Cycle life gets all the attention because it is the number on the datasheet. For a product that ships in volume, calendar ageing during storage often costs more capacity than the cycling does — and unlike cycling, it is almost free to control. Two variables dominate: how charged the cells are, and how warm the room is.

Why a stored cell degrades at all

Degradation continues without any current flowing. The dominant mechanism is slow, self-consuming chemistry at the electrode interfaces, and its rate depends strongly on temperature and on the cell's voltage. A cell held at high state of charge sits at high voltage, which accelerates that chemistry. A warm cell accelerates it further.

This is the same family of mechanisms that drives the capacity loss discussed in lithium battery capacity fade; storage is the case where it operates with no cycling to mask it.

The two rules

  • Store at partial charge, not full. Common manufacturer guidance is in the region of 30–50% state of charge. This is the single biggest lever, and it costs nothing.
  • Store cool, not cold. A stable, moderate room temperature is the goal. Freezing is not required and introduces condensation risk on removal; what matters is avoiding sustained heat.

The failure case to avoid is the combination: fully charged cells in an un-airconditioned warehouse through a hot summer. That is where a batch arrives at the assembly line measurably down on capacity, and nobody can explain why the bench prototypes were fine.

Self-discharge and the over-discharge cliff

Every cell loses charge slowly on the shelf. That is normally harmless — but a cell stored for a long time at a low starting state of charge can self-discharge into deep over-discharge, where copper dissolution can occur and the cell may be unsafe to recharge.

This is why the guidance is 30–50% and not "as low as possible". It is also why long-term storage needs a periodic check rather than a one-time decision. Our note on coin-cell self-discharge and shelf life covers the equivalent problem in primary and small secondary cells.

The shipping rule is not the storage rule

Lithium-ion cells and batteries shipped on their own by air must be at a low state of charge — not more than 30% — under the dangerous-goods rules. That requirement exists to reduce the energy available in a transport incident, and it is a transport rule, not a storage recommendation.

The two happen to be compatible: shipping at ≤30% and storing at 30–50% are close enough that a sensible inbound process satisfies both. But do not reason from one to the other — the obligations come from different places. See DGR basics and lithium air freight for the transport side.

A practical warehouse process

  1. Specify incoming state of charge in the purchase agreement, and measure it on receipt as a sampled check.
  2. Control the room. Moderate, stable temperature matters more than a precise set point. Log it, so you can correlate later if a batch underperforms.
  3. Rotate stock first-in-first-out by manufacture date, not receipt date.
  4. Set a maximum shelf age before the cells are either used or checked, and define what the check is.
  5. Re-check long-held stock: sample open-circuit voltage and reject anything that has drifted below the supplier's stated floor rather than attempting to recover it.
  6. Keep manufacture-date traceability through to the finished product. When a field issue appears, the first useful question is how old the cells were.

What to ask your supplier

  1. Shipping state of charge, and the tolerance around it.
  2. Recommended storage state of charge and temperature for your intended duration.
  3. Self-discharge rate at a stated temperature.
  4. The open-circuit voltage floor below which a cell must be scrapped rather than recharged.
  5. Manufacture-date coding on the cell, so FIFO is actually enforceable.

The fifth one sounds trivial and is the one most often missing.