Battery Pack

A single cell rarely matches a project's needs, so cells are combined into packs. This tool shows what a series and parallel layout gives you.

The theory

• Cells in series (end to end) add their voltage, and the capacity stays the same.
• Cells in parallel (side by side) add their capacity (current-delivering ability), and the voltage stays the same.

The layout is written xSyP: a 3S2P pack has 3 groups in series, each with 2 cells in parallel, for 6 cells. Then:

• Nominal voltage = cell nominal voltage × S
• Capacity = cell capacity × P
• Energy (Wh) = nominal voltage × capacity in amp-hours
• Maximum current = C-rate × pack capacity (for example, 2C on 5 Ah is 10 A)

Typical per-cell voltages used here (check the cell datasheet): Li-ion/LiPo 3.7 nominal, 4.2 full, 3.0 empty; LiFePO4 3.2, 3.65, 2.5; NiMH 1.2, 1.45, 1.0; alkaline 1.5, 1.6, 0.9; lead-acid per 2 V cell 2.0, 2.35, 1.75.

Step by step

1. Open the Battery Pack tab.
2. In Cell chemistry, choose the type of cell.
3. In Capacity of one cell (mAh), type it, for example 2500.
4. In Cells in series (S) and Cells in parallel (P), type your layout, for example 3 and 2.
5. Optionally, in Max discharge rate (C, optional), type the cell's continuous C-rating from its datasheet.
6. Read the layout, the voltages, the capacity, and the stored energy.

Worked example

Six 2500 mAh Li-ion cells as 3S2P: a nominal 11.1 V (12.6 V full, 9.0 V empty), 5000 mAh, and 55.5 Wh. At a 2C rating it can supply 10 A.

Things to watch

• Series lithium packs need a battery management system (BMS) to balance the cells and protect against overcharge, over-discharge, and short circuits.
• Use matched cells: the same type, brand, age, and charge level. Never mix chemistries.
• Parallel cells should be connected at the same voltage first, or large currents will flow between them.
• Lithium cells can catch fire if mistreated. Follow the manufacturer's guidance.