When sizing a battery pack one of the first things to look at is the number of cells in series and pack voltage.
Pack Nominal Voltage = Cell Nominal Voltage x Number of Cells in Series
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.
If we look at the battery packs out there we can see that they cover the range of nominal voltages from 3.2V to 820V in the graph (plotted from the Battery Pack Database).
This also shows two distinct sets of data and that is fundamentally down to the two dominant chemistries currently being used, LFP and NMC/NCA.
LFP has a nominal cell voltage of ~3.2V and NMC/NCA has a nominal of ~3.6V
This post has been built based on the support and sponsorship from: Thermo Fisher Scientific, Eatron Technologies, About:Energy, Quarto Technical Services and Alvatek.
Variations in the NMC/NCA chemistry and hence nominal cell voltage can be seen particularly in the packs with 96 cells in series.
We also need to consider the maximum and minimum pack voltage.
The maximum to minimum voltage swing increases as we increase the number of cells in series.
The maximum voltage is important as the charging system requirements need to be checked to ensure it can charge the pack to the correct State of Charge (SOC).
The minimum voltage is important to understand as elements of the system may not be able to operate or fully operate if the voltage is too low.
Depending on the battery pack application it is normal to restrict the usable state of charge window to improve the lifetime and / or power delivery. This will limit the maximum and minimum pack voltage.
