Cell Balancing

Cell balancing is all about the dissipation or movement of energy between cells. The aim being to align them all with respect to state of charge. Aligning the state of charge of all of the cells in a pack will allow the pack to deliver the most energy and power.

A set of cells connected in series will all see the same current, whether they are generating it when discharged or accepting it when charged. If all of the cells have the same initial SoC and the same capacity then they should charge and discharge equally.

cells in series at different SoC

However, if one cell has a slightly lower capacity it will be discharged further in SoC and it will go slightly overvoltage each time it is charged up. Over time this will result in a further reduction in it’s capacity.

cell balancing

When we then come to recharge this group of cells the centre cell will reach the maximum cell voltage first and the BMS will prevent further charging.

We need to remove charge from this cell so that we can charge the other cells in the string.

If we balance the cells then we can recharge them all to their maximum SoC. This means we need to ideally move charge from the higher SoC cell to the lower SoC cells.

cell balancing

After balancing the centre cell will have a lower SoC, but the 3 cells will all be at the same SoC and so we can now charge them up to the maximum voltage.

cell balancing

Fundamentally there are two methods of cell balancing:

  1. Passive balancing
  2. Active balancing

Passive Balancing

This simple form of balancing switches a resistor across the cells. 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.

Active Balancing

The idea here is to redistribute the energy across the cells. Give energy from the cells with the highest SoC to the cells with the lowest SoC. This is the ideal cell balancing approach.

active balancing

However, this means the system has to be able to move energy between cells in the pack. Ideally between any two cells in the pack. Lots of wires and lots of switches means more weight, complexity and cost.

Thoughts

This immediately raises some questions that we need to think about:

  • How do I size the balancing resistor?
    • Balancing currents are small. In a 100kWh pack they are typically 100 to 200mA for each of the set of parallel cells (~280Ah).
  • How out of balance can the cells be before needing to be balanced?
    • If the cells are within the error of what you BMS can measure then don’t balance. If the error is measurable and say reducing the capacity or power capability by 10% then you should balance the cells.
    • There is a fine line between balancing to improve the pack performance and balancing continuously. Therefore it is important to set limits on when to start and stop balancing. Any algorithm needs testing on new and old packs to ensure that it is stable.
  • Do we balance when charging or discharging?
    • Balance when charging and at a low charge rate. You want the cells to be as close together in terms of temperature and current flow with no extreme or fast changes that may take time to settle.
  • What is the impact on cell lifetime?
    • Balancing will improve the overall pack lifetime as you will not be pushing some cells over voltage in order to charge the pack to 100%.
  • How different are a batch of new cells?
    • Cell production quality is improving all the time. The quantity often measured is cell capacity and this is getting tighter. One reason for this improvement is that cells are sold based on capacity and hence it is important for the manufacturer to control this spread.

SoC from OCV lookup

SoC Estimation Techniques

A look at the estimation of State of Charge (SoC) using voltage profiling and coulomb counting. These two methods give a good overview of the difficulty and errors associated in estimating this critical battery parameter.

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