Why Cells In Series And Parallel Behave Differently

Every battery pack larger than a single cell is an arrangement of smaller ones. Whether they are wired end to end or side by side changes almost everything about the result.

Series adds voltage, parallel adds capacity

Connecting cells end to end stacks their voltages. Four cells of a given voltage produce four times that, while the current capability stays what one cell can supply.

Connecting them side by side keeps the voltage of a single cell and adds their capacities together. The pack can now supply more current for longer.

Large packs use both. A grid of cells arranged in series groups wired in parallel delivers a chosen voltage at a chosen capacity from identical parts.

Voltage exists to reduce current

Delivering a given power at a higher voltage requires less current, and losses in wiring and connectors depend on current.

This is why substantial devices use series stacks rather than one enormous cell. The conductors, connectors and switching components can all be smaller.

It also raises the safety threshold. A stack of many cells reaches voltages that require insulation and isolation that a single cell never does.

A series chain is limited by its weakest cell

The same current flows through every cell in a series string, so the cell with the least remaining capacity empties first.

Once it is empty, the pack must stop, even though every other cell holds charge. Discharging past that point drives the weak cell into damaging territory.

Charging has the mirror problem. The strongest cell reaches full first, and continuing to push current into the string overcharges it while the others are still filling.

Balancing keeps the string usable

Management electronics monitor each cell individually and equalise them, either by bleeding charge from the leading cells or by shuttling it towards the laggards.

Without balancing, a pack loses usable capacity steadily as the cells diverge, even though no cell has failed. The spread rather than the average determines what you can access.

This is why a pack cannot be repaired by replacing one cell with a new one. A fresh cell in an aged string is a mismatch the balancer must fight continuously.

Parallel groups hide their faults

Cells in parallel share a voltage automatically, so a weak one is propped up by its neighbours and produces no obvious symptom.

The stronger cells simply supply more of the current, which ages them faster, and the imbalance grows quietly until capacity has visibly fallen.

A cell that develops an internal short in a parallel group is worse still, because every other cell in the group discharges into it with nothing limiting the current.