Measuring battery life depends on what you’re working with. For a phone or laptop, built-in software tools can show you capacity, health percentage, and cycle count in minutes. For standalone batteries, a multimeter or load tester gives you voltage and real-world performance data. The approach changes depending on the device, but the core idea is the same: compare what your battery can deliver now against what it could deliver when it was new.
Understanding Battery Capacity Units
Battery capacity is expressed in two main units, and knowing the difference matters. Milliamp-hours (mAh) tells you how long a battery can supply current, but it ignores voltage. A 5,000 mAh phone battery and a 5,000 mAh power bank sound identical, but if they run at different voltages, they hold different amounts of total energy.
Watt-hours (Wh) is the more complete measurement because it factors in voltage. You can convert between the two with a simple formula: mAh × voltage ÷ 1,000 = Wh. A 5,000 mAh battery running at 3.7 volts holds 18.5 Wh of energy. This is why smartphones list capacity in mAh (they all use similar voltages) while electric vehicles use kWh, where comparing energy across very different battery architectures requires a unit that accounts for voltage.
Checking Battery Health on an iPhone
On iPhone 15 and later models, open Settings, then go to General, then About, and scroll to Cycle Count. This tells you how many full charge-discharge cycles the battery has completed. You’ll also find your maximum capacity percentage under Settings > Battery > Battery Health, which shows how much charge your battery can hold compared to when it was new.
On older iPhones, the cycle count isn’t directly visible in settings. You can still see the maximum capacity percentage through the Battery Health menu, but getting the exact cycle count requires using a Shortcuts workaround or connecting the phone to a Mac and reading the analytics data. The maximum capacity percentage is the more useful number for most people anyway. Once it drops below 80%, you’ll likely notice significantly shorter battery life between charges.
Checking Battery Health on Android
Android varies by manufacturer. Samsung devices show battery health in Settings > Battery and Device Care > Diagnostics. Google Pixel phones display it under Settings > Battery > Battery Health. Some Android phones don’t surface this information at all, in which case third-party apps like AccuBattery can estimate your battery’s current capacity by tracking charge and discharge patterns over time. These estimates get more accurate the longer you use the app.
Generating a Laptop Battery Report on Windows
Windows 10 and 11 have a hidden battery report tool that gives you detailed health data. Open Command Prompt as an administrator (press Windows + S, type “cmd,” right-click, and choose “Run as administrator”). Then type:
powercfg /batteryreport
Press Enter, and Windows generates an HTML file you can open in any browser. The report shows your battery’s design capacity (what it was built to hold), its full charge capacity (what it actually holds now), and a history of how that capacity has changed over time. If you want to save it to a specific location, use: powercfg /batteryreport /output "C:\battery-report.html"
The most important number in the report is the ratio between full charge capacity and design capacity. If your laptop was designed for 60 Wh and now only charges to 48 Wh, you’re at 80% health. The report also logs recent battery usage sessions with drain rates, so you can see how quickly your battery depletes under your actual workload.
Checking Battery Health on a Mac
On recent versions of macOS, click the Apple menu, open System Settings, and go to Battery. You’ll see a Battery Health section showing your condition (Normal, Service Recommended, etc.) and maximum capacity percentage. For more granular data, including cycle count and the exact design versus current capacity in mAh, go to the Apple menu, click About This Mac, then open System Report and select Power from the left sidebar. This gives you the same level of detail as the Windows battery report.
Using a Multimeter for Standalone Batteries
For batteries that aren’t inside a smart device, a multimeter is the most accessible tool. Set the dial to DC voltage mode, connect the red probe to the positive terminal and the black probe to the negative terminal, and read the voltage on the display. Wait for the reading to stabilize before recording it.
A resting voltage reading tells you the current charge level but not much about overall health. A fully charged AA battery reads about 1.5V, while a healthy car battery reads around 12.6V. If either reads significantly below its rated voltage after a full charge, the battery is degraded.
For a more meaningful health assessment, you need a load test. This applies a controlled electrical load to the battery and measures how the voltage drops under stress. A healthy car battery should stay above 10V during a load test. Batteries that show a sharp voltage drop under load have weakened cells, even if their resting voltage looks fine. Dedicated battery load testers handle this automatically, but you can approximate it with a multimeter by reading voltage while the battery powers a known load.
Why Discharge Rate Affects Capacity
A battery’s rated capacity assumes a specific, steady discharge rate. If you drain it faster, you get less total energy out of it. This relationship, described by Peukert’s Law, matters for any application where you’re trying to predict runtime.
The intuition seems wrong at first. If a battery rated at 100 amp-hours takes 20 hours to drain at a 5-amp draw, you’d expect a 10-amp draw to last exactly 10 hours. In practice, it lasts noticeably less than 10 hours. Higher current generates more heat and internal resistance, which wastes energy. This effect is most pronounced in lead-acid batteries and less severe in lithium-ion cells, but it exists in all battery chemistries. Every battery has a unique constant that determines how sensitive it is to discharge rate.
This is why manufacturer runtime estimates can feel optimistic. They’re usually based on a light, steady load. If you’re gaming on a laptop or running power tools off a battery pack, your actual runtime will fall short of the rated capacity because you’re pulling current faster than the test conditions assumed.
Calibrating Your Battery Gauge
The battery percentage on your phone or laptop isn’t a direct measurement. It’s an estimate from a small chip called a fuel gauge that tracks how much charge flows in and out. Over time, this estimate drifts, and you might see your device jump from 20% to dead, or sit at 100% for an unusually long time.
Recalibrating is straightforward: let the battery drain until the device shows a low battery warning and shuts down, then charge it all the way to 100% without interruption. The full discharge sets a reference floor, and the full charge sets a reference ceiling, giving the fuel gauge two accurate anchor points. You don’t need to do this often. Once every few months is enough to keep the percentage reading accurate.
What “End of Life” Actually Means
Most lithium-ion batteries are considered at end of life when their capacity drops to 80% of the original rating. Some manufacturers use a 70% threshold instead, which makes their cycle life numbers look better on paper. A battery rated for 6,000 cycles to 80% capacity might claim 8,000 cycles if the spec uses 70% as the cutoff. Always check which standard a manufacturer is using when comparing battery longevity claims.
For context, a typical lithium iron phosphate cell can last 4,000 to 6,000 cycles before hitting 80% capacity at room temperature. Heat accelerates degradation significantly, so the same cells in a hot environment will reach end of life much sooner. In practical terms, a phone battery that’s lost 20% of its original capacity doesn’t just last 20% less per charge. Combined with the software overhead of a device that’s aged alongside the battery, the real-world impact on daily use is usually more noticeable than the percentage alone suggests.
Electric Vehicle Battery Health
EV batteries use a metric called State of Health (SoH), expressed as a percentage of the battery’s original capacity. Most EVs display this in the dashboard or infotainment system, though some brands make it easier to find than others. Third-party OBD-II diagnostic tools can pull more detailed data, including individual cell voltages and temperature readings across the pack.
Measuring SoH accurately in an EV is more complex than in a phone because the battery pack contains thousands of cells, and degradation isn’t always uniform. The most common method, called ampere-hour integration, tracks the total charge flowing in and out of the pack over time. More advanced approaches analyze how the battery’s internal resistance changes during charging, particularly at the moments when charging current switches between stages. These resistance patterns correlate closely with aging and can estimate SoH with less than 2% error. For most EV owners, checking the built-in health readout every few months and tracking the trend is sufficient to know when the pack is approaching the point where range loss becomes noticeable.