What does battery health on a phone or laptop actually mean?
An estimate of remaining capacity relative to when new, expressed as a percentage — and it is a calculated figure rather than a direct measurement, which explains why it can move oddly and why it is not the whole story.
How it is estimated. Battery management systems track charge going in and out, voltage under load, internal resistance and temperature, and infer capacity. It is a model, not a reading, which is why it occasionally jumps after a full cycle recalibrates it, and why a figure can appear stuck for months.
What the number means in practice. At 80%, a battery holds roughly four-fifths of its original charge — so a day's use becomes about four-fifths of a day. Manufacturers commonly treat 80% after a defined number of cycles as the warranty threshold.
What it does not tell you: internal resistance. As a cell ages, resistance rises, so it cannot deliver current as readily under load. This is why an ageing phone shuts down suddenly at 30% in cold weather — not because capacity ran out, but because voltage sagged below the threshold under demand. It is also why some devices reduce peak performance on degraded batteries, which caused considerable controversy when it was introduced without explanation.
What actually degrades a battery, in order: heat, which is the dominant factor and the reason charging in a hot car or under a pillow is damaging; time spent at high state of charge, since sitting at 100% ages cells faster than sitting at 50%; deep discharges to empty; and high charge and discharge rates.
What follows practically: use optimised or limited charging features, which hold at around 80% until you need the rest; avoid overnight charging in a warm place; do not leave a laptop permanently at 100% on mains unless it has a charge limit setting, which most now do; and store an unused device at around half charge, not full or empty.
A swollen battery is a safety matter — stop using the device, do not puncture it, and have it replaced professionally.