1. Individual battery-pack cell SOCs tend to diverge from one another over time due to differences in cell characteristics (effect amplified by pack temperature gradients) 2. Unless cells are brought back into balance, SOCs diverge until pack is in a useless state, even though all cells are still perfectly healthy ("weak" cell has SOC too low to discharge; "strong" cell has SOC too high to charge) 3. Passive balancing: Drain charge individually from "strong" cells, dissipating excess energy as heat through a resistor 4. Active balancing: Use intermediate energy-storage/ conversion devices (e.g., switched capacitors, switched inductors/transformer, or more general dc-dc converters) to move charge from "strong" cells to "weak" cells (many possible architectures)
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