Cell Matching

What level of cell matching do you do prior to assembling a battery pack? Assuming the battery pack will be balanced the first time it is charged and in use. Also, assuming the cells are assembled in series.

  1. none, force the cell supplier to deliver cells matched to within +/-0.02V
  2. none, gross balance the pack during first charge once built
  3. preselect and group cells prior to build
  4. pre-charge/discharge all in-coming cells to a set voltage/SOC
  5. average-balance cells in parallel group prior to building in series
  6. average top-balance cells in parallel group prior to building in series

If the cells are very different in State of Charge (SoC) when assembled the Battery Management System (BMS) will have to gross balance the cells on the first charge. This can take a long time as the maintenance balancing currents are generally very small compared to the Ah ratings of the cells (1 to 3mA/Ah).

cells in series 3S1P

When connecting cells in series the negative terminal of the first cell is connected to the positive terminal of the second cell. The negative terminal of the second cell is connected to the positive terminal of the third cell. This continues until we reach the total number of cells required in series.

The nominal voltage of the final set of cells is the number of cells in series times the nominal voltage of a single cell.

1. Supplier Delivers Matched Cells

If the cell manufacturer can deliver cells with a proven quality history of OCV within +/-0.02V then you will be able to assemble and charge these cells without gross balancing.

However, you will need to consider a few things:

  • cell manufacture, formation, ageing end of line testing all have reporting and metrics
  • logistics around transport minimises time and temperature swings
  • local storage of the cells carefully controls storage time and temperature

What is measured at Cell manufacturing end of line should be remeasured at Goods receipt.

Cells will be placed on Sample test, built into Modules & Packs or placed in storage and at a bare minimum, an OCV confirmation check will be necessary prior to any test or build or selection from storage.

The Manufacturer will in general specify OCV, SoC & (delta OCV / time), insulation resistance etc at defined environment in case of anomalies in transport and/or storage or buffered lot builds.

This is the preferred volume build approach, however, a number of smaller pack manufacturers will not get this level of support from the cell manufacturer.

2. Gross Balance Pack

This is what you are probably trying to avoid as it can take hours or even days for the pack balancing to remove large SoC differences.

An SoC difference of 10% on a 100Ah cell will take 100 hours to remove with a 100mA balancing current. This means you will need to put the pack on charge for >4 days at the end of the manufacturing process. This costs in capital equipment, energy required to charge the pack and inventory costs of holding packs post production for 4 days.

3. Select Best Cells

This is the approach used by the satellite industry [2] and adopted by motorsport. The cells undergo a number of checks from visual inspection, capacity and internal resistance measurement before finally selecting the best cells.

This is an expensive approach in both time, equipment and number of cells that are processed and rejected.

4. Preselect and Group Cells

Similar to option 3, but using just OCV to group cells such that the initial SoC of the cells in a pack will not require gross balancing.

This does mean that you need to measure the voltage of all incoming cells and bin then into a voltage range.

5. Pre-charge/Discharge Cells

Prior to assembling the battery packs you can charge/discharge all of the cells to a defined voltage. This ensures all of the cells are matched in SoC prior to assembly.

6. Average-Balance Cells

Before assembling the cells in series you connect them all together in parallel [1]. This will discharge the higher SoC cells and charge the lower SoC cells. However, you need to be careful that the cells are reasonably close in state of charge before connecting them in parallel as cells that are a long way off will be charged/discharged very rapidly.

Doesn’t completely solve the problem as it balances them at a low soc (they will have been delivered close to 30% SoC) and when you first charge the pack it will still need to do a top of charge balance. The overall balance will be a lot closer though.

7. Average Top-Balance Cells

Before assembling the cells in series you connect them all together in parallel with a charger that brings them all to an upper SoC/voltage [1]. This alignment at a higher voltage will produce a better balanced battery pack with less balancing required during the first charge cycle.

Same warning as for option 6, you need to be careful that the cells are reasonably close in state of charge before connecting them in parallel as cells that are a long way off will be charged/discharged very rapidly.

Notes

The best approach is to ensure the cell supplier makes and ships cells to a number of quality metrics, including state of charge.

However, it is still likely that cells will evolve and change batch to batch [4], hence the pack manufacturer needs to take this into account.

References

  1. Cell pre-balancing, Li-Ion BMS
  2. Chris Pearson, Carl Thwaite, Nick Russel, The use of Small Cell Lithium-Ion Batteries for Small Satellite Applications, AEA Technology
  3. Battery Pack Cell Voltage Difference and Solution Part 2, GrePow
  4. M. Schindler, J. Sturm, S. Ludwig, J. Schmitt, A. Jossen, Evolution of initial cell-to-cell variations during a three-year production cycle, eTransportation, Volume 8, 2021

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