Your iPhone will slow down, shut off unexpectedly, and eventually stop holding a charge
When your iPhone battery ages without replacement, the phone itself becomes less reliable. A degraded battery cannot deliver the power your iPhone needs, so the operating system throttles the processor to prevent sudden shutdowns. This makes your phone noticeably slower—apps take longer to open, scrolling lags, and tasks that used to be when ready now pause. You'll also see your battery percentage drop faster than it used to, and the phone may shut itself off even when the battery indicator shows 20 or 30 percent remaining.
Eventually, the battery reaches a point where it cannot power the phone at all. At that stage, your iPhone becomes a brick—it will not turn on, will not charge, and will not respond to any input. This typically happens 2 to 3 years after the battery was manufactured, though the timeline depends on how often you charge it and what temperatures you expose it to.
Key Takeaways
- iOS automatically slows down your processor when the battery degrades, which makes your phone feel sluggish even though nothing is broken.
- Your battery will drain faster and your phone may shut off at 20 or 30 percent charge, making it unreliable for calls or navigation.
- A degraded battery can cause unexpected restarts, app crashes, and failure to charge properly.
- After 2 to 3 years, most batteries lose the ability to power the phone at all, leaving you with a device that will not turn on.
- Replacing the battery costs significantly less than buying a new phone and restores your iPhone to normal performance.
How battery degradation triggers slowdowns
Your iPhone's processor demands a lot of power in short bursts—especially when you open apps, take photos, or use the camera. A fresh battery can deliver that power when ready. As the battery ages, its internal chemistry breaks down and it cannot deliver power as quickly or as much. When iOS detects this, it reduces the maximum speed of the processor to match what the battery can actually supply. This is called performance management, and it is automatic—you do not turn it on or off.
You can see your battery's health in Settings > Battery > Battery Health & Charging. If it shows 80 percent or lower, slowdowns have likely already started. At 50 percent health, your phone will feel noticeably sluggish. The slowdown is not a glitch or a sign that your phone is failing—it is a safety feature that prevents the phone from shutting down without warning.
The problem is that this slowdown affects everything. Your phone takes longer to unlock, photos take longer to save, and games become nearly unplayable. Many people interpret this as a sign they need a new phone, when in reality a new battery would restore the original speed.
Unexpected shutdowns and charging problems
A degraded battery often cannot deliver steady power, which causes your iPhone to shut off suddenly even when the battery percentage shows 20, 30, or sometimes even 40 percent. This happens because the battery voltage drops below what the processor needs to run, even though chemical energy remains in the battery. The phone shuts down to protect itself, and when you plug it in, the voltage recovers and the phone turns back on—but the battery percentage may jump from 1 percent back to 20 percent.
You may also notice that your phone refuses to charge past a certain percentage, charges very slowly, or stops charging altogether. This occurs because the battery's internal resistance has increased, which prevents current from flowing in or out efficiently. Some iPhones will show a "Service Battery" message in Settings, which is Apple's way of telling you the battery is no longer reliable.
These shutdowns are dangerous if you rely on your phone for emergencies, navigation, or work. You cannot trust the battery percentage, so you cannot know when your phone will die.
App crashes and system instability
When the battery cannot deliver stable power, apps crash more often and the operating system becomes unstable. You might see apps close themselves without warning, or the phone might restart on its own. These crashes happen because the processor is being starved of power—the battery voltage dips below the minimum the system needs, and iOS shuts down whatever is running to protect the hardware.
Restarting the phone temporarily fixes the problem because it clears the memory and gives the battery a moment to recover. But the crashes will return within minutes or hours. This makes your phone frustrating to use for anything important, and it can cause you to lose unsaved work.
The cost of waiting versus replacing the battery
Battery replacement through Apple typically costs between $50 and $100, depending on your iPhone model. This is far less than the cost of a new iPhone, which ranges from $300 to $1,200. If you wait until the battery is completely dead, you still have to pay for the replacement—you do not save money by delaying.
More importantly, replacing the battery restores your phone to nearly original performance. The slowdowns disappear, the shutdowns stop, and the battery percentage becomes reliable again. If your iPhone is otherwise in good condition, a new battery can extend its useful life by another 2 to 3 years.
If you do not replace the battery and continue using the phone, you risk data loss from unexpected shutdowns. You also risk being without a phone when it dies unexpectedly, which can be a safety issue if you need to call for help.
When the battery is too degraded to repair
In rare cases, a battery can degrade so severely that it swells inside the phone. This is called battery expansion, and it can damage the screen, the logic board, and other internal components. If this happens, the phone may not be repairable, or the repair cost may exceed the cost of a new phone.
Battery expansion is more common in phones that have been exposed to heat, stored in hot cars, or left plugged in for weeks at a time. If you notice your screen is lifting away from the frame, or if the back of the phone feels puffy, do not use the phone—take it to Apple or a repair shop when ready. A swollen battery can catch fire in rare cases.
How to check your battery health right now
Open Settings, tap Battery, then tap Battery Health & Charging. You will see a percentage labeled "Maximum Capacity." This tells you how much charge your battery can hold compared to when it was new. If it is 80 percent or higher, your battery is still healthy. If it is between 50 and 80 percent, you should plan to replace it soon. If it is below 50 percent, replacement is urgent.
You will also see a "Condition" line that may say "Normal," "Service Recommended," or "Service Battery." If it says anything other than "Normal," Apple is telling you the battery needs replacement.
Some older iPhones do not show the Battery Health screen. If you have an iPhone 5s or earlier, you cannot see the health percentage, but you can still tell the battery is degraded if your phone shuts off unexpectedly or feels slow.
Where to get your battery replaced
Apple Stores and Apple Authorized Service Providers can replace your battery. You can also visit independent repair shops, though prices and warranty terms vary. If you have AppleCare+, battery replacement is covered at no cost if the battery health has dropped below 80 percent.
Replacement usually takes 1 to 3 hours if you drop the phone off, or you can make an appointment for same-day service at many locations. Some Apple Stores offer mail-in service if you do not have a store nearby.
Frequently Asked Questions
Will my iPhone work at all if I don't replace the battery?
Yes, but it will be slow, unreliable, and shut off unexpectedly. You can still use it for basic tasks like texting or calling, but you cannot trust it to stay on when you need it. Most people find a degraded battery so frustrating that they stop using the phone within a few months.
Can I replace the battery myself?
You can, but it is difficult and risky. Opening an iPhone requires special tools and knowledge of how to avoid damaging the screen, the logic board, or the battery itself. If you damage something during the repair, you may end up paying more for repairs than a professional replacement would have cost. Most people should use Apple or a certified repair shop.
How long does a new battery last?
A new battery typically lasts 2 to 3 years before it drops below 80 percent health, depending on how often you charge it and what temperatures you expose it to. Charging your phone in very hot or very cold environments speeds up degradation. Keeping your phone between 50 and 80 percent charge most of the time also extends battery life.
Will replacing the battery make my iPhone feel new again?
Yes. The slowdowns will disappear, the shutdowns will stop, and the battery percentage will become reliable again. Your phone will feel as fast as it did when it was new. The only thing that will not change is the age of the other components, but for most people, a new battery makes the phone feel like a completely different device.
What if my phone is too old to be worth fixing?
That is a personal decision, but remember that battery replacement costs $50 to $100, while a new phone costs $300 to $1,200. Even if your phone is 5 years old, a new battery might give you another year or two of use before you need to upgrade. If the phone has other hardware problems—a cracked screen, water damage, or a broken camera—then replacement might make more sense.