When assembling large battery packs it is necessary to connect cells in series and parallel. Actually the normal method is to assemble them in parallel groups and then to assemble these groups in series.

Firstly it is worth remembering what is meant by parallel and series.

#### Cells in Parallel

Cells that are in parallel have the positive terminals all connected together and the negative terminals all connected together. The voltage of the group of cells in parallel will be the same as a single cell. The nominal capacity of the group of cells will be P multiplied by the nominal capacity of a single cell.

#### 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.

#### Parallel then Series or Series then Parallel

Both of these designs have strengths and weaknesses. Hence both have places where they are optimal.

Parallel and then series will be the lowest cost, but least flexible. Series and then parallel gives flexibility and redundancy and hence is often found in large battery packs.

### 3S3P

If we just expand this idea and first assemble a pack with 3 cells in parallel and then 3 sets of these in series we have the following schematic.

The nominal voltage of this pack would be 3x the nominal voltage of a single cell and the capacity would be 3x the nominal capacity of a single cell.

The nominal capacity of this battery configuration is given by:

Nominal Energy in Wh = S x V_{nomcell} x P x Ah_{nomcell}

The 3p3s battery pack is quite simple to visualise.

Here we see the 9 cells with connections made to bring them together in parallel and then 3 rows connected in series.

This basic principle of series and parallel can be extended to any numbers you wish to create. The diagram below shows the basic principles.

In most pack designs the cells are connected in parallel blocks (when P is greater than 1) and then in series. This is an important factor in managing the battery configuration. However, we will also discuss connecting series strings of cell in parallel as a separate article.

#### Battery Basics

An attempt to walk you through the battery basics from a single cell to multiple cells. Hopefully all of the abbreviations will be obvious, but if you’re stuck there is always a page full of them – Abbreviations.