Understanding Lithium-Ion Degradation
Modern smartphones are powered by lithium-ion batteries. While these batteries are incredibly efficient and capable of holding a large charge in a very small physical space, they are also chemically unstable over long periods of time. Every time you drain your iPhone battery and charge it back up to 100%, it goes through a “charge cycle.”
As these charge cycles accumulate over months and years, the chemical composition inside the battery begins to degrade. This means a battery that could hold enough power for 14 hours of video playback on the day you bought it might only hold enough power for 8 hours of playback two years later.
If your iPhone is suddenly dying halfway through the day, suddenly shutting down when the battery percentage says 20%, or performing noticeably slower than usual (a feature Apple introduced to prevent aging batteries from crashing the phone), the first thing you should do is check the official Battery Health diagnostic page.
How to Check Maximum Capacity on an iPhone
Apple has built a highly accurate diagnostic tool directly into the iOS settings menu. You do not need to download third-party battery diagnostic apps (which are often inaccurate and filled with advertisements).
- Open the Settings app on your iPhone.
- Scroll down the main settings list and tap on Battery. (It has a green icon).
- On the main battery screen, you will see graphs showing your recent usage. Tap on the section labeled Battery Health & Charging (on newer iPhones, this might simply say Battery Health).
- Look for the line item labeled Maximum Capacity.
This percentage number is the most important metric on your phone. It represents the battery’s current capacity relative to when it was brand new.
How to Interpret the Maximum Capacity Percentage
Understanding what that number actually means will help you decide if it is time to buy a new phone or simply pay for a battery replacement.
- 100% to 90%: Your battery is in excellent condition. If your phone is dying quickly, the battery is not to blame. You likely have a rogue app running in the background draining your power, or you are spending too much time playing graphically intense mobile games.
- 89% to 81%: Your battery is experiencing normal wear and tear. You might notice you need to charge your phone slightly earlier in the evening than you used to, but the device should still perform perfectly well.
- 80% or below: This is the critical threshold. Apple officially considers a battery to be “degraded” once it hits 80%. When your capacity drops to this level, iOS will often trigger a warning message at the top of the Battery Health screen suggesting that your battery “service is recommended.”
Understanding “Peak Performance Capability”
Directly beneath the Maximum Capacity percentage, you will see a section labeled “Peak Performance Capability.” This tells you if your degrading battery is actively slowing down your phone.
As batteries age and drop below 80% capacity, they struggle to deliver the sudden, massive spikes of electrical power required by the iPhone’s processor during heavy tasks (like opening a heavy app or recording 4K video). If the battery fails to deliver that power spike, the iPhone will instantly crash and reboot to protect the internal hardware.
To prevent these random crashes, iOS will automatically throttle (slow down) your phone’s processor so it never demands more power than the aging battery can provide. If this has happened to your phone, the Peak Performance section will state that “Performance management has been applied.”
You can tap the small blue “Disable” button to turn this throttling off and regain your phone’s speed, but be warned: doing so significantly increases the risk of your phone randomly shutting down when you need it most.
When to Replace the Battery
If your Maximum Capacity is 80% or lower, and your phone is being artificially slowed down by Performance Management, it is time for a replacement.
Do not throw the entire iPhone away. A battery replacement at an official Apple Store or authorized repair centre usually costs between $69 and $99 (depending on the model). Replacing the battery restores the Maximum Capacity to 100%, completely removes all processor throttling, and makes a three-year-old iPhone feel brand new again for a fraction of the cost of a new device.
