Cells in Series

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.

cells in series 3S1P

Cells are arranged in series to increase the pack voltage.

Applying a load across the terminals of the three cells, a current will flow. As the cells are all in series the same current will flow through all of the cells. There is only one path for current flow in a series circuit.

Three 4.8Ah cells connected in series and fully charged to 4.2V / cell and hence 12.6V would be measured for the string of 3 cells.

cells connected in series

Hence, if all of the cells behave the same they will start with 4.8Ah and if we draw 2.4A for one hour they will finish with 2.4Ah. The cells at half capacity or 50% State of Charge, SoC.

cells in series at 50% SoC

If there are differences in the cells then they might discharge to a different point.

Discharging to 50% SoC doesn’t cause too many issues, but if we then charge the cells back up to their maximum capacity.

cells in series recharge

The 1st and 3rd cells are fully charged and have reached their maximum cell voltage of 4.2V, we cannot charge them any further without risking damage.

The 2nd cell is not completely charged. Hence our overall pack capacity is reduced. There are a few ways of approaching this problem:

  1. Only use perfectly matched cells and operate them in a uniform thermal environment.
  2. Balance the cells electrically to realign them.

This has been described as happening over just one cycle. In reality this is more likely to happen over a number of cycles and smaller differences that accumulate with use.

What could lead to differences between cell capacities?

3s3p connection of cells

Series and Parallel

In battery pack designs it is necessary to connect cells in series and in parallel to meet operating voltage and capacity requirements.

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