A key aspect to consider for any battery cell or system with cell(s) installed are the storage conditions. Key parameters to consider are:
- Safety
- chemical
- electrical
- fire
- Ageing
Safety
Safety is a key aspect with battery cell storage. Chemical leakage, gas emissions, electrical shorts and thermal events that go all the way to fires must be considered.
Ageing
Even if the cells in storage are not being cycled they will be ageing. The rate at which the cells lose capacity will be dependent on chemistry, temperature and state of charge. Ageing in storage is called Calendar Ageing.
Storage Temperature
Temperature is one of the two dominant drivers of Calendar Ageing in stored cells, the other being state of charge, and the relationship is roughly exponential — capacity fade accelerates sharply above 35°C. For long-term storage, keeping cells at 10–15°C keeps this fade close to its minimum, which is why the lower end of that range is generally preferred over ambient room temperature whenever it’s practical to control. This matters most for cells or packs held in storage for weeks or months rather than days: cells sitting in a shipping container, for example, can spend extended periods at elevated ambient temperature in transit and arrive with measurably reduced capacity before they’re ever put into service. Temperature shouldn’t be considered in isolation — a cell stored cool but at high SoC will still age quickly, so the two need to be managed together.
How to Minimise Calendar Ageing
Calendar ageing and self discharge rate decrease at low State of Charge (SOC) and low temperature. Typically a temperature around 15°C and an SOC ~30% work well for storing cells. However, if you have a large quantity of cells or packs being stored it is worth experimenting with conditions to optimise the parameters. In addition the battery monitoring schedule needs to be considered.
See also: Stand Test