In short
Store lithium-ion cells cool, stable and at partial charge. Both heat and a high state of charge accelerate the internal degradation that eventually causes failures, and the two compound each other. The exact temperature range and charge level depend on the cell, so use the manufacturer’s datasheet.
You will find specific numbers quoted widely online — a percentage charge, a temperature band, a storage voltage per cell. Some come from manufacturer datasheets, some from hobbyist practice, and they are frequently repeated without a source. We are not going to add to that, because the correct figure genuinely varies by chemistry and product. What is worth understanding is why the two variables matter, so you can read a datasheet and apply it.
Why temperature matters
Every lithium-ion cell degrades over time through slow chemical reactions inside it — growth of the layer on the electrode surface, gradual electrolyte breakdown. These reactions are temperature-dependent: warmer means faster. That has two consequences for storage.
First, capacity loss. A pack stored warm for a year will hold measurably less charge afterwards than one stored cool. That is a cost issue rather than a safety one.
Second, and more relevant here, that degradation is also what eventually produces internal faults. A cell that has aged hard is closer to the conditions in which thermal runaway can start. Cool storage is therefore not fussiness — it slows the process that makes failures more likely.
Stability matters alongside the absolute figure. Repeated cycling between hot days and cold nights, as happens in an uninsulated container or a vehicle, is harder on cells than a steady temperature at the warmer end of the same range. This is why outdoor storage needs insulation and ventilation addressed rather than just distance from the building.
Why state of charge matters
Two separate reasons, pulling in the same direction.
Ageing. A cell held at high voltage sits under more electrochemical stress, and the degradation reactions run faster. Storing at full charge is the worst case for longevity.
Stored energy. If a pack does fail, the energy released scales with how full it was. A pack at partial charge in runaway is a smaller event than the same pack at full charge. This is the reason partial-charge storage appears in fire safety guidance and not only in battery-longevity advice.
The opposite extreme is also a problem. A cell allowed to over-discharge — below the voltage its chemistry tolerates — can suffer internal changes that make subsequent charging genuinely hazardous. This is why stored packs need a periodic top-up rather than being left indefinitely, and why a pack found flat after long storage should be retired rather than revived.
Putting it together
| Variable | What to aim for | What to avoid | Where the number comes from |
|---|---|---|---|
| Temperature | Cool and stable, at normal indoor temperatures | Direct sun, heating plant, uninsulated containers, vehicles, wide daily swings | Manufacturer datasheet for that cell or pack |
| State of charge | Partial charge, checked periodically | Full charge for long periods; allowing packs to drift into over-discharge | Manufacturer datasheet; a mid-range default where none is published, recorded as an assumption |
| Humidity | Dry, no condensation | Unheated damp stores; bringing cold packs into warm rooms and charging immediately | General good practice |
| Review interval | A recurring inspection and top-up | “We’ll check them when we need them” | Manufacturer guidance; more often in warm locations |
Frequently asked questions
What is the ideal storage temperature for lithium-ion batteries?
Cool and stable, within the range the manufacturer publishes for that product. We do not quote a single band because it varies by chemistry and because a figure without a source is not something you can rely on in an assessment. The practical version: normal indoor temperatures, not next to a heat source, not in direct sun, not in an uninsulated outdoor space that swings twenty degrees in a day.
What voltage should lithium batteries be stored at?
At the storage voltage or state of charge specified by the manufacturer, which is normally partial rather than full. For multi-cell packs this is managed at pack level by the battery management system rather than by measuring individual cells, so in practice you are setting a charge percentage rather than a voltage. Where no figure is published, a mid-range charge is a defensible default.
Does cold weather damage lithium batteries in storage?
Storage in the cold is generally less harmful than storage in the heat, and can slow ageing. The hazards are practical rather than chemical: condensation when packs warm up, and charging a cold cell, which can cause internal damage. Let packs reach room temperature before charging them, and keep the store dry.
How often should stored batteries be recharged?
Often enough that none drifts into over-discharge. Six-monthly is a common interval and works for most workplace stores; warmer locations and older packs need checking more often. Combine the top-up with a visual inspection for swelling or damage, so one task covers both.