What Happens When One Cell in a Pack Falls Behind

Battery packs are built from many individual cells, and they age at slightly different rates. The consequences of that divergence are larger than the divergence itself.

Cells in series must be charged and discharged together

Current flows through every cell in a series string equally. There is no way for one cell to take less charge or deliver less current than its neighbours.

If one cell holds less energy, it reaches empty first while the others still have capacity. The pack must stop there, because discharging that cell further damages it.

The same applies at the top. The weak cell reaches full charge first, and charging must stop before the others are full.

The pack's usable range is set by the weakest member

Combining both limits, the usable window is the window of the weakest cell. Everything else in the pack is carrying capacity that cannot be reached.

This is why a pack can lose range disproportionately as it ages. The average cell may still be healthy while one outlier defines the outcome.

It also explains why range decline is often sudden rather than gradual. Nothing changes noticeably until one cell drifts far enough to become the binding constraint.

Balancing slows the divergence rather than fixing it

Management circuitry monitors each series group and bleeds off small amounts of charge from those running ahead. This keeps the group voltages aligned near the top of charge.

The currents involved are very small, so balancing takes time and works best when a pack is left on charge after it appears full.

Balancing corrects state-of-charge differences. It cannot correct capacity differences, which is what actually develops as cells age unevenly.

Position inside the pack drives the divergence

Cells in the middle of a pack run hotter, because they are surrounded by other cells and have the least path to ambient air.

Heat accelerates ageing, so the interior cells degrade faster. The divergence is therefore largely a thermal design outcome rather than a manufacturing defect.

Packs with better spacing and thermal paths age more evenly. This is a genuine difference between well and poorly engineered packs that is invisible from outside.

Replacing individual cells rarely works

It is tempting to substitute a new cell for a weak one. The new cell has different capacity and internal resistance from its aged neighbours.

The mismatch means the pack is now unbalanced in the other direction, and the new cell is stressed differently. Improvement is usually short-lived.

Rebuilding with matched cells throughout is the approach that works, and it is skilled work involving significant hazards. It is not a task to attempt casually at home.