How to increase capacity of a battery using Parallel Connection?

  • Combining cells in series increases the voltage, yet it has no effect at all on the capacity of the battery. Combining 10 Lithium-Ion cells that are each rated for 3.5 Ah in series will result in a 37V 3.5Ah battery. That’s a decently high voltage, but very low capacity for most applications. In order to increase the capacity of the battery, we must combine cells in parallel.

  • Parallel connections can be thought of as the opposite of a series connection. Instead of connecting the positive terminal of one cell to the negative terminal of the next cell as in series connections, parallel connections are made by connecting the same terminals together. To connect two cells in parallel, you simply connect the positive terminal of the first cell to the positive terminal of the second cell, and then connect the negative terminal of the first cell to the negative terminal of the second cell. This essentially creates one larger cell, because the two cells are now sharing the same terminals and function as one battery cell.

  • One important safety note: before you connect any battery cells or battery packs in parallel, you must ensure that they have nearly identical voltages. If the voltages are off by a large amount, it means that one cell is at a higher state of charge than the other. When you connect cells with mismatched charge levels in parallel, the higher charged cell will try to charge the lower charged cell. If the difference in charge is large, the higher charged cell will try to dump a large amount of energy at once into the lower charged cell. This high current flow will damage both cells and can result in the cells overheating or catching on fire. Always check that cells are very close or identical in voltage before connecting them in parallel.

  • Let’s look at some simple math. Calculating the total capacity in Ah of battery cells connected in parallel is easy: just multiply the number of cells connected in parallel by the capacity of the individual cells.

  • Capacity of parallel cells = The number of cells in parallel × Capacity of a single cell in Ah

  • For example, let’s say that we have two Lithium-Ion cells, each with a nominal voltage of 3.7 V and rated for 3.5 Ah. If we connect them in parallel by joining their positive terminals together and then their negative terminals together, we will have created a 3.7V 7.0Ah battery pack, as shown in the diagram. If we add one more cell in parallel with the first two, that will create a 3.7V 10.5Ah battery pack. If we connect 10 of these cells together in parallel, that will create a 3.7V 35Ah battery pack.