Battery capacity describes how much charge or energy a battery can store and deliver; battery life usually means how long a device runs before it needs recharging. Capacity helps determine runtime, but it does not guarantee a particular number of hours: the device’s power use and the battery’s condition also matter.
What do battery capacity and battery life mean?
Capacity is a battery specification, not a measure of time. It describes the charge a battery can deliver under stated conditions, up to a specified cut-off voltage. Panasonic explains the definition and units in its Battery Terminology.
“Battery life” can mean two different things. In everyday device comparisons, it generally means runtime: the time between charges. It can also mean lifespan: how long the battery remains serviceable before it needs replacement. Apple distinguishes runtime from lifespan in its guide to iPhone battery and performance.
What do mAh, Ah, and Wh tell you?
Ampere-hours (Ah) and milliampere-hours (mAh) measure electric charge. One Ah is 1,000 mAh. Neither unit directly tells you how many hours a device will run, because that depends on how quickly the device draws power.
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Watt-hours (Wh) measure energy and account for voltage. As a practical approximation, Wh = Ah × nominal voltage. That makes Wh a better comparison when batteries have different voltages: two batteries with the same mAh rating can store different amounts of energy. Keysight explains the distinction between charge capacity and energy ratings in Simplify Validating a Battery’s Capacity and Energy Ratings.
Why doesn’t a capacity rating predict runtime?
Runtime depends on the usable energy available and the device’s average power demand. A simplified estimate is runtime in hours ≈ usable energy in Wh ÷ average power in W. Treat this as an estimate, not a promise: power draw changes with workload, screen brightness, network activity, and power management, while real usable energy depends on discharge conditions and conversion losses.
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- RELIABLE PERFORMANCE: 24-pack of AA rechargeable NiMH batteries (2,000 mAh)
- DEVICE COMPATIBLE: Ideal for remote controls, flashlights, clocks, and more
- LONG LASTING: Can be recharged up to 1000 times with minimal power loss
- LOW SELF DISCHARGE: Maintains 80% capacity for 2 years
- EASY USE & STORAGE: Has a shelf-life up to 5 years for everyday or emergency use; arrives pre-charged and ready to use
That is why a high-capacity battery can still drain quickly during demanding use. Dell describes runtime as depending on capacity and operating conditions in its laptop battery guidance; Microsoft likewise notes that power-hungry workloads can drain a device quickly even when its battery capacity is high in its Battery Health Report guidance.
How to compare batteries or device runtime
- Comparing battery energy: Compare Wh when voltage or pack configuration differs; do not rank batteries by mAh alone.
- Comparing device runtime: Compare hours only when the devices and test conditions are reasonably similar, including workload, brightness, network use, and test method.
- Checking battery condition: Distinguish current full-charge capacity from design capacity when new. A percentage of original capacity does not, by itself, tell you how many hours remain.
For example, a battery health report may show how much energy the battery can hold now compared with its design capacity. Microsoft defines those measures as current full-charge capacity and the battery’s original capacity when new in its battery health documentation. It is useful for understanding wear, but runtime still depends on the device’s power draw.
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- 4 pack of Energizer Power Plus Rechargeable AA Batteries
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Why power-bank mAh labels can mislead
A power bank’s advertised mAh may describe its internal cells at one voltage, while the USB output uses another. Voltage conversion and circuit losses mean the energy delivered to a device is not identical to the energy stored in those cells, and output-side mAh can be lower. For a clearer energy comparison, look for Wh and remember that conversion still affects what reaches the device. TP-Link explains the voltage conversion and efficiency issue in Understanding the capacity (mAh) and the charge efficiency of a power bank.
Quick Recap
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