When buying a used iPhone, a seller's claim that "the battery's still great" usually isn't reliable. What actually matters are two numbers found on screen: the maximum capacity percentage and the cycle count. A tutorial from Engadget points out that both figures can be found directly in the iPhone's built-in Settings—no need to pay for a repair shop check or buy a diagnostic tool.
The path is straightforward: open the Settings app, tap Battery, and find Battery Health. On older models like the iPhone 13, this option is labeled "Battery Health & Charging." The screen shows the current battery capacity as a percentage of its original, brand-new state—this is the first indicator of how much the battery has degraded.
Once Maximum Capacity Drops Below 80%, Brace Yourself
According to Apple's official documentation, batteries are designed to retain up to 80% of their capacity under normal usage conditions; the iPhone 15 and later models base this estimate on 1,000 complete charge cycles, while older models use 500 cycles. Once health drops below 80%, the iPhone still functions, but battery life shortens noticeably.
If the battery has degraded significantly, iOS may activate performance management to prevent unexpected shutdowns caused by unstable power delivery—in this case, a warning message appears under the "Peak Performance Capability" section. If no warning shows up, the battery is currently functioning normally and running at full performance.
Settings also lets you turn on "Optimized Battery Charging," which learns your charging habits and pauses charging at 80% before topping up to 100% closer to when you're likely to need the phone; iPhone 15 and later models even let you manually set a charging limit—again to help extend battery lifespan.
Native Cycle Count Only Started with iPhone 15—Older Models Need a Third-Party App
How you check the cycle count depends on your model. On iPhone 15 or later, simply scroll down on the Settings > Battery > Battery Health page to see the number. Apple doesn't build this stat into older models, so you'll need a third-party Mac app like coconutBattery to read it.
A single charge cycle means using up 100% of the battery's current capacity—not necessarily in one go. For example, using 50% today, charging, then using another 50% later adds up to one full cycle. Based on this, 1,000 cycles roughly corresponds to two to three years of actual use.
Hitting the cycle limit or dropping below 80% health doesn't mean the battery will fail immediately, but capacity typically drops noticeably and battery life feels worse. Apple recommends considering a battery replacement in this situation, which means contacting Apple Support to arrange a repair. For anyone about to buy a used iPhone, checking these two numbers is far more practical than taking a seller's word for it.
