Two batteries can both read 100 percent and have different amounts of work left in them. The number on a charge display describes how full each battery is relative to what it can presently hold. It does not say how much that container has shrunk, how readily it can deliver a sudden demand, or how quickly its performance may change next year. To call a battery “healthy,” we first have to ask: healthy for what job?
For a lithium-ion cell, state of charge is the short-term question: how much charge remains now? Capacity asks how much the cell can deliver through a defined charge-and-discharge test. A common capacity-based state of health is the measured capacity of an aged cell divided by its initial capacity, under comparable conditions. A multi-institution study of commercial cells used exactly that ratio, measuring capacity by integrating current through a controlled discharge. The method gives a meaningful comparison because it says what was measured and how. It also takes time; an operating battery cannot always be taken out of service for a full laboratory cycle.
Holding energy and delivering it
A battery can lose room for charge and also become less able to supply power. Those losses need not march together. The National Petroleum Council's technical assessment, hosted by the US Department of Energy, distinguishes capacity fade from power fade. It describes unwanted reactions between electrolyte and electrode materials that consume usable lithium and form resistive layers. Fewer lithium ions available for the intended reaction can reduce stored charge; greater internal opposition to current can make high-power delivery harder. The exact mix depends on chemistry, temperature and use.
The units help separate the questions. Ampere-hours describe how much electrical charge a cell delivers under stated conditions. Watt-hours describe energy, taking the changing voltage during that delivery into account. Watts describe the rate at which energy is supplied. A long, gentle discharge and a brief, demanding pulse therefore test different abilities. A cell can retain enough energy for one job while its voltage falls too sharply under the other. Saying only that it has “80 percent health” leaves the job and the test unstated.
This is why measuring resistance or impedance is attractive. Impedance describes how a cell responds to a small changing electrical signal across frequencies. It can reveal internal processes without the long wait of a complete discharge. In the commercial-cell comparison, several impedance-based methods estimated capacity-derived health with average errors of roughly 0.75 to 1.5 percentage points in that dataset. The result demonstrates a useful, faster proxy once a model has been calibrated. It does not turn an impedance reading into a direct measurement of capacity in every battery design.
The distinction grows sharper outside a controlled test. Researchers at the Technical University of Munich compared new cells, laboratory-aged cells and cells taken from two similar electric vehicles. Capacity and impedance were related, but the correlation depended strongly on operating and storage conditions; high temperature was a particular source of deviation. A quick test built from one aging history could misread a cell that arrived by another. The number needs a history, or at least a calibration that resembles the battery's likely use.
The reading has a temperature
Some apparent change in a health indicator comes from the conditions of measurement rather than a new increment of permanent aging. In experiments on five lithium-ion cells, equivalent-circuit resistance and capacitance indicators shifted as the researchers varied temperature and state of charge, even while comparing cells at the same aging stage. They built a model that corrected for those effects within their tested range. Their fitted relationships are useful evidence that a reading needs context; they are not universal calibration curves for every chemistry and pack.
This matters in ordinary language because “the battery is weak today” may refer to at least two things. The cell may be aged, or the test may be taking place under conditions that temporarily change its output. Those possibilities can coexist. A fair comparison specifies temperature, charge level and load instead of treating a single number as an intrinsic property detached from the test.
Time itself belongs in the record. Battery life is often counted in cycles, but cells also age while resting. In a Sandia-led study of silicon-anode cells, researchers varied the spacing of test cycles during long rests. Their cells showed mechanical effects from cycling, yet capacity fade in the longer-rest protocols tracked elapsed time more strongly than the number of those test cycles. Post-test analysis pointed chiefly to loss of usable lithium and rising impedance. The cells and protocols were specialized; the finding is not a claim that cycling matters less than storage for every battery. It is a warning against using a cycle count as the whole biography of a cell.
The future is another measurement problem
Even a carefully measured capacity percentage describes a present state. It may conceal different paths to that state. In a 2022 study of 88 commercial coin cells, researchers varied charging and discharging currents and tested whether an impedance reading could forecast later discharge capacity when the future cycling protocol was known. Their result challenged the idea that one scalar state-of-health value contains everything needed to predict what happens next. Different internal changes can produce similar capacity today and diverge under later use.
The study was done on controlled coin cells at regulated temperature, not on a fleet of working vehicles. Its deeper point is still practical: a forecast must specify the intended duty. The same cell may be asked to deliver short bursts of power, hold energy for long periods, or cycle daily. A capacity test, a power test, an impedance spectrum and an aging record illuminate different parts of that request. None is interchangeable with the charge icon.
A useful battery report therefore begins with the job and the conditions. How much energy must be delivered, at what rate, at what temperature, and for how long? A battery that has lost some capacity may still serve a modest application; a cell with adequate capacity may struggle with a demanding pulse. “Health” is a summary only after those questions have names and measurements. Otherwise it is a percentage waiting for its meaning.
A thought worth keeping?
