Formation, aging and capacity grading: how to specify and accept the equipment

The last steps of cell production take days and much of the floor space. The equipment quote should say exactly which recipe it can run.

By BatteryEquip Editorial Team · 2026-11-04

After a cell is filled with electrolyte and sealed it still cannot be sold. It has to be charged for the first time under control, stored, measured and sorted. These steps are called formation, aging and capacity grading. They are slow, they need many channels, and they decide much of the cell's later life, which makes the equipment specification important.

What formation does

On the first charge, part of the electrolyte decomposes on the anode surface and forms a thin layer called the solid electrolyte interphase (SEI). A stable SEI lets lithium ions pass while protecting the electrolyte from further reaction. Schomburg and co-authors, in their 2024 review, describe formation as the step that creates and stabilises this layer and survey the many factors that influence it. In practice the formation protocol is set by the cell maker and treated as its own know-how.

Formation usually starts with low currents and may include rest periods, temperature control and, for some formats, pressure on the cell. Gas is produced, so pouch cells are often formed with a gas pocket that is cut off and resealed afterwards, and prismatic cells may be formed under negative pressure through an open fill port.

Why it takes so long

Wood, Li and An (Joule, 2019) report that wetting and formation can take three to seven days and aging up to two further weeks, and that these steps can occupy up to a quarter of a plant's floor space. Engineers are working to shorten that time, but a buyer should assume the recipe is set by the cell maker, not by the equipment supplier. The equipment has to run that recipe reliably across thousands of channels.

Aging and grading

During aging the cells rest, often at controlled temperature. OCV is measured at the start and end, and cells with an unusual voltage drop are rejected as possible internal shorts. The OCV, ACIR and DCIR page explains these measurements and why the voltage signal is small.

Capacity grading discharges and charges each cell to measure its capacity, then sorts cells into grades so that cells in one module match. It is a full cycle per cell and so needs as many channels as formation.

What to specify

Start with the recipe: the current and voltage steps, the cutoffs, the rests, the temperature and any pressure. Then translate it into equipment terms.

Channels: the number of cells in parallel, the current range per channel and the accuracy on the range you will actually use. A channel rated for 300 A measures a 5 A step less precisely than a 10 A channel.

Temperature: whether control is per bay, per tray or per cell, and the uniformity across a tray. Blade and large prismatic cells generate heat during formation, so some systems use water cooling.

Pressure and gas: for pouch cells, the clamping force and its uniformity; for prismatic cells, the negative-pressure level and how electrolyte carried out with the gas is handled.

Energy: regenerative channels return discharge energy to the site supply instead of burning it in resistors. Ask for the conversion efficiency and its conditions.

Data and safety: every cell needs a record linked to its identifier, and the system needs smoke detection, isolation and a plan for a cell that overheats in a closed bay.

Examples on BatteryEquip

Production systems include the Hangke prismatic negative-pressure formation system, the Hangke blade-cell formation machine with independent bay temperature, the Geesun modular formation and capacity-grading family and the Repower high-voltage series formation system. For pouch cells, compare the TMAX heated force-controlled formation station and the Gelon post-formation degassing sealer. For laboratory work, the WinAck heated negative-pressure cabinet covers small prismatic cells. Each figure here is the maker's own; ask for confirmation for your cells.

Acceptance

Accept the equipment with your own cells and recipe. Check channel accuracy with calibrated references, temperature uniformity with sensors in a full tray, and data integrity by comparing the stored records with an independent meter. The acceptance testing page explains how to split these checks between the factory and your site, and the RFQ guide shows how to turn the recipe into supplier questions.

Sources

1. Wood, Li and An, Formation challenges of lithium-ion battery manufacturing, Joule 3 (2019) 2884-2888 · checked 2026-11-04
2. Schomburg et al., Lithium-ion battery cell formation: status and future directions towards a knowledge-based process design, Energy & Environmental Science (2024) · checked 2026-11-04
3. Keysight, measure self-discharge using OCV on lithium-ion cells (Electronic Design) · checked 2026-11-04