Remaining capacity is the capacity a cell shows straight after a storage or rest period, measured with a single standard discharge and no intervening cycling. It is sometimes called retained capacity. Comparing it against Recovery Capacity – the capacity measured after the same storage period but following one or more full charge/discharge cycles – separates capacity loss that is genuinely permanent from capacity that looks lost but comes back once the cell is exercised.
Like recovery capacity, remaining capacity is normally captured as part of the periodic Reference Performance Tests built into calendar-life and storage-ageing test programmes: a baseline is recorded before storage, the cell rests at a target temperature and state of charge for a defined interval, and its capacity is checked with a single discharge immediately on removal – before any recovery cycling is applied.
Why the two numbers are measured separately
Recovery capacity on its own can be a misleading health indicator. Krupp et al. note that if a cell is losing capacity in storage mainly through parasitic side reactions that cause a fast self-discharge, that loss shows up in the remaining capacity reading but can be masked in the recovery capacity reading once the cell is recharged and cycled a couple of times [1]. A cell that looks fine on recovery capacity alone could still be quietly degrading through a self-discharge mechanism that only remaining capacity would catch.
Running both tests side by side gives a more complete picture: a large gap between remaining and recovery capacity points to a reversible effect (lithium trapped in anode overhang, or temporarily electrically isolated lithium reconnecting during cycling – see Recovery Capacity for the detail), while a remaining capacity that stays low even after recovery cycling points to genuine, irreversible fade.
Practical implications
- Calendar-ageing datasets that only report a single post-storage capacity number should specify which one it is – remaining or recovery – since the two can diverge meaningfully after long storage intervals
- Second-life grading and warranty assessment based on a single discharge straight after a rest period risks reading a temporary, recoverable dip as end-of-life fade
- A storage-ageing test plan that measures both values is better placed to distinguish a self-discharge problem (visible in remaining capacity) from ordinary calendar fade (visible in both)
References
- Krupp, A., Beckmann, R., Diekmann, T., Ferg, E., Schuldt, F., & Agert, C. (2022). Calendar aging model for lithium-ion batteries considering the influence of cell characterization. Journal of Energy Storage, 45, 103506
- Idaho National Laboratory / U.S. Department of Energy. (2003). FreedomCAR Battery Test Manual for Power-Assist Hybrid Electric Vehicles