Life and ageing
State of health (SoH)
The current condition of a cell expressed as a percentage of its original capability, usually capacity, sometimes resistance.
What it changes in practice
SoH is the number that decides whether a pack is still fit for its application. Capacity-based SoH captures how much energy the cell holds; resistance-based SoH captures how well it can still deliver current. A cell can pass a capacity test and still fail a power application because its impedance has grown. This is why impedance growth over cycling matters as much as the cycle count itself.
Why it matters when you specify a cell
SoH combines capacity fade and impedance rise, and the two do not move together. A pack can hold its capacity and still deliver less power, or lose capacity while its impedance stays flat. Define which one your warranty measures, and at what temperature and rate, before publishing a retention percentage.
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 charge (SoC)
The remaining charge in a cell at a given moment, expressed as a percentage of its current usable capacity.
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 →