Where the number lives
Settings → Battery → Battery Health & Charging. Maximum Capacity is the headline figure: the percentage of its original charge the battery can still hold. A brand new iPhone reads 100%, and it declines from there with use.
Below that is Peak Performance Capability, which tells you whether iOS is throttling the processor to prevent unexpected shutdowns. That message is the one that actually matters for how the phone feels.
What the percentage means in practice
Above 90% is effectively new. Between 80 and 90 is normal for a phone one to two years old and you will not notice much. Below 80% is where Apple considers the battery consumed — it is the warranty threshold, and it is usually where people start describing the phone as not lasting the day.
Apple's design target is retaining 80% capacity after 500 complete charge cycles, so reaching that in two years of heavy use is expected behaviour rather than a fault.
Peak Performance Capability is the important line
If it reads that the battery is supporting normal peak performance, the phone is not being throttled. If it reports an unexpected shutdown occurred and performance management has been applied, iOS is deliberately limiting the processor to stop the phone dying under load.
That is when an iPhone feels suddenly, dramatically slower. You can turn the management off from that screen, but the shutdowns come back — the real fix is a new battery.
Cycle count, which tells you more than the percentage
On recent iPhones, Settings → General → About lists Cycle Count alongside the manufacture date. A cycle is one full charge's worth of use, not one plug-in, so charging from 50% twice counts as one.
Under 500 cycles a battery should still be healthy; well over 800 explains a lot regardless of what Maximum Capacity claims. Our cycle count guide covers where to find it on older models too.
When the number is misleading
Maximum Capacity is an estimate iOS refines over time, and it can jump after an update or after the phone has been left at a very low charge. A sudden drop of several percent overnight is usually recalibration rather than sudden failure.
A battery replaced with a non-genuine part often shows no health data at all, or an Important Battery Message warning that Apple cannot verify it. That is not necessarily a bad battery, but the reporting will not be reliable.
Deciding whether to replace it
Two things justify it: capacity below 80%, or performance management being active. Either one means the phone will keep getting worse, and a replacement typically costs a fraction of a new device and restores it completely.
Our replacement guide covers the cost and the process, and our device pages note what to expect from each model's battery at its age.
Frequently Asked Questions
Where do I check battery health on an iPhone?
Settings → Battery → Battery Health & Charging. Maximum Capacity is the main figure.
What is a bad battery health percentage?
Below 80%. That is Apple's threshold for a consumed battery and where most people notice the phone not lasting the day.
What does Peak Performance Capability mean?
Whether iOS is throttling the processor to prevent shutdowns. If performance management is active, the battery needs replacing.
Why did my battery health drop suddenly?
Usually recalibration after an update or a deep discharge. iOS estimates the figure and refines it over time.