A battery module is a chain of cells. When one cell loses capacity, the whole module loses it with that cell, even if every other cell is in perfect health.

Why one cell sets the limit

Most battery modules connect their cells in series, so the same current flows through every cell. During discharge, the cell with the least capacity empties first. The battery management system then has to stop the whole module, because pushing that cell any lower would damage it.

Charging works the same way in reverse. The weakest cell is full first, so charging stops while the other cells still have room. The module ends up working between the limits of its weakest member.

Why the weak cell gets weaker

Cells are never perfectly identical. Small differences from manufacturing, and temperature differences inside a rack, mean some cells age faster than others. Once a cell falls behind, it has a harder life:

• It swings through a deeper share of its own capacity on every cycle.
• Its internal resistance rises, so it runs warmer than its neighbours.
• Heat and deep cycling speed up its ageing, so the gap keeps widening.

Cell balancing helps keep the cells at the same state of charge, but it cannot give a cell back capacity it has lost. Most balancing circuits simply bleed a little charge from the stronger cells, using small currents, and turn it into heat.

What it means at module and system level

• Lost energy you paid for. The healthy cells carry charge that can never be used.
• Earlier end of life. Warranties are usually tied to module capacity, so one weak cell can bring the replacement date forward.
• Difficult repairs. Cells are welded and matched inside the module. Replacing one cell is rarely practical, so the whole module is usually replaced.
• Mismatch across racks. When modules and racks are connected together, the weakest one again limits the rest, and uneven modules share current unevenly.
• Safety margin. A weak cell is the one most likely to be pushed too high or too low, which is why careful monitoring of every cell matters.

How a flow battery avoids the problem

A vanadium redox flow battery also has many cells connected in series in a stack. The difference is where the energy is stored. In a flow battery, the energy is in the electrolyte, held in tanks outside the stack. Every cell is fed from the same two tanks, so every cell sees the same state of charge. No single cell can run empty before the others.

A worn cell in a flow battery shows up differently: its resistance rises, so the stack loses a little efficiency. It does not strand stored energy. Capacity loss in a flow battery comes mainly from the two electrolytes slowly becoming unbalanced. That affects the whole system evenly, and it can largely be recovered by rebalancing the electrolyte, instead of replacing hardware.

Flow batteries have engineering challenges of their own, such as spreading the flow evenly across every cell and limiting stray currents through the shared electrolyte. These are design problems solved once in the stack, rather than a slow loss that grows cell by cell over the life of the system.