Life and ageing
State of charge (SoC)
The remaining charge in a cell at a given moment, expressed as a percentage of its current usable capacity.
What it changes in practice
SoC is a relative measure, so it depends on the capacity figure it is measured against, and that figure falls as the cell ages. It is also not directly measurable; it is estimated from voltage, current integration and temperature, with the estimate drifting between full-charge references. Peak power capability varies strongly with SoC, which is why power figures for high-drain cells are quoted at a stated SoC.
Why it matters when you specify a cell
State of charge is an estimate, not a measurement. Coulomb counting drifts and needs periodic correction; voltage-based estimation is unreliable on a flat curve such as LFP. For a product that displays remaining runtime, decide which method the BMS uses before promising an accuracy figure to the customer.
Related terms
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More in Life and ageing
Other terms in this group
How cells age, and which numbers describe it honestly.
Cycle life
The number of charge and discharge cycles a cell completes before its capacity falls below a stated fraction of its original value, usually 80%.
Read the entry →Depth of discharge (DoD)
The fraction of a cell capacity removed during a discharge, expressed as a percentage.
Read the entry →State of health (SoH)
The current condition of a cell expressed as a percentage of its original capability, usually capacity, sometimes resistance.
Read the entry →Calendar life
Ageing that occurs with time rather than with use, driven mainly by temperature and state of charge during storage.
Read the entry →