Passive Balancing

Passive balancing is perhaps the simplest form of cell balancing with a resistor that is switched on and off across the cell. In the example shown with the 3 cells the balancing resistor would be switched on for the centre cell. Discharging this cell and losing the energy to heat in the balance resistor (typically 30Ω to 40Ω).

passive balancing

This is ok when the balancing requirements are small. However, as the cells age the amount of balancing required to optimise the available energy is likely to increase. Resulting in increasing amounts of energy being lost to heat. This can also increase charge times when trying to reach maximum SoC for the pack.

  • Balancing currents are small. In a 100kWh pack they are typically 100 to 300mA for each of the set of parallel cells (~280Ah). This equates to 1 to 3mA/Ah.
  • This does depend on the quality of the cells and the electrical and thermal design of the system.

The Tesla Model S has a balancing resistor of 39.5Ω for a system capacity of 229.4Ah [1]. Blömeke et al [1] also look at the sizing of the balance resistor based on self-discharge currents.

References

  1. Blömeke, A., Zappen, H., Ringbeck, F. et al. Balancing resistor-based online electrochemical impedance spectroscopy in battery systems: opportunities and limitationsCommun Eng 3, 62 (2024)
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