Simple 3S1P battery pack for 21700 cells
The purpose of this battery was to make a simple and small 3S battery which helps when testing components, such as electric motors. Before building a bigger battery pack dedicated to the project. The 3D models were developed in Onshape.
Capacity and discharge calculations
Each cell is a Samsung INR21700-53G, rated for 5,140 mAh with a nominal voltage of 3.63V. Since the pack is 3S1P (three cells in series, one in parallel), the capacity stays the same as a single cell while the voltages add up across the three cells in series:
- Nominal pack voltage: 3 × 3.63V = 10.89V
- Fully charged: 3 × 4.2V = 12.6V
- Energy: 5.14Ah × 10.89V ≈ 56 Wh
The cell is rated for a continuous discharge current of 1C (5.3A) without affecting its cycle life, and up to 3C (15.9A) for short bursts. Since the pack is 1P, that rating applies directly to the whole pack rather than being split between parallel cells. That gives enough headroom to test small motors without pushing the cells past their rated limits.
Building
It is important to not solder directly on the battery packs since the heat can damage the cells. Therefore the wires was soldered first to the nickel strips and then spot welded to the cells.
