Battery life is measured using a combination of capacity units, discharge rates, and standardized test cycles that together describe how much energy a battery stores, how quickly it delivers that energy, and how long it lasts before needing replacement. The two most common units you’ll encounter are milliampere-hours (mAh) for small devices and watt-hours (Wh) for larger ones like laptops and electric vehicles. But those numbers only tell part of the story.
mAh vs. Wh: Two Ways to Express Capacity
Milliampere-hours (mAh) and watt-hours (Wh) both describe battery capacity, but they measure different things. mAh tells you how much electrical charge a battery can supply at a given voltage over time. A 5,000 mAh battery can deliver 5,000 milliamperes of current for one hour. This is the number you’ll see on phone batteries and portable chargers.
Watt-hours measure total energy, which accounts for both current and voltage. A 50 Wh battery can deliver 50 watts of power for one hour. Wh is more useful for comparing batteries that run at different voltages, which is why laptops, power stations, and electric vehicles use this unit instead of mAh.
You can convert between the two if you know the battery’s voltage:
- Wh to mAh: mAh = (Wh × 1,000) / Voltage
- mAh to Wh: Wh = (mAh × Voltage) / 1,000
A 5,000 mAh phone battery operating at 3.7 volts, for example, holds about 18.5 Wh of energy. This conversion matters when you’re comparing power banks to laptops or trying to figure out airline carry-on limits, which are set in Wh.
C-Rate: How Fast a Battery Drains
Capacity alone doesn’t tell you how long a battery will actually last, because that depends on how fast you draw power from it. This is where C-rate comes in. C-rate expresses the discharge (or charge) speed relative to the battery’s total capacity. A 1C rate means the battery fully discharges in one hour. A 0.1C rate means it takes 10 hours. A 5C rate drains it in just 12 minutes.
For a 10 Ah battery, those rates translate to real-world currents:
- 1C: 10 amps of current, fully drained in 1 hour
- 5C: 50 amps of current, fully drained in 12 minutes
- 0.1C: 1 amp of current, fully drained in 10 hours
Higher C-rates generate more heat and reduce the usable capacity you actually get from a single charge. This is why a manufacturer’s rated capacity, tested under gentle lab conditions, often exceeds what you experience when running demanding applications.
Cycle Life: Measuring Long-Term Durability
Battery life also refers to how many times you can recharge a battery before it loses meaningful capacity. The industry standard defines end of life as the point when a battery retains only 80% of its original capacity. One cycle equals one full discharge followed by one full recharge, though partial cycles count proportionally (two half-discharges equal one cycle).
When manufacturers quote cycle life, they record the “initial” capacity after the first one to five charges, since battery chemistry needs a few cycles to stabilize. From that baseline, they count how many full cycles it takes to hit the 80% threshold. A typical smartphone battery might be rated for 500 to 800 cycles, while an electric vehicle battery might last 1,000 to 2,000 or more. After hitting 80%, the battery still works. It just holds noticeably less charge than when it was new.
How Your Device Tracks Remaining Charge
The battery percentage on your phone or laptop comes from a method called coulomb counting. The battery’s management chip continuously monitors the current flowing in and out, integrating that flow over time to estimate how much charge remains. Think of it like a water meter: it tracks how much has been poured out and how much has been added back, then calculates what’s left in the tank.
Coulomb counting is simple and requires very little processing power, which is why it’s the standard approach in consumer electronics. Its weakness is that small measurement errors accumulate over time, causing the percentage display to drift. This is why your phone might jump from 15% to dead unexpectedly, or why a full discharge and recharge occasionally “recalibrates” the reading. The chip resets its reference point when the battery hits a known empty or full state.
Temperature Changes Everything
All battery capacity ratings assume testing at room temperature, typically around 22°C (72°F). Real-world temperatures can dramatically alter what you actually get. Lab testing on lithium-ion cells shows that capacity drops about 15% at roughly negative 10°C (14°F) and plummets by 35% at negative 20°C (negative 4°F). Even moderate cold, around 2°C (36°F), reduces usable capacity by about 5%.
Heat is a different problem. At around 52°C (126°F), capacity also dips by roughly 5% in the short term, but sustained high temperatures accelerate permanent degradation, shortening cycle life over months and years. This is why EV manufacturers include thermal management systems and why your phone charges more slowly on a hot day. The rated mAh or Wh on the label represents ideal conditions that rarely match the environment your battery actually lives in.
Internal Resistance as a Health Indicator
As batteries age, their internal resistance rises, which means more energy is lost as heat during charging and discharging. Measuring this resistance is one of the most reliable ways to assess battery health beyond simple capacity numbers. There are two main approaches.
DC methods (sometimes called current interrupt testing) work by briefly cutting the current and measuring how quickly the voltage recovers. This gives a snapshot of resistance at a single moment. AC methods, particularly electrochemical impedance spectroscopy, send tiny alternating currents at various frequencies through the battery and map how it responds. The AC approach is more detailed because it can detect specific aging effects that a single DC measurement would miss, like degradation at the electrode surfaces versus changes in the electrolyte.
For most consumers, you won’t perform these tests yourself. But when your phone’s settings report “battery health” as a percentage, or when an EV service center evaluates your pack, they’re using some version of these resistance and capacity measurements behind the scenes.
How EV Range Gets Its Official Number
Electric vehicle range testing is one of the most visible applications of battery life measurement, and two competing standards produce different numbers for the same car. The EPA test used in the United States starts with a fully charged battery and drives the vehicle continuously over standardized city and highway cycles until the battery is completely dead. The distance is recorded for each cycle separately.
Those raw distances then get multiplied by a factor of 0.7 to account for real-world conditions like climate control, aggressive driving, and terrain. The adjusted city and highway ranges are blended together using a 55% city, 45% highway weighting to produce the combined range number on the window sticker. The European WLTP standard uses a different driving cycle with generally less aggressive adjustment factors, which is why the same EV often shows a higher range number in Europe than in the United States.
Automakers can also opt for a more complex multi-cycle EPA test that includes four city cycles, two highway cycles, and two constant-speed segments. This longer sequence sometimes produces slightly different results than the simpler single-cycle test.
Energy Density: Comparing Battery Technologies
When comparing different battery chemistries, energy density is the key metric. It’s measured in watt-hours per kilogram (Wh/kg) for weight or watt-hours per liter (Wh/L) for volume. Current lithium-ion cells in EVs typically deliver 250 to 300 Wh/kg. Next-generation solid-state batteries are targeting 400 to 500 Wh/kg for commercial products, with some developers claiming potential for 600 Wh/kg. At 400 to 500 Wh/kg, an EV could theoretically achieve over 1,200 km of range from a single charge.
Higher energy density means either a lighter battery for the same range or more range for the same weight. It’s the single number that best captures how far battery technology has advanced, and it’s the reason your current smartphone lasts longer than one from ten years ago despite being thinner.