Battery health is a measurement of how much charge your battery can hold compared to when it was brand new. It’s expressed as a percentage: a new battery starts at 100%, and the number drops over months and years as the battery’s internal chemistry wears out. If your phone shows 87% battery health, that means it can now store only 87% of the energy it originally held, which translates directly to shorter time between charges.
This is different from the battery percentage you see on your home screen every day. That daily percentage tells you how much charge is left right now. Battery health tells you whether “full” today is the same as “full” the day you bought the device.
Battery Health vs. Battery Life
These two terms get mixed up constantly, but they describe different things. Battery life is how many hours your device runs on a single charge. Battery health (also called “maximum capacity” on iPhones or “state of health” in technical contexts) is the long-term measure of how much total energy the battery can store. As battery health declines, battery life shrinks with it, because there’s simply less energy available each time you plug in.
Think of it like a water bottle with a slow leak that makes the bottle itself smaller over time. Battery life is how long you can sip before the bottle is empty. Battery health is the size of the bottle. A new phone might last 10 hours on a full charge. Two years later, with battery health at 85%, that same full charge only gets you about 8.5 hours under the same conditions.
What Causes Battery Health to Drop
Every lithium-ion battery degrades over time through a process called chemical aging. This isn’t just about calendar time. It’s driven by charge cycles, temperature exposure, and charging habits. A charge cycle is one full discharge of the battery’s total capacity, but it doesn’t have to happen all at once. If you drain your phone from 100% to 50% one day and then 50% the next, that counts as one cycle.
At the chemical level, a thin layer forms on the battery’s internal surfaces every time it charges and discharges. This layer gradually thickens, blocking the flow of ions that carry energy. The result is twofold: the battery holds less total energy, and its internal resistance increases. Higher internal resistance means the battery struggles more to deliver power during demanding tasks like gaming, video recording, or launching apps. In high-demand moments, this can cause voltage drops that the device has to manage.
Heat is the single biggest accelerator of this process. Research cycling lithium-ion cells at temperatures from 25°C (77°F) to 55°C (131°F) found that degradation rates climb sharply at elevated temperatures. The protective layer on the electrodes grows faster, the electrode materials themselves break down more quickly, and the overall capacity loss compounds. Leaving your phone on a car dashboard in summer or charging it under a pillow at night creates exactly these conditions. Cold extremes also increase internal resistance temporarily, though the effect reverses once the battery warms up.
How Your Device Measures It
Your phone or laptop doesn’t crack open the battery and inspect the chemistry. Instead, the built-in battery management system uses software algorithms to estimate health over time. The most common approach is coulomb counting, which tracks exactly how much electrical charge flows in and out of the battery during each session. By comparing the total charge the battery actually accepts against its original rated capacity, the system calculates the health percentage you see in your settings.
Some systems also monitor voltage behavior and internal resistance patterns to refine the estimate. These readings become more accurate over time as the system gathers more data from your real-world usage. The number you see isn’t a direct physical measurement. It’s an informed estimate, and it can occasionally fluctuate by a percentage point or two.
What the Percentage Actually Means for You
A battery health reading above 90% is typical for a device under a year old, and you’re unlikely to notice any difference in daily use. Between 80% and 90%, you’ll start to feel the decline: shorter screen-on time, maybe needing a midday top-up when you didn’t before. The phone still works fine, but the gap between charges is noticeably tighter.
The industry-standard threshold for replacement is 80%. Below that point, most manufacturers consider the battery significantly degraded, and you’ll likely experience real frustration with how quickly it drains. Some devices also begin managing performance at low health levels, subtly reducing processing speed during peak power demands to prevent the weakened battery from causing unexpected shutdowns. Apple made this approach widely known in 2017, and most major manufacturers now use similar strategies.
For electric vehicles, the same concept applies on a much larger scale. EV battery health directly correlates to remaining driving range. Most EV warranties guarantee the battery will stay above 70% health for 8 years or 100,000 miles. The European Union is making battery health transparency mandatory through a digital battery passport requirement for new EVs by 2027, though there’s still no universal standard for how vehicle-level health should be measured.
How to Keep Battery Health Higher for Longer
The single most effective habit is avoiding shallow and deep extremes. Keeping your battery roughly between 20% and 80% for everyday use reduces the stress on its internal chemistry. Shallower discharge cycles put less strain on the battery than deep ones. Draining to zero regularly wears it out faster than topping off more frequently throughout the day. Partial charges are perfectly fine for lithium-ion batteries and actually preferable to full cycles.
Temperature management matters just as much. Avoid charging your device when it’s hot, take it out of thick cases during long charging sessions, and don’t leave it in direct sunlight. If your phone feels warm to the touch while charging, that heat is actively accelerating degradation.
Most modern phones now include features that help automatically. Optimized charging learns your schedule and holds the battery at 80% overnight, only completing the charge to 100% right before you typically wake up. Some Android phones and newer iPhones let you set a hard charging limit at 80% if you want maximum longevity. Enabling these features is one of the easiest things you can do. Over two or three years, the difference between a phone charged to 80% daily and one charged to 100% every night is meaningful, often 5 to 10 percentage points of preserved health.
Where to Check Battery Health
On an iPhone, go to Settings > Battery > Battery Health & Charging. You’ll see Maximum Capacity as a percentage and a Peak Performance Capability status that tells you whether the battery can still handle full processing speed.
On most Android phones, the path varies by manufacturer. Samsung uses Settings > Battery > Battery Status (or you can use the Samsung Members app for a diagnostic). Google Pixel phones show it under Settings > Battery > Battery Health. For other Android devices, third-party apps like AccuBattery can estimate health by tracking charge and discharge patterns over time, though built-in tools are more reliable when available.
On MacBooks, hold the Option key and click the Apple menu, then choose System Information > Power. You’ll see cycle count and condition. Windows laptops can generate a battery report by running “powercfg /batteryreport” in the command prompt, which shows design capacity versus current full charge capacity. The ratio between those two numbers is your battery health.