell—effectively limits the run time of the stack. When the stack is charged, the weakest cell reaches its full charge voltage before stronger cells, so stronger cells are not charged to capacity. Likewise, when the stack is discharged, the weakest cell reaches its cutoff voltage sooner, limiting run time.
The capacity of the stack and its run time can be improved by balancing the state of charge between cells within the stack. Figure 1 shows a simplified schematic of a 12-cell balancer using two LTC3300-1 cell balancing controllers.
Figure 1. Simplified schematic of how the LTC3300 actively balances individual cells in a 12-cell battery stack
For a full discussion of the LTC3300-1 and how it dramatically improves the capacity, run time and lifetime of battery stacks, read the full article from the January 2013 issue of LT Journal.
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