Battery health, cycle count, and what the numbers mean
The short answer
Battery health is an estimate of how much charge the cell still holds against the day it was made. A charge cycle is one full battery's worth of energy used, not one plug-in. Most manufacturers design a cell to keep about 80 per cent of its capacity after 500 to 1,000 cycles.
What the percentage is actually measuring
Battery health is not a measurement, it is an estimate. The phone watches how the voltage behaves under load, how long a full charge lasts and how the cell responds to temperature, and it works backwards to a figure for how much capacity is left compared with a new cell.
Because it is an estimate, it moves in steps rather than smoothly, it can jump after an update recalibrates it, and two identical phones can report different numbers on the same day. None of that means the figure is useless; it means it is a trend rather than a gauge.
The number that matters more is what the phone actually does. A cell reading 88 per cent that shuts the phone off at 20 per cent on a cold morning is finished, whatever the readout says. A cell reading 79 per cent that comfortably lasts a working day is not.
A cycle is not a plug-in
Those are the shape of it rather than a promise. A phone that lives on a car dashboard in summer will get there faster than the table suggests, and one that never gets hot will beat it.
| Cycles | Typical health | What people notice |
|---|---|---|
| 0 to 200 | 95 to 100 per cent | Nothing. This is a new phone. |
| 200 to 500 | 85 to 95 per cent | A slightly earlier evening charge. |
| 500 to 800 | 80 to 88 per cent | Noticeably shorter days, and cold weather hurts. |
| 800 and up | Under 80 per cent | Shutdowns under load, and the phone throttling to avoid them. |
Why 80 per cent is the number everybody quotes
It is the point manufacturers design to, and it is where the behaviour changes rather than just the endurance. Below roughly 80 per cent a cell struggles to deliver peak current, and a phone asked for more power than the battery can supply will simply switch off.
Rather than let that happen, phones throttle. They cap the peak performance of the processor so the demand never exceeds what the cell can give. That is the real reason an old phone feels slow, and it is why a new battery in a three-year-old phone often feels like a bigger upgrade than the battery alone should be.
So 80 per cent is not a cliff the cell falls off. It is the point where the phone starts protecting itself, and where a replacement stops being cosmetic.
What actually wears a battery out
What is not on the list: charging overnight, charging to 100 per cent occasionally, using the phone while it charges in a cool room, and letting it drop to 20 per cent. Those are all normal and none of them is what kills cells.
- Heat, by a distance. Charging fast in a hot car, or gaming while plugged in, does more damage than anything else on this list.
- Sitting at 100 per cent for hours, particularly warm. Overnight charging in a cool room is fine; overnight on a wireless pad under a duvet is not.
- Running to zero regularly. Deep discharges are harder on lithium than partial ones.
- Long storage at a full or empty charge. If a phone is going in a drawer for months, leave it near half.
- Age itself. A cell degrades slowly even unused, which is why a five-year-old phone that was barely used still has a tired battery.
Where to find the number
On an iPhone: Settings, Battery, Battery Health and Charging. It shows maximum capacity and, on recent models, cycle count.
On a Pixel and on recent Samsung phones: Settings, Battery, and then battery information or device care depending on the version. Some Android phones report cycle count and some do not.
On anything that does not report it, we can read it at the counter with a tester, and that is part of the free diagnostic rather than a separate charge.
Common questions.
Around 80 per cent is where phones start throttling performance to avoid shutdowns, so that is the usual point to replace. The better test is behaviour: a cell that shuts the phone off before zero, or that will not last a working day, is finished whatever the number says.
No. A cycle is one full battery's worth of energy used, however it accumulated. Using 40 per cent today and 60 per cent tomorrow is a single cycle.
Not on its own. Modern phones stop drawing current at full and most learn your routine to finish charging near when you wake. What does damage a cell is heat, so overnight on a wireless pad under a pillow is a different matter from overnight on a bedside table.
Often, yes, and not because the battery is a speed part. Phones throttle the processor when the cell cannot deliver peak current. Replace the cell and the throttling stops.
Usually recalibration rather than sudden damage. The figure is an estimate the phone refines over time, and an update or a full discharge can move it in one step. A drop that keeps going is different and worth checking.
A fixed price per model, listed on that model's page, including the cell, the fitting, the testing and the 360-day warranty. It is the cheapest repair that makes an old phone feel new.
Where this leads.
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