Building a Battery Pack: Series & Parallel Cells

Wiring multiple cells together to raise voltage, raise capacity, or both — and the one safety rule that matters most.

You'll need: several identical battery cells (same chemistry, same capacity, similar charge level — AA or AAA cells are the easiest and safest to practice with), a battery holder, and a multimeter.

Connecting cells in series — positive terminal of one to the negative terminal of the next — adds their voltages together while keeping the same capacity as a single cell: four 1.5V AA cells in series give 6V. This is exactly the same principle as the series LED circuit from Lesson cb-03, just applied to power sources instead of loads.

Connecting cells in parallel — all positive terminals joined, all negative terminals joined — keeps the same voltage as a single cell but adds their capacities together, letting the pack run longer or supply more current before depleting.

The one rule that matters most: never mix cells of different types, ages, or charge levels in the same pack, especially in series. A weaker or different cell forced to match a stronger neighbor's current can be driven into reverse charging or overheating — a real safety hazard, particularly with rechargeable chemistries. Always build packs from cells that started matched, and ideally replace or recharge them as a full set.