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The battery health percentage is not a fuel gauge. It is how much of the battery’s original capacity is left — so 85% does not mean the battery is nearly full, it means a full charge now holds 85% of what a full charge held when the phone was new.

Apple states it plainly: maximum battery capacity “measures the device battery capacity relative to when it was new.” Google’s version of the same idea is “an estimate of your device’s remaining battery capacity compared to its original capacity.”

Where to find it

On iPhone — open Settings, tap Battery, then tap Battery Health. Apple’s own instructions are exactly that sequence. The same screen tells you whether the performance-management feature is on, which Apple says is “enabled only after an unexpected shutdown first occurs on a device with a battery that has diminished” ability to deliver peak power.

On a Pixel — Settings → Battery → Battery health, which reports a status of “Normal” or “Reduced” rather than a number. For the number, Google says to go one level deeper: Battery → Battery health → Battery capacity.

On other Android phones the information exists in different places depending on the manufacturer, and on some older devices it is not exposed at all. There is no single Android path, and any guide that gives you one is wrong on most phones.

What the number should be

This is the part that turns the reading into a decision, and both companies publish it.

Apple designs batteries to “retain 80 percent of their original capacity at 500 complete charge cycles under ideal conditions” — and for iPhone 15 models, 80 percent at 1000 complete charge cycles. That is a considerable change, and it means a 2023-or-later iPhone showing 90% after two years is behaving as designed rather than degrading unusually.

Google does not publish a percentage threshold in the same way. It moves the status to “Reduced” and says that when it does, “Google recommends replacing your battery.”

So the rule of thumb worth carrying:

Reading	What it means
Above 85%	Normal. Nothing to do.
80–85%	Ageing as expected. Worth watching, not worth acting on.
Below 80%	Past the capacity both makers design toward. A replacement will be noticeable.
Pixel says “Reduced”	Google’s own advice is to replace it.

What it does not tell you

It is not a fault. Apple’s framing is that all rechargeable batteries “are consumable components that become less effective as they chemically age”, and Google’s is that batteries “aren’t meant to last indefinitely”. A declining number is the battery working normally, in the way tyres wear.

It is not the reason your phone feels slow — unless the performance-management feature has engaged, which on iPhone only happens after an unexpected shutdown. If your phone is slow and battery health is 92%, the battery is not your problem.

It is not precise. Google calls its figure an estimate, and Apple notes the exact percentage “depends on how the devices are regularly used and charged”. Treat a two-point move between readings as noise.

What to actually do

Read it once, write down the number and the date. One reading is meaningless; the useful signal is the slope over six months.
Do not replace on the number alone. Replace when the phone stops lasting the day you need it to last. Capacity is a proxy for that, not a substitute.
If you are below 80% and keeping the phone, a battery replacement is the cheapest performance upgrade available to you, by a wide margin.
If you are shopping second-hand, ask the seller for this screen. It is the single most useful number about a used phone and it takes them ten seconds.