Whether we are storing cells or packs we want to reduce the capacity loss vs time in storage.
Battery cells are an electrochemical system that ages whether being used or just stored on a shelf. The ageing when in storage is termed Calendar Ageing. The calendar ageing results in capacity loss and an increase in internal resistance. Hence it is important to understand how to minimise calendar ageing.
Calendar Ageing vs Chemistry
Here we see that for all chemistries the lowest capacity loss vs time is at low state of charge and low temperature.
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Temperature
As we reduce the temperature the calendar ageing in terms of resistance increase and capacity loss decreases. Wherever the cells are stored we need to consider the heat lost to and absorbed from the environment.
Hence there will be an optimum temperature for the cell lifetime and an optimum depending on the storage facility, location and building thermal capability.
State of Charge
Again, as we reduce the state of charge the calendar ageing in terms of resistance increase and capacity loss decreases. However, if we are going to think about long term storage we also need to consider the self discharge rate.
Self discharge of cells is dependent on the chemistry, temperature and age of the cell. These reactions occur in any electrochemical systems and are very difficult to avoid completely. This is because some self discharge reactions involve chemical reactions which will always take place at a rate proportional to temperature.
Typically this ranges between 2 to 5%/month (Cell Database).
Summary
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.